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Witchweed (Striga asiatica): A destructive crop plant parasitic weeds
Nweze Agatha Chidiebere1, Nweze Justus Amuche
2 and Nweze Julius Eyiuche
2
1Department of Plant Science and Biotechnology
2Department of Microbiology
Faculty of Biological Sciences,
University of Nigeria, Nsukka
(How to cite: Nweze, A.C., Nweze, J.A., and Nweze J.E. (2015). Witchweed (Striga asiatica): A destructive crop
plant parasitic weeds. Retrieved from https://nwezejustus.wordpress.com/2015/07/28/witchweed-striga-asiatica-a-
destructive-crop-plant-parasitic-weeds/)
ABSTRACT
The parasitic weed, Striga asiatica, is a major biotic constraint and a serious threat to
subsistence cereal crop productions especially in sub-Sahara Africa. Severity of the parasitic
weed in this area is aggravated by the inherent low soil fertility, recurrent drought and natural
resources degradation. They causes symptoms like stunting, wilting and chlorosis which are
similar to those seen from severe drought damage, nutrient deficiency and vascular disease.
Striga produces numerous minute seeds which can remain dormant in the soil for as long as 10
years. Dispersion of the seeds is primarily by water or wind, or by human movement of soil,
plant, or machinery. The germination of Striga seeds depends on the perception of germination
stimulants released by nearby host roots. Striga unlike most weeds which merely compete with
crops, do their damage more directly. They rob nutrients and moisture by tapping directly into
the host’s root system using haustoria.
In tackling the negative effect of this weed, different control measures have been
recommended: use of cultural and mechanical control, biological control, chemical control,
increasing soil fertility, use of resistant host crops and integrated control management. However,
an integrated control management is recommended as the best method in controlling Striga
asiatica.
TABLE OF CONTENTS
Title Page _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Dedication _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Preface _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Summary _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Table of Contents _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _
CHAPTER ONE: INTRODUCTION _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
1.1 Introduction _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
1.2 Background Information _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _
1.3 Definition of Terms _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
CHAPTER TWO: ORIGIN, OCCURRENCE AND DISTRIBUTION _ _ _ _ _ _ _
2.1 The Genus Striga_ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2.2 Distribution _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2.3 Description and Biology of Striga asiatica _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2.4 Host Crops and other Plants _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _
CHAPTER THREE: IMPACT OF WITCHWEED INFESTATIONON THE HOST
AND LIFE CYCLE _ _ _ _ _ __ _ _ _ _ __ _ _ _ _ __ _ _ _ _ __ _ _ _ _ __ _ _ _ _ __ _ _
3.1 The Impact - Distribution and Host Range _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
3.2 Conditions favouring Striga asiatica growth _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _
3.3 Soil Fertility and Striga weed _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _
3.4 The Parasitic Life Cycle of Striga asiatica _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
3.2.1 Germination and location of a host root _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _
3.2.2 Haustorium development _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _
3.2.3 Establishment of parasitism and life cycle completion _ _ _ _ _ _ _ _ _ __ _ _ _
CHAPTER FOUR: WITCHWEED (Striga asiatica) CONTROL METHODS _ __ _
4.1 Control Strategies targeting Germination/Host locations _ _ __ _ __ _ __ _ __ _ __ _ __ _
4.2 Cultural and Mechanical Control Methods _ _ __ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
4.3 Biological Control Methods _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
4.4 Chemical Control Methods _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
4.5 Integrated Striga Management _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
CHAPTER FIVE: CONCLUSION AND RECOMMENDATIONS _ _ _ _ _ _ _ _ _
5.1 Next-Generation Striga Research _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
5.2 Recommendations _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
5.3 Conclusion _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
REFERENCES
CHAPTER ONE: INTRODUCTION
1.1 Introduction
Parasitic plants are a major threat to today’s agriculture and provide an intriguing case of
pathogenesis between species of relatively close evolutionary ancestry. Almost all crop species
are potential hosts for parasitic plants, but severe disease outbreaks are usually restricted to
certain host–pathogen combinations. The evolutionary strategy of exchanging autotrophy for
dependence on host plants (parasitism) may seem odd, but it has proven to be evolutionarily
successful for several plant species. Plant parasitism has arisen at least 12 times independently,
generating more than 4000 parasitic dicotyledonous plant species (Westwood et al., 2010).
According to Parker (2009), some parasitic plants are still photosynthectically active
(hemi parasitic), others are not, and depend entirely on a host (holophrastic). The establishment
of parasitism is essential for holoparasites and several hemi parasites, and therefore these species
are called obligate parasites. Depending on which host organ is infected, parasitic plants are
grouped into stem or root parasites. In both cases, the parasite connects to the host vascular
system via a specialized feeding organ, the haustorium. Unlike the haustoria of plant-pathogenic
fungi or oomycetes, plant haustoria are always multicellular organs with complex anatomies and
multiple cell types. The genus Striga consists of obligate hemi parasitic root parasites, some of
which are serious agricultural pests.
Striga is a parasitic weed which is popularly called ‘witch weed’ attacking a wide range
of crops. Striga asiatica is amongst the world’s worst weeds (Nail et al., 2014), reducing the
value of grain crops, particularly in Africa. Striga asiatica is an obligate parasite, drawing
moisture, nutrients and photosynthetic products from its graminoid host plants (mostly C3
plants) (Figure 1 below). Host plants are typically subsistence crops, including wheat, corn
(maize), sorghum, rice, sugarcane and cowpeas. Striga asiatica is typically found in dry, infertile
soils in semi-arid tropical grasslands and savannahs (Spallek et al., 2013). Thus, its effects are
disproportionately felt by poorer farmers on marginal lands. Striga spp. are prolific seed
producers. The fine dust-like seed can last more than 15 years, and consequently, eradication and
control attempts are extremely difficult and prolonged. As with other Striga spp., S. asiatica
reduces crop yields by extracting water, nutrients (particularly nitrogen), and photosynthetic
from the root system of its host plant, resulting in stunting and yield reduction (Parker, 2009).
Figure 1. Witchweed, Striga asiatica (L.). Photograph from USDA APHIS PPQ Archive, USDA APHIS PPQ, Bugwood.org.
http://www.forestryimages.org/browse/detail.cfm?imgnum=1148114#sthash.38odDuRw.dpuf
Striga parasitism according to review by Yoshida and Shirasu (2012), causes severe
chlorosis, wilting, and stunting of susceptible hosts, resulting in yield losses that range from
slight to 100%. The attack of this weed causes a lot of economic losses. ). Four species out of
thirty are known to be of economic importance in Nigeria, these are Striga hermonthica, Striga
gesnerioides, Striga asiatica and Allectra vogelli. . Some of the visible damages caused by this
weed include blotching, scorching, wilting, loss of vigour and finally death of the plant. Other
adverse effects on crops are a reduction in the ear size, plant height, stem diameter and weight of
the whole plant. In addition, severe damage on roots as well as stem lodging may also be
observed. It could be observed that Striga spp that attack cereals are distinct from those on
legumes. For example, studies in Nigeria have shown that Striga hermonthica and Striga
gesnerioides dominate attack mostly cereals. While Striga asiatica and Allectra vogelli attack
mostly legumes. Striga species exhibit variation in their mode of reproduction. S. hermonthica
and S. gesneriodes are allogamous that is they observe cross pollination and usually rely on
vectors such as bees and other agents of pollination for pollen transfer (Dugje et al., 2008). S.
asiatica on the other hand is autogamous that is it observes self pollination and so, no vectors are
needed for pollination instead pollens are picked by the elongation of style and fertilization takes
place (Spallek et al., 2013).
1.2 Background Information
According to review by Spallek et al. (2013), Nail et al. (2014) and Teka (2014), Striga
asiatica is known with the following:
Common Names: Witchweed, red witch weed, Asiatic witch weed, bury, common mealie
witchweed, isona weed, Matabele flower, mealie poison, mealie witchweed, scarlet lobelia, yaa
mae mot.
Scientific Name: Striga asiatica (L.) Kuntze
Synonyms: Buchnera asiatica L., Stiga hirsuta Benth., Striga coc-cinea Benth., Striga lutea
Lour., Striga parvula Miq., Striga pusilla Hochst., Striga spanogheana Miq., Striga
zangebarica Klotzsch
Taxonomy: Kingdom: Plantae; Class: Magnoliospida; Order: Lamiales; Family: Orobanchaceae
Primary Crop Hosts: Maize (Zea mays), rice (Oryza sativa L.), sorghum (Sorghum spp.),
millets (Pennisetum spp., Panicum spp., Eleusin spp., Digitaria spp., etc.), sugarcane
(Saccharum spp.), e.t.c.
Description: Witchweeds are characterized by bright-green stems and leaves and small, brightly
colored and attractive flowers. They are obligate hemi parasites of roots and require a living host
for germination and initial development, though they can then survive on their own. The number
of species is not certain, but exceeds 40 by some counts.
Hosts and symptoms: Although most species of Striga are not pathogens that affect human
agriculture, some species have devastating effects upon crops, particularly those planted by
subsistence farmers. Three species cause the most damage: Striga asiatica, S. gesnerioides, and
S. hermonthica. Witchweed parasitizes maize, millet, sorghum, sugarcane, rice, legumes, and a
range of weedy grasses. It is capable of significantly reducing yields, in some cases wiping out
the entire crops. Host plant symptoms, such as stunting, wilting, and chlorosis, are similar to
those seen from severe drought damage, nutrient deficiency, and vascular disease.
1.3 Definition of Terms
A parasitic plant: is one that derives some or all of its nutritional requirements from another
living plant. All parasitic plants have special organs, named haustoria (singular: haustorium),
which connect them to the conductive system of their host and provide them with the ability to
extract water and nutrient from the hosts. Parasitic plants have a modified root, the haustorium
that penetrates the host plant and connects to the xylem, phloem, or both (Nickrent, 2007).
Striga: ‘Striga’ is a Latin word for ‘witch’. Striga is known as witch weed because plants
diseased by Striga display stunted growth and overall drought-like phenotype long
before Striga plants appear (Nail et al., 2014).
Obligate parasite – a parasite that cannot complete its life cycle without a host.
Holoparasite – a plant that is completely parasitic on other plants and has virtually no
chlorophyll.
Hemi- parasite – a plant that is parasitic under natural conditions and is also photosynthetic to
some degree. Hemiparasites may just obtain water and mineral nutrients from the host plant.
Many obtain at least part of their organic nutrients from the host as well (Nickrent, 2007).
CHAPTER TWO: ORIGIN, OCCURRENCE AND DISTRIBUTION
2.1 The Genus Striga: Plant Parasites among Plant Parasites
Striga species are annual plants and most of their life cycle occurs underground. The
genus Striga was previously grouped within the family Scrophulariaceae, but more recent
analyses have placed Striga as a monophyletic group in the family Orobanchaceae Vent. The
family Orobanchaceae contains the highest number of parasitic species. Although most
Orobanchaceae species are root parasites, ranging from facultative hemiparasitic plants (such as
Triphysaria Fisch), to holoparasitic Orobanche L. (broomrapes), 12 known species in the genus
Lindenbergia Lehm are not parasitic (Spallek, et al., 2013). This offers an opportunity to study
successive stages in plant parasitism within the relatively confined evolutionary boundaries of
one plant family (Westwood et al., 2010). Parasitism is believed to have evolved once within
this family and the divergence of the Lindenbergia linage predates this event. Specialization
towards holoparasitism then followed in several genera independently, often leading to closely
related species with different degrees of parasitism (Spallek, et al., 2013).
Approximately 30 Striga species have been described and most parasitize grass species
(Poaceae). Striga gesnerioides (Wild.) Vatke is the only Striga species that is virulent to dicots
(Mohamed and Musselman, 2008). Striga possibly originates from a region between the Semien
Mountains of Ethiopia and the Nubian Hills of Sudan (Atera and Itoh, 2011). This region is also
the birthplace of domesticated sorghum (Sorghum bicolor L.), which is a major host species for
several Striga species, including S. hermonthica (Delile) Benth. and S. asiatica (L.) Kuntze, and
is believed to be the host on which monocot-parasitizing Striga species have evolved and spread
throughout Africa and Asia (Westwood et al., 2010).
Striga asiatica is morphologically similar to S. hirsuta Benth., S. lutea Lour. and S.
elegans Benth., and therefore they are grouped into one Striga cluster. A few S. asiatica races or
ecotypes occur outside Africa, mainly in Asia (Mohamed and Musselman, 2008). Because the
evolutionary relationship between African and Asian S. asiatica populations is not well
understood, the populations are often treated separately. Striga hirsuta, S. lutea and S. elegans
are not considered to be serious agricultural pests (Mohamed and Musselman, 2008). Striga
asiatica is autogamous like most Striga species, but S. hermonthica and S. aspera (Willd.)
Benth. are obligate outcrossers and occasionally hybridize (Mohamed and Musselman, 2008).
Allogamy probably contributes to the genetic variation between subpopulations of S.
hermonthica, and also restricts spread outside the geographical distribution of available
pollinators (Dugje et al., 2008).
A recent phylogenetic analysis using six chloroplastic loci has suggested a closer
relationship of S. gesnerioides to S. aspera and S. hermonthica than S. asiatica to S. hermonthica
and S. aspera, despite the similar host specificities of S. asiatica, S. hermonthica and S. aspera.
Striga gesnerioides is morphologically distinct relative to other Striga species. The haustoria
differ especially in size and morphology from those of monocotparasitizing Striga species. The
haustoria of S. gesnerioides, in contrast with those of other Striga species, such as S.
hermonthica, exhibit a branched vascular system and lack the so-called hyaline body, which is a
specialized tissue surrounding the xylem bridge connecting the vascular systems of host and
parasite (Estep et al., 2012).
2.2 Known Distribution
Striga asiatica is the most widespread of the 42 or so Striga species (Timko et al., 2007).
It is native to sub-Saharan Africa, and many countries in tropical Asia (Figures 2 and 3). It has
been introduced to the USA (North Carolina, South Carolina), New Zealand, Papua New Guin-
ea, and most recently Australia. The distribution records in Northern Sudan reported by Atera et
al. (2011), the Namibian records noted in GBIF, and the Egyptian report by Timko et al. (2007)
are all presumably reliant on irrigation.
It has been reported that five of the Striga spp. cause devastating effects on crops: S.
hermontica, S. asiatica, S. forbsii, S. aspera and S. gesnerioides (Westwood et al., 2010). Table
1 shows the distribution of Striga in Africa and S. asiatica is said to have a wide world
geographic distribution as compared to others (Timko et al., 2007). Dugje et al. (2008) stated
that in Nigeria three major Striga species have been found to be infecting crops: S. hermonthica
(sorghum, rice and maize), S. aspera (rice) and S. gesnerioides (cowpea). In the savannas of
guinea, S. aspera occurs in the hydromorphic areas where rice is grown, while S. hermonthica
and S. asiatica are found in the free draining upland areas and are regarded as the most
infectious. Notably S. aspera is predominantly found in West Africa and sporadically exists in
Ethiopia and Tanzania overlapping with S. hermonthica.
Generally Striga spp. grows in areas with annual rainfall ranging from 25-150 cm per
year with decrease in severity of infestation in areas of high rainfall. However, S. forbisii mainly
occurs in wet areas and even in water logged conditions infecting wild grasses in swamps and
irrigated crops in Cote d’Ivore and Tanzania.
Table 1. Distribution and occurrence of Striga spp. in sub-Saharan Africa (Timko et al., 2007; Atera et al.,
2011)
There are records indicating S. hermonthica and S. aspera infections on rice in Northern
Cameroon, Northern Nigeria, Benin, Togo and westwards. It has also been reported that S.
hermonthica infects upland rice in Western Kenya and S. asiatica causes serious losses in upland
rice along the Indian Ocean Islands (Mohamed and Musselman, 2008).
2.3 Description and Biology of Striga asiatica
Striga asiatica is an annual obligate hemi-parasite of monocotyledonous plants. It
reproduces by seed, producing tens of thousands of minute seeds per plant. The seeds are quite
cold-tolerant, able to withstand prolonged storage at -7 °C. However, the minimum temperature
for germination is a relatively high 20 °C and the optimum temperature for growth appears to be
approximately 32 °C (Ejeta, 2007; Hearne, 2009; Mahmoud et al., 2013). Hearne (2009)
noted that S. asiatica can withstand temporary water-logging.
Seedlings are not visible above ground, but have white succulent shoots that attach to
host roots via a horstorium. By this means the parasitic plant develops underground until it
produces a stem that surfaces. The above ground parts of mature plants have green foliage
sparsely covered with coarse, short, white, hairs. Plants are normally 15-30 cm tall but can grow
to 60 cm. Small flowers (less than 1.5 cm in diameter) occur in summer and fall, with colours
varying regionally, from red, orange, or yellow in Africa to pink, white, yellow, or purple in
Asia. The flowers develop into swollen seeds pods, each containing thousands of microscopic
seeds. Dispersal is primarily by wind or water, or by human movement of soil, plants, or
machinery (Fen et al., 2007; Waruru, 2013).
2.4 Host Crops and other Plants
Primary crop hosts include wheat, corn (maize), sorghum, rice, and sugarcane. However,
S. asiatica is also known to infest other grasses and some broad leaf crops (e.g. sunflower,
tomatoes, and some legumes) (Mahmoud et al., 2013).
CHAPTER THREE: WITCHWEED INFESTATION AND LIFE CYCLE
3.1 The Impact - Distribution and Host Range
Striga is an ‘Old World’ parasite, and several species were already recognized as cereal
pests in Africa and India at the beginning of the last century. Roughly 80% of the described
Striga species are endemic to Africa, nine species are found outside Africa and three species, S.
curvilflora Benth., S. multiflora Benth. and S. parviflora Benth., are present on the Australian
continent (Nail et al., 2014). Striga species are predominantly found on open grasslands and
savannahs in semi-arid tropical regions. Infestations are more pronounced in infertile soils, but S.
asiatica can grow in a wide range of different soils (Atera et al., 2011). An increase in
monoculture in some parts of Africa has led to reduced soil fertility, thus further worsening the
situation with regard to Striga infestations (Nail et al., 2014). In addition to the presence of host-
derived germination stimulants, temperature is an important factor affecting the distribution of
Striga, as prolonged exposure to high temperatures and humid conditions is required to break
seed dormancy in Striga (Ejeta and Gressel, 2007).
An estimated cereal production area of 50 million hectares, approximately the size of
Spain, shows different levels of Striga infestation in Africa (Westwood et al., 2010). In total, 25
African countries reported Striga infestations in 2005 (De Groote et al., 2008). The
socioeconomic consequences are difficult to measure, but a few estimations have suggested that
Striga affects the life of more than 100 million people in Africa and causes economic damage
equivalent to approximately 1 billion $US per year (Labrada, 2008; Waruru, 2013). Host plants
include sorghum, millet, maize, upland rice, sugarcane, cowpeas—representing the most
important stable crops grown by subsistence farmers in affected areas. Farmers have reported
losses between 20% and 80%, and are eventually forced to abandon highly infested fields (Atera
and Itoh, 2011).
The extent of yield losses cannot be explained solely by competition for nutrients and
water. When disease progresses, very severe symptoms, such as water-soaked leaf lesions,
chlorosis, necrosis and leaf desiccation, occur. An unknown phytotoxin has been proposed to at
least partially contribute to the disease phenotype, but still awaits biochemical identification.
Interestingly, Striga extracts are rich in secondary metabolites and find broad use in traditional
medicine, especially as a result of their antimicrobial activity (Koua and Babiker, 2011). Only
five Striga species are currently of economic importance, with S. hermonthica causing by far the
most serious damage to sub-Saharan cereal production, followed by S. asiatica, S. gesnerioides
and, to a far lesser extent, S. aspera and S. forbesi Benth. (Parker, 2009).
Figure 2: Striga affects millions of smallholder farmers in sub‐Saharan Africa
Facultative parasitic plants of the sister genus Buchnera L. are sometimes mistaken for
Striga, but cause far less damage on host plants such as sorghum, maize or millet. The obligate
parasitic species Alectra vogeli (Benth., Orobanchaceae) is commonly also referred to as yellow
witchweed and, similar to S. gesnerioides, is a major biological constraint to cowpea production
in eastern and southern Africa (Westwood et al., 2010).
3.2 Conditions favouring Striga asiatica growth
Striga infestation is steadily increasing as a result of continuous cultivation of cereal
crops. Over used, depleted and infertile soils have resulted to high infestation of Striga. Pressure
on land for continuous cropping of high yielding cereal crops without rotation or moving to
other new areas has resulted to exhausted soils. These are the soils that favour Striga infestation
in addition to soil moisture stress conditions (Khan et al., 2008). Less shading due to poor
growth of the host crop on poor soils contributes to heavy infestation. This has compounded the
problem for small-scale farmers who can least afford inputs on unproductive land, and thus
continues mono-cropping (planting of the same crop on the same area) for several years.
Infestation in some areas has reduced yield to the extent that abandonment and migration is
necessary. Improper management of Striga weed has contributed to its existence in Sub-Saharan
Africa (SSA) for a long time (Yoshida, S. and Shirasu, 2012).
Poverty level of small scale farmers has enhanced the spread of Striga through sharing of
seeds collected from the previous crop harvest. In addition, Striga pandemic in Sub-Saharan
Africa has increased due to non advocacy of nutrient replenishment of the soils as a result of
mono-cropping, a factor for increased infestation of the weed in size and severity (Westwood et
al., 2010). Striga produces several seeds, and during tillage the seeds are incorporated into the
soil where they can be dormant for many years. Over time they are spread to new areas by
human beings through the tools used for land preparation and weeding. The seeds are also spread
by animals moving from one field to another for grazing purposes. This has culminated to a
complex system of spreading the weed to new areas thus reducing crop yield of farmers who are
not aware of the devastating effect (Hearne, 2009).
3.3 Soil Fertility and Striga weed
Parasitic weeds such as Striga establish preferentially in poor nutrient fields which have
been exhausted by continuous cropping (De Groote et al., 2010). Most Striga infested areas are
characterized by Agricultural Production Systems exhibiting low productivity. These areas tend
to be managed traditionally with low inputs and continuous cereal cropping without crop
rotation. The use of inorganic nitrogen and organic fertilizers such as manure and compost has
been reported to reduce Striga infestations (Ayongwa et al., 2010). According to De Groote et
al. (2010), manure applications have been shown to be as effective as fallowing in maintaining
soil productivity. The positive benefits of applying manure include an increase in pH, water
holding capacity, hydraulic conductivity, infiltration rate and decrease in bulk density. Manure is
also an important source of N, P and K. To enhance the quality and effectiveness of traditional
soil fertility maintenance strategies such as manure application, a fertilizer augmented soil
enhancement strategy need to be adopted to reduce the infections of Striga.
3.4 The Parasitic Life Cycle of Striga asiatica
Parasitic plants have evolved specific traits which allow parasitism or reflect the
consequences of adaptation to a parasitic life style. Critical stages in the life of an obligate root
parasite as shown in figure 4, are as follows (Parker, 2009; Atera and Itoh, 2011; Cardoso et al.,
2011): (i) the identification of a suitable host, thus coupling germination and seedling growth
with the presence and direction of a potential host; (ii) gain of access to the host’s nutrients and
water supply; this process involves the production of a functional haustorium; (iii) completion of
the life cycle on the host; this includes the establishment of parasitism and its maintenance until
seeds are set.
3.2.1 Germination – location of a host root
Striga and other root-parasitic plants have evolved highly efficient strategies to ensure
successful reproduction. Key strategies include the dispersal of an enormous amount of tiny
seeds (Fig. 4 below) of high longevity to establish an extremely persistent seed bank. These
dust-like seeds are easily dispersed by wind, crop seeds, water and people. In addition, Striga
seeds can survive for more than 10 years before germination (Atera and Itoh, 2011).
Germination is linked to the presence of a nearby host, because the endosperm of Striga
seeds can sustain growth/life only for the first 3–7 days. Within that time, Striga must
successfully establish a parasitic relationship with the host plant or otherwise die. This aspect
was successfully exploited during an S. asiatica eradication programme in the USA, when so-
called ‘suicide germination’ was induced by fuming farmland with ethylene to trigger Striga
germination in the absence of host plants (Parker, 2009).
The germination of Striga depends on the perception of germination stimulants released
by host roots. In order to be responsive to germination stimulants, Striga seeds must go through
a phase of moisture and high temperatures for 7–14 days, called ‘conditioning’. If, during that
Figure 4: Life cycle of Striga witchweed
Figure 5: Striga‐infested cowpea
time, no germination stimulant is perceived, Striga seeds fall into a secondary dormancy
(Cardoso et al., 2011). Several germination stimulants have been isolated and include
strigolactones, dihydrosorogoleone, sesquiterpene, kinetin, coumarin, jasmonate, ethylene and
fungal metabolites (reviewed in Cardoso et al., 2011). Strigolactones are certainly the best
studied and extremely potent inducers of Striga germination. Strigolactones are associated with
the negative regulation of root and shoot branching (tillering). They also induce hyphal
branching of arbuscular mycorrhizal (AM) fungi, presumably to attract them in low-nutrient
environments (Xie and Yoneyama, 2010). Major discoveries in biosynthesis and perception have
been made in recent years, and key players have also been predicted to be present in Striga
(Cardoso et al., 2011; Yoshida and Shirasu, 2012). Strigolactones have been shown to induce the
germination of Striga at concentrations as low as 10-16 M (reviewed in Cardoso et al., 2011).
The first strigolactone was interestingly isolated from the root exudates of a nonhost
plant, cotton according to Atera and Itoh, (2011); indeed, the use of nonhost plants producing
high levels of Striga germination stimulants is a promising strategy in Striga control. In
particular, intercropping with the legume Desmodium has been proven to be successful in some
parts of Africa (Khan et al., 2008). Alternatively, low strigolactone-producing host plants reduce
Striga germination and thus infection (Umehara et al., 2008). Low Striga germination stimulant
activity is controlled in sorghum by one single recessively inherited gene, lgs (low germination
stimulant) (Satish et al., 2012). Lines showing low germination-inducing activity have been
shown to have good tolerance towards S. asiatica and S. hermonthica, but tolerance mediated by
low strigolactone production is less reliable when the Striga seed pool in the soil is high (Atera
and Itoh, 2011).
3.2.2 Haustorium development
The radical tip grows chemotropically towards potential host roots after germination. On
contact, Striga radicals stop growing, attach to host roots, form a haustorium and penetrate into
the root cortex of potential hosts. Most plants, including many nonhost plants, do not resist
attachment and penetration. An exception to this is Phtheirospermum japonicum (Thunberg)
Kanitz, a hemiparasitic plant commonly found in East Asia and relatively closely related to
Striga. The root exudate from P. japonicum is able to induce the germination of S. hermonthica,
but S. hermonthica radicals rarely penetrate to P. japonicum roots (Yoshida and Shirasu, 2009).
It is currently unknown whether P. japonicum actively inhibits the attachment of Striga or
whether it is lacking a factor required for Striga penetration.
Within 12 h of attachment, reorganization of the S. asiatica meristem is initiated.
Essential for this step is the perception of haustoria-inducing factors. Several naturally occurring
haustoria-inducing factors have been isolated and their mode of action is best studied by
following 2,6-dimethoxy-p-benzoquinone. DMBQ is a product of lignin oxidation and
decarboxylation of phenolic acids found in plant cell walls. The current model of DMBQ
perception is mainly based on work performed on S. asiatica and Triphysaria versicolor Fisch.
& C.A. Mey. (Yoshida and Shirasu, 2009; Bandaranayake et al., 2010). In summary according
to Bandaranayake et al. (2010), this model proposes that DMBQ is released from host cell walls
and eventually enters parasite cells. The NAD(P)H-dependent quinine reductase QR1 reduces
DMBQ to produce an unstable semiquinone intermediate, which is required for haustorium
development. Triphysaria QR1 is transcriptionally up-regulated in response to host root extracts
and QR1 knock-down roots are compromised in haustoria formation. A second quinone
reductase (QR2) does not respond to host root extracts, but to DMBQ, and could act as a parallel
detoxification pathway. A balance between the detoxification and accumulation of the
haustorium-inducing semiquinone might create an equilibrium-dependent threshold mechanism,
whereby a continuous exposure to DMBQ is required for haustoria formation.
In addition to chemical signals, a thigmotropic response is required for Striga to produce
morphologically normal haustoria. Within 24 h after contact, rapid cell division of the radical tip
stops and a hypertrophic growth phase begins. Penetration of the host epidermis is mediated by
the elongation of distal cells in the protoderm or epidermis and underlying ground tissue,
followed by rounds of periclinal and anticlinal divisions of these cells, leading to growth into the
cortex of host plants. When the host endodermis is reached, the most distal cells of the
haustorium elongate and divide, thus forming a palisade arrangement of cells. Break through the
endodermis is often delayed, but, once accomplished, vascular connections are established. In
general, penetration is completed 48–72 h after contact with a host root (Yoshida and Shirasu,
2009; Cardoso et al., 2011). A detailed scanning electron microscopy study by Yoshida and
Shirasu (2009) showed that invading Striga cells perforate the host vascular system using a
specialized structure, the osculum. Interestingly, no phloem-to-phloem connections have been
observed between Striga and host plants. Once xylem-to-xylem connections are established, the
cotyledons of Striga enlarge and break free from the seed coat within 24 h (Reviewed by
Cardoso et al., 2011).
Many nonhost plants allow the penetration of S. hermonthica and the early events of
haustorium formation. Although infection is mainly terminated in the cortex of Lotus japonicus
(Regel) K. Larsen, S. hermonthica reaches the stele in Arabidopsis and cowpea, but fails to
develop beyond the six-leaf-pair stage. Also, nonhost resistance in lettuce, marigold and cowpea
against S. asiatica is typically established in the cortex within 72 h post-infection (Yoshida and
Shirasu, 2009).
It is currently unknown which Striga genes are required to successfully infect susceptible
host plants. Haustorium development uses cellular processes similar to organogenesis processes
known in other autotrophic plants. For example, cycline promoter B1 is activated within 24 h
after DMBQ treatment in P. japonicum, and localized auxin and ethylene accumulation are
important for haustoria formation in T. versicolor (Ishida et al., 2011).
Haustoria constitute the interface between host and parasite. Although all parasitic plants
develop haustoria, haustoria differ anatomically between different species. Although Striga lacks
phloem-to-phloem connections, direct connections between sieve elements of Orobanche
crenata (Forsk.) and Vicia narbonensis (L.) were observed by electron microscopy (Reviews by
Yoshida and Shirasu, 2009)
The transmission of phloem-localized viruses or RNA molecules has been reported for
several parasitic plants, but not for Striga species (Leblanc et al., 2012). However, interspecies
plasmodesmata between S. gesnerioides and pea raise the possibility of symplastic transport of
nutrients and signalling molecules between Striga and host plants. The movement of DNA
molecules across graft junctions also occurs via cell-to-cell movement and does not involve
phloem connections (Stegemann and Bock, 2009). So far, there is no direct evidence of mRNA
transit between Striga species and host plants. However, host-induced silencing of b-
glucuronidase (GUS) gene expression in T. versicolor and the identification of several horizontal
gene transfer (HGT) events between S. hermonthica and monocot hosts suggest that mRNA and
other RNA molecules could travel between host and root parasite (Tomilov et al., 2008; Yoshida
et al., 2010)
Nevertheless, HGT has also been reported in several other parasitic plants and seems to
be more frequent in parasitic plants than in nonparasitic plants (Leblanc et al., 2012). It remains
to be shown whether and to what extent RNA molecules travel between Striga and host plants
and, if so, whether these molecules can function in trans.
Figure 6: General life cycle of Striga species (Ishida et al., 2011)
3.2.3 Establishment of parasitism and life cycle completion
After xylem-to-xylem connections have been established, Striga grows upwards and
adventitious roots are produced. These adventitious roots are able to form lateral (secondary)
haustoria on the same or other host plants. Facultative hemiparasitic plants, such as Triphysaria
or Phtheirospermum, produce exclusively lateral haustoria.
Secondary haustoria according to Westwood et al. (2010) are believed to be
evolutionarily older than primary or terminal haustoria. Under natural conditions, host plants are
usually parasitized by several Striga plants, and the parasites quickly become a metabolic sink
for photo-assimilates and nutrients. Nitrogen levels are at least twice as high in Striga as in host
plants. Depletion of nitrogen almost certainly affects host physiology and provokes lower host
photosynthesis rates, which are frequently associated with Striga infections. Several
photosynthetic parameters are reduced in sorghum plants infected with S. hermonthica,
including the electron transport rate through photosystem II and photochemical quenching
(Rodenburg et al., 2008).
After emergence from the soil, Striga plants begin to photosynthesize. However, the low
CO2 fixation and high dark respiration rates of S. asiatica result in a negative carbon gain over
the 24-h period, thus making Striga still host dependent when growing above ground. In
addition, Striga leaves are characterized by a degenerated palisade cell layer and a relatively
small number of chloroplasts per cell. Low photosynthesis in Striga is supported by
transcriptome data from RNA isolated from the above-grown Striga tissue. A relatively low
expression of chlorophyll biosynthesis- and photosynthesis-related genes was observed when
compared with the expression of these genes in the facultative hemiparasitic plant T. versicolor
(Wickett et al., 2011). The high transpiration rates of Striga suggest that most host
photoassimilates are obtained by transpirational pull, explaining why high humidity is inhibitory
to Striga growth. Indeed, Striga stomata show high conductance and respiration rates and little
response to dark-induced closure. Relative to the host plant, Striga has a disadvantageous leaf
surface ratio and might compensate for this with higher stomatal. Surprisingly, when water
depletion was investigated under controlled experimental conditions of Striga-infected maize
plants, no significant increase in water use was observed until the very late stage of infection (63
days post-infection). Before that time, maize plants had already established disease symptoms
and showed stunted growth. However, in the final stage of infection, maize plants used nearly
50% more water than control plants (Rodenburg et al., 2008; Satish et al., 2012).
The fact that disease symptoms appear before Striga emerges illustrates how ineffective
the biocontrol of above ground-grown Striga by hand weeding or herbicides is likely to be.
Nevertheless, these techniques are important to avoid the reproduction of Striga. Striga asiatica
and S. hermonthica flower about 4 weeks after emergence. Striga gesnerioides has been reported
to flower earlier (Satish et al., 2012). Inflorescences are arranged in spikes or racemes, each
carrying several flowers. Flower colour varies between species and sometimes within species
from blue and pink (e.g. S. hermonthica and S. gesnerioides) to white, yellow or red (e.g. S.
asiatica). After pollination, seeds mature within 4 weeks in seed pods, which contain 250–500
dust-like seeds of 200–300 mm in size. Under optimal conditions, each Striga plant can produce
up to 50 000–500 000 seeds (Rodenburg et al., 2008). When the seed pods crack, seeds are
spread into the soil and quickly build up in numbers. Striga seeds require a certain time of after-
ripening, about 6 months at elevated temperatures. According to Westwood et al. (2010), this
could be an adaptation to prevent germination during the last rains of the seasons, when no hosts
are in the field.
CHAPTER FOUR: WITCHWEED (Striga asiatica) CONTROL METHODS
The most and recent control methods of Striga seem as follows:
4.1 Control Strategies targeting Germination/Host locations
Considerable research has examined the possibility of exploiting germination stimulants
for control of Striga. Control strategies include: (1) inducing “suicidal germination” and (2)
reducing the production of germination stimulants by crop plants. In addition, the newly
discovered role of strigolactones in the recruitment of symbiotic arbuscular mycorrhizal fungi
(AMF) (Ejeta and Gressel, 2007) has opened new possibilities for modifying the production of
germination stimulants by host plants.
Inducing the germination of Striga seed in the absence of a suitable host plant results in
“suicidal germination,” and subsequent reduction in numbers of parasitic-plant seeds in soil.
Both man-made and natural compounds have been investigated for their ability to induce
germination. Analogs of strigol have been synthesized (e.g., GR 24 and Nijmegen 1) and are
potent elicitors of germination in Striga (Khan et al., 2008); however, their instability in soil,
and the high cost of producing large quantities of these compounds, have so far prohibited their
use in agriculture (Teka, 2014). Ethylene has been a valuable component of the eradication
program targeting Striga asiatica in the United States, where it induces about 90% germination
when injected into the soil (Ejeta and Gressel, 2007). However, fumigating soil with ethylene is
likely to negatively influence AMF and other nontarget soil microorganisms. It has been
proposed that ethylene-producing non-pathogenic bacteria could be used to induce suicidal
germination of Striga, but a better understanding of bacteria/ethylene/crop interactions is needed
before this method can be used in agriculture. Other natural compounds, including fungal toxins
and methyl jasmonate have been shown to induce germination of Striga seed, but their potential
uses in agriculture remain largely unexplored (Fen et al., 2007).
According to Khan et al. (2007) and Hearne (2009), planting nonhost trap crops that
induce suicidal germination is perhaps the most effective strategy currently available for Striga
control. Recent studies in this area have focused on identifying and assessing the effectiveness of
potential trap crops and the possibility of breeding for increased production of germination
stimulants. Use of nitrogen-fixing legumes as trap crops has the added benefit of increasing soil
fertility, which can further assist in Striga control because Striga thrive in poor soils. The
efficacy of legume rotations could potentially even be improved by inoculating crops with
supplemental nitrogen-fixing rhizobia, in combination with ethylene-producing bacteria, to
simultaneously increase suicidal germination and soil fertility.
4.2 Cultural and Mechanical Control Methods
A number of cultural practices have been recommended for Striga control such as crop
rotation (Fasil and Verkleij, 2007); intercropping (Hooper et al., 2009); transplanting (Fasil and
Verkleij, 2007); soil and water management (Fasil and Verkleij, 2007), use of fertilizers (Jamil
et al., 2011); and hand weeding (Khan et al., 2008) to reduce the production of further Striga
seed. These methods should also reduce the density of Striga seeds already in the soil seed bank
(Fasil and Verkleij, 2007). Some of these practices improve soil fertility, which will stimulate
the growth of the host but also adversely affects germination, attachment and subsequent
development of the juvenile Striga plants (Fasil and Verkleij, 2007). However, this approach has
only limited success for small-scale farmers, largely due to socio-economic and financial
constrints.
A. Hand-weeding and Sanitation
This is the most used control method against Striga. It is recommended to prevent seed
set and seed dispersal. Weeding the small Striga plants is a tedious task and may not increase the
yield of already infected plants, it is necessary to prevent seed production and re-infestation of
the soil. Due to high labour costs in repeated hand-pulling of Striga, it is recommended that hand
pulling should not begin until 2-3 weeks after Striga begins to flower to prevent seeding (Khan
et al., 2008). New shoots may sprout out below the soil from infected plants requiring a second
weeding before crop maturity. Sanitation consists of taking care to note infested areas and to
isolate them. Seeds in the soil can be spread by wind, rainwater, plowing, and soil on tools or
root crops. Seed pods on Striga plants attached to maize or sorghum plants pulled for forage will
infest manure and feeding areas (Khan et al., 2008). Crop stubble should also be uprooted or
burned to prevent the continued growth and seeding of the parasite. This weed competes for
water and nutrients as a root parasite. In so doing, crop growth is stunted and yields are generally
reduced. It is not practical to hand weed dense infestations, and weeding is often ineffective,
particularly since it is time consuming and labor-intensive. It is practical, at a low level of
infestation before Striga flowers and in combination with herbicides or fertilizer (Ayongwa et
al., 2010).
B. Crop rotation
Crop rotation of infested land with non-susceptible crops or fallowing is theoretically the
simplest solution. Rotation with non-host crops interrupts further production of Striga seed and
leads to decline in the seed population in the soil. The practical limitations of this technique are
required more than three years for rotation. The choice of rotational crop should therefore be
based first on its suitability to the local conditions and only secondarily on its potential as a trap
crop (Fasil and Verkleij, 2007).
Rotating the infested maize or sorghum areas to wheat/barley, pulses, or groundnuts are
viable and effective options. In Ethiopia two years of cropping to a non-host was reported to
reduce Striga infestation by 50%. However small-holder farmers desiring to maximize the grain
production potential of their land may be difficult to be persuaded to grow other crops. Practical
control measures are effective when a combined program of crop rotation, weeding, sanitation
and, resistant varieties is included (Mahmoud et al., 2013).
C. Trap crops and catch crops
Trap-crops cause suicidal germination of the weed, which reduces the seed bank in the
soil. Some varieties of cowpea, groundnut and soybean have potential to cause suicidal
germination of Striga and improve soil fertility. The use of trap crops such as soybean causes
suicidal germination of the Striga seedlings which do not attack the soybean consequently; the
Striga is ploughed off before flowering thereby reducing the seed density of Striga in the soil
(Khan et al., 2008). De Groote et al. (2010) also found that the use of leguminous trap crops that
include varieties of groundnut (Arachis hypogaea), soybean (Glycine max), cowpea (Vigna
unguiculata), and sesame (Sesamum indicum) stimulate the suicidal germination of Striga is
another technology to control Striga. Also, soybean triggers suicidal germination of Striga and
reduces the Striga seed bank in the soil when intercropped with maize.
Catch crops: Catch crops are planted to stimulate a high percentage of the parasite seeds to
germinate but are destroyed or harvested before the parasite can reproduce. A thick planting of
Sudan grass at 20-25 kg seed per hectare should be sown and either ploughed in or harvested for
forage at 6-8 weeks before Striga seeds. The main crop could then be planted during the main
rains. From the available studies, it can be concluded that trap crops should be cultivated for at
least three consecutive years in order to reduce parasite seeds. In other research findings also
reported the effectiveness of the combined use of trap-cropping, fertilization and host plant
resistance to control Striga (Dugje et ali., 2008).
D. Intercropping
Intercropping cereals with legumes and other crops is a common practice in most areas of
Africa, and has been reported as influencing Striga infestation. Intercropping is a potentially
viable, low-cost technology, which would enable to address the two important and interrelated
problems of low soil fertility and Striga (Hooper et al., 2009). Recent result shows that
intercropping maize with cowpea and sweet potato can significantly reduce the emergence of
Striga in Kenya (Fasil and Verkleij, 2007). The mechanisms by which D. uncinatum reduce
Striga infestation in intercropping was found to be the allelopathic effect inhibiting the
development of haustoria of Striga (Khan et al. 2009). Identification of the compounds released
from D. uncinatum involved in the Teka 495 suppression of the parasite may give more
exploitation for developing reliable intercropping strategies, as well as new approaches for
molecular biology in Striga (Fasil and Verkleij, 2007).
E. Soil fertility
Nitrogen and phosphorus deficiency as well as water stress accentuate the severity of
Striga damage to the hosts. Striga is particularly a pest of low fertile soil and usually the
infection decreases if mineral nutrients, especially nitrogen and phosphorus, are applied in
sufficient quantities. Fertilizer application had significant effect on height, vigour score, reaction
score of sorghum as well as shoot count, days to emergence, dry matter of production and dry
weight of Striga. The application of high nitrogen (N) increases the performance of cereal crops
under Striga infestation. This is due to the fact of that nitrogen reduced the severity of Striga
attack while simultaneously increasingly the host performance (Lagoke and Isah, 2010).
Application of high dosage of nitrogen fertilizer is generally beneficial in delaying Striga
emergence and obtaining stronger crop growth. Also other advantageous effects of fertilizers
include increasing soil nitrogen and other nutrients, replenishing the organic matter of the soil
and increasing soil moisture holding capacity (Dugje et al., 2008)
F. ‘Push–pull’ technology
The ‘push-pull’, as a tool in integrated pest management which involves the use of
behaviour modifying stimuli to manipulate the distribution and abundance of a pest and/or
beneficial insects for management of the pest (Cook et al., 2007). This technology was first
developed to control stem borers but was later found to also suppress Striga weed in the field
depending on which push component the main crop has been intercropped. In a ‘push–pull’
strategy, pests are repelled or deterred away from the target crop (push) by stimuli that mask
host appearancy. The pests are simultaneously attracted (pull) to a trap crop where they are
concentrated, leaving the target crop protected (Cook et al., 2007; Khan et al., 2010). Secondary
metabolites with Striga seed germination stimulatory and post-germination inhibitory activities
are present in the root exudates of D. uncinatum, which directly interferes with parasitism (Khan
et al., 2008). This combination thus provides a novel means of in situ reduction of the Striga
seed bank in the soil through efficient suicidal germination even in the presence of cereal hosts
in the proximity (Khan et al., 2008; Hooper et al., 2009).
Desmodium also fixes atmospheric nitrogen (110 kg N/ha), adds organic matter to the
soil, conserves soil moisture and enhances soil biodiversity, thereby improving soil health and
fertility, which directly contribute to Striga control. It therefore improves agro-ecosystem
sustainability, resilience, and has great potential to mitigate the effects of climate change.
Desmodium has also been reported to have additional soil improvements such as; increasing of
soil nitrogen, organic matter and conserving moisture (Khan et al. 2009; Cook et al., 2007).
According to a study done by Khan (2010), push-pull technology helps controlling both
Striga and stem borers with at least 2 tons per hectare higher grain yield. The technology is
currently being disseminated among smallholder farmers in eastern Africa and adoption rates are
raising.
4.3 Biological Control Methods
The objective of weed biological control is not the eradication of weeds but the reduction
and establishment of a weed population to a level below the economic threshold (Atera et al.,
2011). Means of biological control of weeds comprise herbivorous insects, microorganisms
(especially fungi), and smother plants (Ejeta, 2007). The method, involves importation,
colonization, and establishment of exotic natural enemies, which include predators and
parasitoids. Efforts to manage weeds using biological control have been gaining momentum
throughout the world, especially in the recent past. Biological control is considered as a potential
cost-effective, safe and environmentally beneficial alternative mean of reducing weed
populations in crops, forests or rangelands (De Groote et al., 2010). Disadvantages of weed
biological controls include it will usually require a long period (5 to 10) years of research and a
high initial investment of capital and human resources (De Groote et al., 2008).
This is because the intensive use of chemical herbicides came under scrutiny due to
several areas of concern, which include the development of herbicide resistant or tolerant weeds
and environmental contaminations, comprehending effects on non-target organisms as well as
the pollution of soil, underground water and food. Strong public criticism due to health concerns
arose from such contaminations (Koua et al., 2011). These limitations of chemical herbicides
encouraged researchers to look for alternative systems of weed control.
A. Biological control using insects
The insects that attack Striga can be classified according to their damage as defoliators
such as Junonia spp., gall forming as Smicronyx spp. (Coleoptera: Curculionidae) in India and
Africa; shoot borers as Apanteles sp., miners as Ophiomyia Strigalis, Spencer (Diptera:
Agromyzidae) in East Africa; inflorescence feeders as Stenoptilodes taprobanes and fruit feeders
as Eulocastra spp. (Lepidoptera: Noctuiidae) in India; (Koua et al., 2011; Mahmoud et al.,
2013).
The potential of the weevils Smicronyx guineanus and Smicronyx umbrinus and the
butterfly Junonia orithya as biocontrol agents for Striga have been investigated and proved to be
effective as reported by Koua et al., (2011). Koua et al. (2011) concluded that the use of
herbivorous insects could play a role in an integrated control package, lowering the Striga
population by reducing its reproduction capabilities and spread. However, the augmentation of
native insect populations through inundative releases is not applicable in the third world, mainly
due to the infeasibility of mass rearing.
B. Biological control using pathogens
Most organisms have natural enemies that balance their populations, avoiding excessive
abundance. Biological control of S. hermonthica using Fusarium oxysporum is considered as one
of the novel management strategies (Sauerborn et al., 2007). Fungi are preferred to other
microorganisms as bio-herbicides Teka 497 because they are usually host specific, highly
aggressive, and easy to mass produce and are genetically diverse (Rebeka et al., 2013). Field and
laboratory tests showed that F. oxysporum is highly effective in hindering germination, growth
and development of Striga and thus may lead to reduction of Striga seed bank in the soil
(Sauerborn et al., 2007).
Extensive surveys in Burkina Faso, Mali and Niger also demonstrated the occurrence of
highly pathogenic and Striga specific isolates of F. oxysporum. Among this isolate virulent
isolate of F. oxysproum M12-4A provided more than 90% control of Striga, and a three-fold
increase in sorghum biomass (Hearne, 2009). The use of a myco-herbicide, that is F. oxysporum
coated seeds and host plant resistance reportedly reduced Striga emergence by 95% and
increased sorghum yield by 50% (Teka, 2014).
Recent findings indicated the effectiveness of integrated use of F. oxysporum compatible
and Striga resistant sorghum genotypes to control Striga in Ethiopia (Rebeka et al., 2013). To
realize the full potential of this approach it is important to recombine traits of Fusarium
compatible and Striga resistant sorghum lines. This would allow continued selection of targeted
progenies with combined resistance and Fusarium compatibility and for subsequent seed
treatment of suitable hybrid(s) for direct use. Thus effective Striga control would be possible
through synergistic effect of biocontrol and host resistance (Teka, 2014; Rebeka et al., 2013).
Recently, the combined application of two or more control measures has been promoted
for effective Striga management. The use of bio-control agent such as virulent isolate of F.
oxysporum f.sp. Strigae as a component of integrated Striga management was identified to have
several advantages (Fen et al., 2007). Sauerborn et al. (2007) and Teka (2014) also found that
the application of integrated Striga management package combining a mycoherbicide based on
F. oxysporum isolate and host plant resistance has been demonstrated on farmers fields as
effective Striga control approach. There is other agreed combined use of resistant varieties with
the application of Fusarium oxysporum as pest granules or as a seed coating was reported to be
effective to controlling Striga (Julien et al., 2009).
According to Hearne (2009) and Julien et al. (2009), various Fusarium spp. and vesicular
arbuscular mycorrhizal (VAM) fungi have been found which can reduce Striga infestations
significantly on sorghum and maize when used together with resistant host. These control
options when applied individually are not effective and sometimes affected by environmental
conditions. Therefore the use of F. oxysporum in combination with other cost effective control
methods may provide an effective and sustainable control option for subsistence farmers.
However, integrated Striga management approach relies on the use of resistant host genotypes
and Striga pathogenic F. oxysporum application to control S. hermonthica emergence and
growth lead to effective results.
4.4 Chemical Control Methods
A. Germination stimulants
Certain chemicals such as ethylene, ethephon, strigol and strigol analogues can induce
germination of Striga seeds in the absence of a suitable host and therefore reduce seed reserves
in the soil (Timko et al., 2007). In dicotyledonous plant species there is evidence that the
production of strigolactone by the host plant could be reduced if sufficient minerals are available
(Xie and Yoneyama, 2010).
B. Pre emergence herbicides
Technology currently being deployed as a complement to Striga resistance in maize
involves use of herbicide as a seed coating. The parasite competes with its host for resources;
changes host plant architecture and reduce the photosynthetic rate and the water use efficiency of
the host (Kabambe et al., 2008a). This has led to the emergence of a new technology known as
imazapyr-resistant maize (IRM) which has proven to be efficient for Striga control (De Groote et
al., 2008). The International Maize and Wheat Improvement Center (CIMMYT), Badische Anlin
and Soda Fabrik (BASF), African Agricultural Technology Foundation (AATF) and other
stakeholders have made efforts in bringing imazapyr-resistant maize (IRM) technology to
farmers as assistance for Striga control. Result of experiments also proved that herbicide seed
treatment using imazapyr appears to be a promising approach for the control of Striga in maize
or sorghum (Kabambe et al., 2008b). Satish et al. (2012) has also reported coating sorghum seed
with herbicide reduced Striga infestation, Striga flowering and Striga seed set, and it is
considered as the most effective approach as it does not affect sorghum biomass. Many
herbicides are useful in preventing the build-up of Striga seeds in the soil but may not prevent
damage prior to their emergence (Xie and Yoneyama, 2010). The sustainability of many
technologies will only be maintained when integrated with other technologies.
C. Post emergence herbicides
Herbicides tested for the selective control of Striga mostly acts through the foliage,
although some have soil residual effects. Among the herbicides tested, 2, 4-D has been the most
selective and is the cheapest. 2-methyl-4-chlorophenoxyacetic acid (MCPA), a compound
closely related to 2, 4-D, has also been effective especially when mixed with bromoxynil (Ejeta
et al., 2007). Post emergence application of 2,4-D (1 L product/ha), Glufosinate (2 L product/ha)
and Oxyflourfen (1 L product/ha) was effective in preventing the top growth of Striga.
Unfortunately, most of those products had narrow window of application and the only safe
treatment for the crop was targeted spray of 2,4-D (Fasil et al., 2007). Satish et al. (2012)
reported that a combination of urea and dicamba effectively controlled Striga between 62-92%
on sorghum, while chlorsulfuron in combination with dicamba controlled Striga as much as 77-
100% on sorghum. However, results of the experiments showed that pre and post emergence
herbicides do not prevent crop yield loss, because they cause their impact after Striga has
already attached and damaged the host.
Research efforts on the identification of systemic herbicides, which could ideally
translocate through the host crop to prevent initial stages of parasite development, were not
successful. So Research efforts should therefore be directed towards identifying herbicides that
persist in the soil, allowing the germination of Striga seeds but killing the seedlings before
attachment to the host. Herbicides must also be compatible with the mixed cropping systems
practiced by farmers and be profitable to use with low initial capital outlay.
D. Host plant resistance
Host plant resistance would probably be the most feasible and potential method for
parasitic weed control. Using biotechnological approaches (including biochemistry, tissue
culture, plant genetics and breeding, and molecular biology) significant progress has been made
in developing screening methodologies and new laboratory assays, leading to the identification
of better sources of parasitic weed host resistance (Ejeta et al., 2007; Ishida et al., 2011). It is
potentially an acceptable Striga control option to resource-poor farmers (Ejeta et al., 2007).
However, reliance on host resistance alone is not ideal because so far complete resistance against
Striga cannot be attained through breeding, and usually the newly developed varieties may not
fulfill farmers preference traits (Labrada, 2008).
Reports of genetic resistance to Striga have been documented in rice (Stegemann and
Bock, 2009; Cissoko et al., 2011), sorghum (Sorghum bicolor) (Hooper et al., 2009) and maize
(Lagoke et al., 2010). Identifying source germplasm with different resistance mechanisms can
facilitate combining several resistance genes to obtain more durable and stable polygenic
resistance to Striga in cereals (Ejeta et al., 2007; Kabambe et al., 2008a). The International
Institute for Tropical Agriculture (IITA) has released Striga resistant, drought-tolerant, and low
soil nitrogen-tolerant extra-early maturing white maize varieties in Striga and drought resistance,
were combined by classical breeding (Cissoko et al., 2011). Basically the resistant varieties were
low yielding and not desirable in other agronomic characteristics. However, integrating genetic
resistance with other control measures is the smartest option possible both for effectiveness of
control as well as for increasing durability of resistance genes (Ejeta, 2007; Cissoko et al.,
2011).
4.5 Integrated Striga Management
Striga has a high fecundity, it uses the host plants nutrients and the seed is asynchronous.
These characteristics make the weed difficult to control (Andrianjaka et al., 2007). It is also
difficult to control effectively because most of its damage to the host plant occurs underground
before the parasitic plant emerges (De Groote et al., 2010). The rate of infestation needs
therefore to be managed through different control methods. Today there are several control
options have been recommended to reduce Striga damage such as the use of resistant cultivars,
crop rotation, intercropping with pulse crops, late planting, deep planting, using trap crops, use
of organic and inorganic fertilizers, herbicides, and biological control (Hearne, 2009). Although
the level of Striga infestation and damage is increasing, farmers rarely adopt Striga control
methods either due to limitations associated with the technology itself, access and costs of the
technology or due to lack of information about available technology options (Ejeta, 2007;
Hearne, 2009). Furthermore, available options when applied individually are not effective and
sometimes affected by environmental conditions.
Integration of weeding with high urea application, appropriate sowing date, and effective
control of weeds which may serve as alternative hosts, will further enhance the long-term control
of Striga (Fen et al., 2007). Combined use of row planting, fertilizers and hand pulling (during
flowering) registered 48% higher grain yield and over 50% reduction in Striga shoot counts.
However, from this result of research experiment showed that the best solution in the control of
Striga is an integrated approach that includes a combination of methods that are affordable and
acceptable by farmers (Hearne, 2009).
According to the research findings, the integration of multiple control options is
suggested as a better approach to combat Striga problem (Cook et al., 2007; Fen et al., 2007;
Hearne, 2009; De Groote et al., 2010). Cook et al. (2007) and Hearne (2009) also proved that the
best options for successful Striga control lies in an integrated Striga management (ISM)
approach.
CHAPTER FIVE: CONCLUSION AND RECOMMENDATIONS
5.1 Next-Generation Striga Research
In recent years, efforts have been undertaken to elucidate the molecular events
underlying Striga infections using next generation and conventional sequencing technology
(Yoshida et al., 2010). For example, comparative studies on repetitive regions in five Striga
species generated a total of about 2200 Sanger sequence reads and about 10 000 454 reads
(Estep et al., 2012). Partially assembled and identified repeats were most similar to the most
closely related plant species. Overall, the authors came to the conclusion that the analysed Striga
genomes have a rather typically complex angiosperm genome. Estimated haploid genome sizes
range from 615 Mb for S. asiatica to 1425 Mb for S. hermonthica and 2460 Mb for S. forbesii,
suggesting several polyploidization events. Polyploidization is also an important factor for
speciation in the sister genus Orobanche (Yoshida et al., 2012). No evidence of large transfers of
repetitive DNA regions from the host genomes was observed, which is in contrast with the
observed HGT events between monocot genes and S. hermonthica (Yoshida et al., 2010), and
favours the hypothesis that HGT events originate from Mrna species rather than from large
pieces of genomic DNA.
Next-generation sequencing technology has led to an increase in available transcriptional
data for S. hermonthica and related species. For example, Wickett et al. (2011) analysed
sequence data obtained from Illumina short reads of mRNA isolated from above-ground tissue
of three Orobanchaceae species: the facultative hemiparasite T. versicolor, S. hermonthica and
Phelipanche aegyptiaca (pers.) Pomel. The expression of photosynthesis-related genes was
much lower in S. hermonthica than in Triphysaria, and no expression of these genes was
detected in Phelipanche. This study also revealed that chlorophyll a synthesis gene expression
was conserved and detectable in all three species, even in the nonphotosynthetically active Ph.
aegyptiaca.
Next-generation sequencing technology will almost certainly provide detailed
transcriptional information for Striga at different stages of infection and on different hosts, and
will allow the simultaneous detection of host and pathogen transcriptomes. So far, host
transcriptome data are mainly based on microarray studies or similar methods. Overall, these
data, although sometimes very difficult to compare, draw a common picture, in which Striga is
actively recognized by resistant plants and triggers a defence-like response. This response
appears to be very similar to that observed for other nonhost or race-specific resistance responses
to other plant pathogens. It also shows that Striga actively manipulates host transcription to
foster parasitism by either up-regulating host genes associated with nutrient supply or by down-
regulating defence-related genes. It is not known how Striga manipulates transcription in host
plants. Avirulence gene products are interesting candidates, but difficult to isolate as a result of
limited genetic resources in parasitic plants. Ultimately, candidate genes will need to be tested in
planta. Several hairy root transformation systems for members of the Orobanchaceae family,
including T. versicolor, P. japonicum and Ph. Aegyptiaca, are available (Fernandez-Aparicio et
al., 2011; Ishida et al., 2011; Tomilov et al., 2007).
5.2 Recommendations
According to Andrianjaka et al. (2007), Tomilov et al. (2007), De Groote et al. (2008),
Hearne (2009), Fernandez-Aparicio et al. (2011), Ishida et al. (2011), and Teka (2014), the
following short and long term recommendations should receive an immediate attention.
A. Short term
Some of the points that should receive an immediate attention include:
i) Identify and mark the farms classified as to level of infestation and develop treatment
plans according to cost and return potential.
ii) Generate information from which farmers can make optimum decisions on choice of
cereal species and variety, time and method of planting, mixed cropping, herbicide and
hand pulling as relevant to the farming system.
iii) Use clean crop seeds to avoid Striga.
iv) Improve soil fertility by using fertilizers.
v) Crop rotation with non host crops or crops that induce suicidal germination.
B. Long term
To alleviate the alarming problem of Striga in the long-run emphasis should be placed on:
i. Research efforts should be focus on controlling the production of new Striga seeds and
reducing the number of seeds in the soil.
ii. Demonstration of existing improved technologies that are effective and feasible for the
small scale farmers.
iii. Striga control approaches, namely cultural, chemical, genetic, and biological options
should be widely investigated and developed.
iv. Practices and measures should be easily affordable, economical, and practicable to poor
farmers.
v. Finding suitable companion and trap crops that fit into the farming systems of target
communities.
vi. The use of trap crops as an intercrop with susceptible hosts to reduce the seed bank needs
prolonged investigations.
vii. Effective preventive measures need to be taken through seed quarantine and Striga free
equipment.
viii. Developing and use of resistant crop varieties.
ix. Demonstration and training should be strongly focus in integrated Striga control
x. Need to launch an action program for the control of Striga. This program should cover all
aspects of the problem.
5.3 Conclusions
In spite of intensive research, adequate strategies for controlling parasitic plants remain
elusive, and these weeds continue to threaten agricultural crops worldwide. Chemically mediated
interactions between early-stage parasitic plants and their hosts play a key role in infestation and
may be exploited for control. Recent advances in this area suggest a number of potentially
fruitful approaches, including the prospect of simultaneously managing beneficial symbionts and
parasitic weeds. For example, implementing cultural practices that favor AMF, such as reducing
tillage and fungicide application, could improve growth and increase drought tolerance in crops
and potentially reduce Striga infestations.
Additional research is needed to understand the mechanisms underlying strigolactone
perception and responses in both parasitic plants and AMF. Integrated measures seem to be the
best option for Striga control in Nigeria and Africa. Variability in farming systems, literacy
level, ecological peculiarities and farmers’ resources will go a long way in the choice of method
to apply. The important thing is to control this devastating parasitic weed, so as to enhance
higher crop yield.
REFERENCES
Andrianjaka, Z., Ball, R., Lepage, M., Thioulouse, J., Comte, G., Kisa, M. and Duponnois, R.
(2007). Biological control of Striga hermonthica by cubitermes termite mound
powder amendment in sorghum culture. Applied Soil Ecology, 37, 175-183.
Atera, A.E., Itoh, K.
and Onyango, J.C. (2011). Evaluation of ecologies and severity of Striga
weed on rice in sub-Saharan Africa. Agriculture and Biology Journal of North
America, 2(5), 752-760.
Ayongwa, G.C., Stomph, T.J., Hoevers, R., Ngoumou, T.N., Kuyper, T.W. (2010). Striga
infestation in northern Cameroon: Magnitude, dynamics and implications for
management. NJAS – Wageningen Journal of Life Sciences, 57, 159-165.
Bandaranayake, P.C.G., Filappova, T., Tomilov, A., Tomilova, N.B., Jamison-McClung, D., Ngo,
Q., Inoue, K. and Yoder J.I. (2010). A single-electron reducing quinone
oxidoreductase is necessary to induce haustorium development in the root parasitic
plant Triphysaria. Plant Cell, 22, 1404–1419.
Cardoso, C., Ruyter-Spira, C. and Bouwmeester, H.J. (2011). Strigolactones and root infestation
by plant-parasitic Striga, Orobanche and Phelipanche spp. Plant Sciences, 180, 414–
420.
Cissoko, M., Boisnard, A., Rodenburg, J., Press, M.C. and Scholes, J.D. (2011). New Rice for
Africa (NERICA) cultivars exhibit different levels of post-attachment resistance
against the parasitic weeds Striga hermonthica and Striga asiatica. New Phytol. 192,
952–963.
Cook, S.M., Khan, Z.R. and Pickett, J.A. (2007). The use of ‘push–pull’ strategies in integrated
pest management. Annu. Rev. Entomol. 52, 375-400.
De Groote, H., Rutto, E., Odhiambo, G., Kanampiu, F., Khan, Z., Coe, R. and Vanlauwe, B. (2010).
Participatory evalutaion of integrated pest and soil fertility management options
using ordered categorical data analysis. Ag. Sys. doi:10.1016/j.agsy.2009.12.005.
De Groote, H., Wangare, L., Kanampiu, F., Odendo, M., Diallo, A., Karaya, H. and Friesen, D.
(2008). The potential of a herbicide resistant maize technology for Striga control in
Africa. Agric. Sys. 97(1-2), 83-94.
Dugje, I.Y., Kamara, A.Y. and Omoigui, L.O. (2008). Influence of farmers’ crop management
practices on Striga hermonthica infestation and grain yield of maize (Zea mays L.) in
the savanna zones of northeast Nigeria. Journal of Agronomy, 7(1):33-40.
Ejeta, G. (2007). The Striga scourge in Africa: a growing pandemic. In: Ejeta, G. and Gressel, J.
(eds). Integrating New Technologies for Striga Control: Towards ending the witch-
hunt. World Scientific Publishing Co. Pte Ltd, 5 Tol Tuck Link, Singapore, pp. 3-16.
Ejeta, G. and Gressel, J. (2007). Integrating New Technologies for Striga Control: Towards
ending the witch-hunt. World Scientific Publishing Co. Pte Ltd, 5 Tol Tuck Link,
Singapore, pp. 3-16.
Estep, M.C., Gowda, B.S., Huang, K., Timko, M.P. and Bennetzen, J.L. (2012). Genomic
characterization for parasitic weeds of the genus Striga by sample sequence
analysis. Plant Genome-Us, 5, 30–41.
Fasil, R. and Verkleij, J.A. (2007). Cultural and cropping systems approach for Striga
management-a low cost alternative option in subsistence farming. In: Ejeta, G. and
Gressel, J. (eds). Integrating New Technologies for Striga Control: Towards Ending
the Witch-hunt. World Scientific Publishing Co., Singapore. pp.229-240.
Fen, D.B., Steven, G.H., Venne, J. and Watson, A.K. (2007). The Striga scourge in Africa- a
growing pandemic. p.3-16. In: Ejeta, G. and Gressel, J. (eds). Integrating New
Technologies for Striga Control: Towards Ending the Witch-hunt. World Scientific
Publishing Co., Singapore.
Fernandez-Aparicio, M., Rubiales, D., Bandaranayake, P.C.G., Yoder, J.I. and Westwood, J.H.
(2011). Transformation and regeneration of the holoparasitic plant Phelipanche
aegyptiaca. Plant Methods, 7, 36.
Hearne, S.J. (2009). Control - the Striga conundrum. Pest Management Science, 65, 603-614.
Hooper, A.M., Hassanali, A., Chamberlain, K., Khan, Z. and Pickett, J.A. (2009). New genetic
opportunities from legume intercrops for controlling Striga spp. parasitic weeds.
Pest Manag. Sci. 65, 546-552.
Ishida, J.K., Yoshida, S., Ito, M., Namba, S. and Shirasu, K. (2011). Agrobacterium rhizogenes-
mediated transformation of the parasitic plant Phtheirospermum japonicum. PLoS
ONE, 6, e25802.
Jamil, M., Charnikhova, T., Cardoso, C., Jamil, T., Ueno, K., Verstappen, F., Asami, T. and
Bouwmeester, H.J. (2011). Quantification of the relationship between
strigolactones and Striga hermonthica infection in rice under varying levels of
nitrogen and phosphorus. Weed Resources, 51, 373-385.
Kabambe, V. H., Kanampiu, F. and Ngwira, A. (2008a). Imazapyr (herbicide) seed dressing
increases yield, suppresses Striga asiatica and has seed depletion role in maize (Zea
mays L.) in Malawi. African Journal of Biotechnology, 7(13), 3293-3298.
Kabambe, V., Katunga, L., Kapewa, T. and Ngwira, A.R. (2008b). Screening legumes for
integrated management of witchweeds (Alectra vogelii and Striga asiatica) in
Malawi. African Journal of Agricultural Research, 3 (10), 708-715.
Khan, Z.R., Amudavi, D.M., Midega, C.A.O., Wanyama, J.M., Pickett, J.A. (2008). Farmers‘
perceptions of a 'push–pull' technology for control of cereal stemborers and Striga
weed in western Kenya. Crop Prot. 27, 976-987.
Khan, Z.R., Midega, C.A.O., Bruce, T.J.A., Hooper, A.M. and Pickett, J.A. (2010). Exploiting
phytochemicals for developing a 'push-pull' crop protection strategy for cereal
farmers in Africa. J. Exp. Bot. 61(15), 4185-4196. DOI: 10.1093/jxb/erq 229.
Khan, Z.R., Pickett, J.A., Hassanali, A., Hooper, A. and Midega, C.A.O. (2008). Desmodium for
controlling African witchweed: present and future prospects. Weed Resources, 48,
302-306.
Koua, F.H.M. and Babiker, H.A.A. (2011). Phytochemical and biological study of Striga
hermonthica (Del.) Benth callus and intact plant. Res. Pharm. Biotechnol. 3, 85–92.
Labrada, R. (2008). Farmer training on parasitic weed management. In: Progress on farmer
training in Parasitic Weed Management (Labrada, R., ed.), pp. 1-5. Rome: FAO.
Lagoke, S.T.O. and Isah, K.M. (2010). Reaction of maize varieties to Striga hermonthica as
influnced by food legume intercrop, spacing and split application of compound
fertilizer. Nigeria Journal Weed Sciences, 23, 45-58.
Leblanc, M., Kim, G. and Westwood, J.H. (2012). RNA trafficking in parasitic plant systems.
Front. Plant Sci. 3, 203.
Mahmoud, B.A., Hamma, I.L., Abdullahi, S. and Adamu, Y. (2013). Common striga control
methods in Nigeria: A review. International Journal of Agronomy and Agricultural
Research (IJAAR), 3(9), 26-29.
Mohamed, K.I. and Musselman, L.J. (2008). Taxonomy of agronomically important Striga and
Orobanche species. In: Progress on Farmer Training in Parasitic Weed Management
(Labrada, R., ed.), Rome: FAO. pp. 7-14.
Nail, K., Kriticos, D.J., Scott, J.K., Yonow, T., and Ota, N.(2014). Striga asiatica. HarvestChoice
Pest Geography, 2, 1–6.
Nickrent, D.L. (2007) Parasitic plant genera and species. Parasitic plant connection.
http://www.parasiticplants.siu.edu/
Rodenburg, J., Bastiaans, L., Schapendonk, A.H.C.M., van der Putten, P.E.L., van Ast, A.,
Dingemanse, N.J. et al. (2008). CO-assimilation and chlorophyll fluorescence as
indirect selection criteria for host tolerance against Striga. Euphytica, 160, 75–87.
Satish, K., Gutema, Z., Grenier, C., Rich, P.J. and Ejeta, G. (2012). Molecular tagging and
validation of microsatellite markers linked to the low germination stimulant gene
(lgs) for Striga resistance in sorghum [Sorghum bicolor (L.) Moench]. Theor. Appl.
Genet. 124, 989–1003.
Sauerborn, J., Müller-Stöver, D. and Hershenhorn, J. (2007). The role of biological control in
managing parasitic weeds. Crop Prot. 26, 246-254.
Spallek, T. Mutuku, J.M. and Shirasu, K. (2013). The genus Striga: a witch profile. Molecular
Plant Pathology, 3(9), 1-9.
Stegemann, S. and Bock, R. (2009). Exchange of genetic material between cells in plant tissue
grafts. Science, 324, 649–651.
Teka, B.H. (2014). Advance research on Striga control: A review. African Journal of Plant
Science, 8(11), 492-506.
Timko, M.P., Gowda, B.S., Ouedraogo, J. and Ousmane, B. (2007). Molecular markers for
analysis of resistance to striga gesnerioides in cowpea. In: Ejeta G. and Gressel J.
(eds). Integrating new technologies for Striga control: Towards ending the witch-
hunt. World Scientific Publishing Co. Pte Ltd, 5 Tol Tuck Link. Singapore, 1-14.
Tomilov, A.A., Tomilova, N.B., Wroblewski, T., Michelmore, R. and Yoder, J.I. (2008). Trans-
specific gene silencing between host and parasitic plants. Plant Journal, 56, 389–
397.
Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N. (2008).
Inhibition of shoot branching by new terpenoid plant hormones. Nature, 455, 195–
200.
Waruru, M. (2013). Deadly Striga weed spreading across Eastern Africa. Available at:
http://www.scidev.net/en/sub-suharan-africa/news/deadly-striga-weed-
spreadingacross-eastern-africa.html: SciDev.Net [accessed on April 2, 2015].
Westwood, J.H., Yoder, J.I., Timko, M.P. and dePamphilis, C.W. (2010). The evolution of
parasitism in plants. Trends in Plant Science, 15, 227–235.
Wickett, N.J., Honaas, L.A., Wafula, E.K., Das, M., Huang, K., Wu, B.A. et al. (2011).
Transcriptomes of the parasitic plant family Orobanchaceae reveal surprising
conservation of chlorophyll synthesis. Curr. Biol. 21, 2098–2104.
Xie, X. and Yoneyama, K. (2010). The strigolactone story. Annu. Rev. Phytopathol. 48, 93–117.
Yoshida, S. and Shirasu, K. (2012). Plants that attack plants: molecular elucidation of plant
parasitism. Current Opinion in Plant Biology, 15, 708–713.
Yoshida, S., Maruyama, S., Nozaki, H. and Shirasu, K. (2010). Horizontal gene transfer by the
parasitic plant Striga hermonthica. Science, 328, 1128.