Diseases of Begonia

Cristina Rosa and Gary W. Moorman

Contents1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Fungal and Fungus-Like Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Fusarium Wilt (Fusarium foetens) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Gray Mold (Botrytis Blight; Botrytis cinerea Pers.: Fr.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Powdery Mildew (Oidium begoniae, Asexual Stage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 Pythium Root Rot (Pythium sp.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Bacterial and Phytoplasma Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1 Bacterial Spot (Bacterial Blight; Xanthomonas campestris pv. begoniae

(Takimoto) Dye) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Viral Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.1 Clover Yellow Mosaic Potexvirus (ClYMV) Described by Johnson (1942) . . . . . . . . . 104.2 Cucumber Mosaic Virus (CMV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.3 Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV) . . . . 104.4 Tobacco Mosaic Virus (TMV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.5 Tobacco Necrosis Necrovirus (TNV) and Carnation Mottle Carmovirus

(CarMV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.6 Tobacco Ringspot Virus (TRSV) and Arabis Mosaic Nepovirus (ArMV) . . . . . . . . . . . . 124.7 Broad Bean Wilt Virus (BBWV) Described by Stubbs (1947) . . . . . . . . . . . . . . . . . . . . . . . 134.8 Zucchini Yellow Mosaic Virus (ZYMV) Described by Lisa et al. (1981) . . . . . . . . . . . . 13

5 Nematode Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.1 Foliar Nematode (Aphelenchoides fragariae) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Abiotic Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

C. Rosa (*) • G.W. Moorman (*)Department of Plant Pathology & Environmental Microbiology, The Pennsylvania State University,University Park, PA, USAe-mail: czr2@psu.edu; gmoorman@psu.edu

# Springer International Publishing Switzerland 2016R.J. McGovern, W.H. Elmer (eds.), Handbook of Florist's Crops Diseases, Handbook ofPlant Disease Management, DOI 10.1007/978-3-319-32374-9_30-1

1

AbstractBegonias are susceptible to a wide variety of fungi, bacteria, and viruses, as wellas nematodes and abiotic diseases. The systemic nature of some of the pathogensmakes it likely that the diseases they cause can be found wherever vegetativelypropagated begonias are shipped. Management of these pathogens is paramountfor specialty propagators while growers purchasing plants must inspect incomingplants for symptoms and understand the biology of the pathogens involved inorder to manage them effectively.

KeywordsFusarium wilt • Botrytis blight • Powdery mildew • Pythium root rot • Bacterialspot • Viruses • Foliar nematodes • Abiotic diseases

1 Introduction

Begonias are a very diverse and complex group of plants. The name “Begonia” wasfirst used by Charles Plumier, a Franciscan monk and botanist, in 1703. Linnaeusdescribed all the species known to him in 1753, and by the end of the 1700s, begoniaswere being cultivated widely in Europe. By the early 1800s, hybrids were beingdeveloped. Based on their dominant horticulture characteristics, begonias can begrouped in categories including cane-like, shrub-like, semperflorens, rhizotamous,rex cultorum, tuberous, and trailing-scandent (Thompson and Thompson 1981).

2 Fungal and Fungus-Like Diseases

2.1 Fusarium Wilt (Fusarium foetens)

Geographic occurrence and impact. This disease has been found in Europe, Japan,and North America causing wilt and sometimes stem rot (Elmer and Vossbrinck2004; Schroers et al. 2004; Sekine et al. 2008; Tian et al. 2012; Tschope et al. 2007;Van der Gaag and Raak 2010).

Symptoms/signs. Leaves become dull green and the leaf veins may yellow. Theleaves wilt and die. The vascular tissue of infected plants turns brown. A basal stemrot may develop as the disease process proceeds (Fig. 1). Large numbers of paleorange spores develop on the dying tissue, particularly if the tissue is placed on a wetsurface in a container and incubated.

Biology and epidemiology. Fusarium foetens is related to the Fusariumoxysporum complex that contains many form species responsible for vascular wiltsof various plants. It is thought that the pathogen was introduced to Europe on plantmaterial brought there for breeding (Schroers et al. 2004), indicating that thepathogen may be spread long distances associated with plant shipments.

2 C. Rosa and G.W. Moorman

Management. Infected plants should be destroyed. The various types and culti-vars of begonia differ in relative susceptibility (Brand and Wienberg 2005; Tianet al. 2012; Tian and Zheng 2012). The biological control agents Bacillus subtilis,Streptomyces griseoviridis, Gliocladium catenulatum, Streptomyces lydicus, andTrichoderma harzianum have been reported to provide protection of begoniasagainst Fusarium foetens in greenhouse experiments (Tian and Zheng 2013). Toreduce the presence of the fungus in production systems, chlorine bleach, hydrogenperoxide, and quaternary ammonia sanitizers have been found effective (Elmer2008).

2.2 Gray Mold (Botrytis Blight; Botrytis cinerea Pers.: Fr.)

Geographic occurrence and impact. This disease is very common and occurswherever plants are grown if the environmental conditions are conducive to itsdevelopment. The pathogen, Botrytis cinerea, can infect almost any plant in agreenhouse including begonias.

Symptoms/signs. Cuttings rot at their base. Tan spots develop on leaves.Established plants rot at the crown or a dry shriveled area develops along a branch(Tompkins 1950). Abundant, dusty, gray masses of spores form on infected tissue ifthe humidity is high or if infected tissue is placed in a container with moisture.

Biology and epidemiology. Seedlings and all above ground parts of matureplants are susceptible. The fungus can infect intact tissue or through wounds.Infection occurs most readily when the fungus has a food base from which it attacks,such as fading flowers or senescent leaves that have fallen onto healthy leaves. Highrelative humidity, wetness on the plant, and temperatures between 18 �C and 25 �Cgreatly favor Botrytis infection (Jarvis 1980). The spores are readily spread by aircurrents (Hausbeck and Pennypacker 1991).

Fig. 1 Fusarium wilt onbegonia (Photo courtesy ofWade Elmer)

Diseases of Begonia 3

Management. Maintain low humidity within the crop canopy by spacing plantswell and venting in a greenhouse setting to improve air circulation. Remove dead anddying flowers and leaves from the area around the plants, from the potting soilsurface, and from the plant. Remove this debris from the greenhouse promptly orput it in a closed waste container. Avoid unnecessarily damaging plants. Apply afungicide preventively but note that the exclusive use of one class of chemical,particularly systemic chemicals, can result in the selection of fungicide resistanceto the chemical class. Resistance to benzimidazole fungicides is very widespread inBotrytis populations Do not rely on only one chemical class (Elad et al. 1992; Gullinoand Garibaldi 1987; Hausbeck and Moorman 1996; Moorman and Lease 1992).

2.3 Powdery Mildew (Oidium begoniae, Asexual Stage)

Geographic occurrence and impact. This disease is widespread, probably foundwherever begonias are grown, and it significantly damages the aesthetic quality ofaffected plants. The obligate plant pathogenic organism exhibits dimorphism. Thatis, the asexual phase of growth looks significantly different from the sexual stage(Bélanger et al. 2002). The asexually reproducing phase of the pathogen is namedOidium begoniae (Fig. 2). The sexual stage is thought to be a member of the genusMicrosphaera because the conidia of the asexual stage are not formed in chains(Quinn and Powell 1981), but the structures formed as a result of sexual reproduction(chasmothecia; formerly cleistothecium) are rarely found associated with begonia. Itis known that Sphaerotheca fuliginea (Powell 1985) and Golovinomycescichoracearum (formerly Erysiphe cichoracearum) (Sammons et al. 1982) mayalso cause powdery mildew in begonia.

Symptoms/signs. White, mealy fungal growth develops on leaves, flowers, andstems. Tissue beneath the fungus may die.

Fig. 2 Powdery mildew onbegonia (Photo courtesy ofThe Pennsylvania StateUniversity Dept. of PlantPathology and EnvironmentalMicrobiology)

4 C. Rosa and G.W. Moorman

Biology and epidemiology. Disease develops most rapidly when temperaturesare 20–21 �C and is greatly inhibited if temperatures are above 28 �C (Quinn andPowell 1982). Unlike essentially all other fungal plant pathogens, wetness has littleinfluence on disease development of this powdery mildew. In fact, it flourishes whenleaf surfaces are relatively dry. Severe powdery mildew outbreaks should be antic-ipated when cool, damp night conditions alternate with warm, dry days.

Management. Examine plants carefully and frequently to detect the onset ofdisease. Maintain a fungicide program to protect plants. It is known that horticulturaloil can kill powdery mildew fungi when it comes in direct contact with the patho-gens. Once the oil dries on the plant surface, it has no activity against powderymildew. That is, it has no residual activity. Applied under the wrong conditions orwhen the plant is wilting, oils are phytotoxic.

2.4 Pythium Root Rot (Pythium sp.)

Geographic occurrence and impact. Several species of Pythium have beenreported to cause root rot and lower stem rot of begonia around the world includingP. ultimum (Globisporangium ultimum) and P. splendens (Globisporangiumsplendens) (Farr et al. 1989). Phytopythium helicoides (formerly, Pythiumhelicoides) has also been found on begonias (Yang et al. 2013).

Symptoms/signs. Infected seedlings die. Shiny tan, water-soaked areas developon the stems and petioles of established plants at or just above the soil line as plantscollapse and die (Middleton 1942). Under very high humidity conditions, thepathogen may be seen as a fluffy, white mass near the soil line but this is unusualto see.

Biology and epidemiology. Pythium species are often found in field soil, sandtaken from streams and rivers, and can be found in pond and lake sediments, anddead roots of previous crops (Ivors and Moorman 2014). Excessively wet pottingmixes greatly favor the development of root rot.

Management. Plant in pasteurized, not sterilized, potting media. If the pottingmix has been sterilized (killing all living organisms in it) by heating it to too high atemperature or heating it too long, a biological vacuum will have been created. IfPythium then contaminates the sterile potting mix, it can cause very severe croplosses, because it has no competition and no natural inhibition by other microbes.Keep hose ends off the ground in order to avoid picking up Pythium-contaminatedsoil and then spraying it onto the crop. Use a well-drained potting mix and do notover-water the plants, particularly if plants are not utilizing a great deal of waterbecause of prevailing weather conditions. Apply a fungicide or biological controlagent at planting for the best protection. If the crop is grown for several months, anychemical or biological control agent will have to be applied repeatedly.

Listed below are additional fungus or fungus-like organisms which have beenassociated with Begonia (Farr et al. 1989). For the most up-to-date listing, searchhttp://nt.ars-grin.gov/fungaldatabases/fungushost/fungushost.cfm:

Diseases of Begonia 5

Aecidium begoniaeAgrobacterium tumefaciensAlternaria sp.Alternaria tenuis (Alternaria alternata)Armillaria melleaBartalinia begoniaeBotryosphaeria sp.Botryotinia fuckeliana (Botrytis cinerea)Ceratobasidium sp.Cercospora begoniaeCercospora sigesbeckiaeChoanephora cucurbitarumCladosporium inconspicuumCladosporium sphaerospermumColeosporium begoniaeColletotrichum capsici (Colletotrichum truncatum)Colletotrichum gloeosporioidesColletotrichum sp.Corticium solani (Rhizoctonia solani)Corynespora cassiicolaCurvularia inaequalisErysiphe begoniaeErysiphe begoniicolaErysiphe communis (Erysiphe pisi var. pisi)Erysiphe orontii (Golovinomyces orontii)Erysiphe polygoniErysiphe polyphaga (Golovinomyces orontii)Fusarium begoniaeFusarium equisetiFusarium roseumFusarium solaniGloeosporium begoniaeGlomerella cingulata (Colletotrichum gloeosporioides)Golovinomyces orontiiHelminthosporium sp.Lasiodiplodia theobromaeMacrophoma sp.Macrophomina phaseoli (Macrophomina phaseolina)Marssonina maliMeliola begoniaeMicrosphaera begoniae (Erysiphe begoniicola)Moniliopsis aderholdii (Rhizoctonia solani)Mycena citricolorMyrothecium roridumOidium begoniae (Golovinomyces orontii)

6 C. Rosa and G.W. Moorman

Oidium ellipticumPenicillium bacillisporum (Talaromyces bacillisporus)Pestalotiopsis sp.Phoma sorghina (Epicoccum sorghi)Phoma sp.Phomopsis sp.Phyllachora begoniaePhyllosticta begoniaePhyllosticta sp.Phyllostictina sp.Phytophthora cactorumPhytophthora cryptogeaPhytophthora cryptogea f. sp. begoniaePhytophthora nicotianae var. nicotianae (Phytophthora nicotianae)Phytophthora niederhauseriiPhytophthora parasitica (Phytophthora nicotianae)Pseudoidium sp.Pucciniastrum boehmeriaePythium aphanidermatumPythium debaryanum (Globisporangium debaryanum)Pythium intermedium (Globisporangium intermedium)Pythium irregulare (Globisporangium irregulare)Pythium vexans (Phytopythium vexans)Rhizoctonia solaniSclerotinia sclerotiorumSclerotium rolfsii (Athelia rolfsii)Septonema sp.Septoria begoniaeShrungabeeja begoniaeSphaeropsis begoniicolaStemphylium sp.Thanatephorus cucumeris (Rhizoctonia solani)Thielaviopsis basicolaVerticillium albo-atrumVerticillium dahliae

3 Bacterial and Phytoplasma Diseases

Where possible, the scientific names used are those accepted by the InternationalSociety of Plant Pathology Committee on the Taxonomy of Plant PathogenicBacteria (Bull et al. 2010).

Diseases of Begonia 7

3.1 Bacterial Spot (Bacterial Blight; Xanthomonas campestrispv. begoniae (Takimoto) Dye)

Geographic occurrence and impact. This disease, one of the most damaging tobegonias, was first described in the 1920s (Powell 1985) (Fig. 3). It causes a leaf spotor blighting of some cultivars but can be systemic in Rieger elatior begonias (Harriet al. 1977). In some cases such as a slight infection, the disease is easily overlooked.Thus, the pathogen can be inadvertently shipped with plants if infection goesundetected.

Symptoms/signs. Water-soaked circular lesions surrounded by yellow halosdevelop on leaves. In wax begonias (semperflorens types), the spots may be verysmall (1 mm diameter) (Daughtrey et al. 1995). Plants slowly die one leaf at a time. Ifinfection begins at the leaf margin, wedge-shaped brown dead areas develop with thewide part of the wedge at the margin and the point of the wedge pointing toward theplant stem. When infected tissue is cut, placed in a clear drop of water on amicroscope slide, and observed with a bright field microscope with the iris dia-phragm mostly closed, bacteria can be seen to stream from the tissue. When Riegerbegonias are infected systemically, they wilt and die.

Biology and epidemiology. The bacteria can enter injured roots or leaves andbecome systemic in some cultivars of Rieger begonias. Eventually wilting and deathof the plant occurs. In some other begonias such as Rex, the pathogen remainslocalized in leaf spots (Jodon and Nichols 1974). The pathogen can survive forextended periods of time if the leaf debris is dry. The pathogen can be spread fromplant to plant on workers’ hands, clothing, and tools and has been shown to bedisseminated in recycled irrigation water (Atmatjidou et al. 1991).

Management. Purchase plants free of the pathogen. Discard infected plants,particularly Rieger-types which are systemically infected. Remove infected leavesfrom Rex and tuberous types because they are not systemically infected. Irrigate plantsin a manner that keeps water off the foliage. Do not propagate from infected plants.Remove the debris from infected plants from the greenhouse or place it in a closed

Fig. 3 Bacterial blight onbegonia (Photo courtesy ofThe Pennsylvania StateUniversity Dept. of PlantPathology and EnvironmentalMicrobiology)

8 C. Rosa and G.W. Moorman

container. Workers should wash their hands thoroughly periodically when performingtasks on the plants and should disinfect any tools that come in contact with the plants.

Additional Diseases with Bacterial PathogensCrown gall (Rhizobium tumefaciens, formerly Agrobacterium tumefaciens) (Powell1985)

Bacterial fasciation (Rhodococcus fascians) (Powell 1985)Soft rot (Pectobacterium carotovorum, formerly, Erwinia carotovora) (Powell

1985)

Additional Diseases with Phytoplasma PathogensPhytoplasmas induce symptoms such as phyllody, shoot proliferation, small leavesand flowers, and stunted plant growth. These bacteria are vectored primarily byleafhoppers, planthoppers, and psyllids.

“Candidatus Phytoplasma asteris”A phytoplasma in the 16SrI group was reported to infect begonia (Powell 1985).Shoot proliferation.A phytoplasma belonging to the 16SrIII group was associated with shoot prolif-

eration in begonia in Brazil in 2006 (Ribeiro et al. 2006).

4 Viral Diseases

Viruses of begonia are associated with mild to severe mosaic and mottling, ringspots,leaf malformation, stem necrosis, chlorosis, and stunting. Generally viruses can becontrolled by using clean propagative material, by discarding diseased plants andisolating surrounding plants and controlling vector populations for viruses that arevector transmitted. Clean potting soil needs to be used to control viruses whosevectors are soil inhabitants. The presence of viruses can be diagnosed by use ofmolecular tests such as reverse transcriptase PCR or immune-based assays such asELISA or rapid immunostrips, or by indicator hosts.

The EPPO Panel on “Certification of Ornamentals” developed procedures for theproduction of healthy carnation, pelargonium, lily, narcissus, chrysanthemum, tulip,crocus, iris, begonia, impatiens, rose, freesia, hyacinth, kalanchoe, and petunia.

In the USA, certification programs are managed by the major commercial prop-agators. Pelargonium and other major flower crops are tested for the presence of themost prevalent viruses either by in house plant pathologists or by testing servicescontracted with by the propagators. In the USA, phytosanitary certificates arerequired for intra- and interstate plant movement. Data on pathogens of highconsequence, including viruses, are recorded by the National Plant DiagnosticNetwork.

Where available, information was taken from Adams and Antoniw (2006).

Diseases of Begonia 9

4.1 Clover Yellow Mosaic Potexvirus (ClYMV) Described byJohnson (1942)

Geographic occurrence and impact. It is common in the Western USA and Canada(Agrawal et al. 1962; Pratt 1961). It has occasionally being imported to the UK, but ithas not established in the EPPO region.

Symptoms. ClYMV induces color variations and poor vigor.Biology and Epidemiology. ClYMV is in the family Alphaflexiviridae. ClYMV

is transmitted by infected sap but no vector is known. Reports indicate that it is alsoseed transmitted (Hampton 1963). Clover yellow mosaic virus is serologicallydistantly related to White Clover mosaic virus.

Management. Use of healthy propagative material and sanitation isrecommended.

4.2 Cucumber Mosaic Virus (CMV)

Geographic occurrence and impact. CMV is found worldwide, has an extremelybroad host range, and can infect species in more than 100 plant families (Zitter andMurphy 2009).

Symptoms/signs. Symptoms observed on infected begonia are mosaic, leafdeformation and curling, and vein clearing.

Biology and epidemiology. CMV (Jacquemond 2012; Palukaitis and García-Arenal 2003) belongs to the family Bromoviridae. CMV is a ss + RNA virus with atripartite genome, is aphid transmitted in a noncirculative (nonpersistent) manner(Hoggan 1933; Simons 1955; Watson and Roberts 1939), and can be seed transmit-ted in some plant species (Neergaard 1977).

Management. Control for this virus is particularly difficult, since CMV is acommon virus on many plant Families and it can be transmitted by multiple aphidspecies. Because aphids transmit CMV readily during probing, aphid control ispartially efficacious at reducing the spread of the virus (Zitter and Murphy 2009).Mineral oil has been proposed to delay viral symptoms in different crops (Simonsand Zitter 1980).

4.3 Impatiens Necrotic Spot Virus (INSV) and Tomato SpottedWilt Virus (TSWV)

Geographic occurrence and impact. INSVand TSWVare prevalent in many plantfamilies and species, making their control particularly challenging. INSV can infectaround 800 plant species and TSWV more than 1,000.

Symptoms/signs. Symptoms associated with INSV and TSWV are mosaic,mottling, stem necrosis, ringspots on leaves, and leaf deformation (Fig. 4).

Biology and epidemiology. INSV (Law and Moyer 1990) and TSWV (Samuelet al. 1930) are in the Family Bunyaviridae. These ssRNA viruses have tripartite

10 C. Rosa and G.W. Moorman

genomes, with negative or ambisense orientation. INSV and TSWV are transmittedby thrips in a circulative and propagative manner (Ullman et al. 1993; Wijkampet al. 1993); thus, they can replicate in their plants as well as in their insect hosts.

Management. One of the best ways to manage these viruses is to control thripsnumbers, since thrips need a relatively long acquisition and transmission time, andsince the viruses encounter a latent period in the vector prior to becoming transmis-sible. For instance, in the Netherlands, where INSV and TSWV have been reportedon begonia, the incidence of disease has decreased significantly thanks to theimplementation of a rigorous thrips management program. The Netherlands hasalso established a certification program for many ornamental plants, including bego-nia. To complicate matters, many thrips are insecticide resistant (Brødsgaard 1994;Zhao et al. 1995); thus, care needs to be taken to rotate different classes of insecti-cides. Weed management is also necessary (Bond et al. 1983; Cho et al. 1986;Kobatake et al. 1984), since weeds can serve as reservoirs for tospoviruses and thrips.

4.4 Tobacco Mosaic Virus (TMV)

Geographic occurrence and impact. TMV can infect a variety of plants in 30 Fam-ilies worldwide (Shew and Lucas 1991).

Symptoms/signs. Symptoms on begonia are reported to be yellowing, necrosis,mosaic, leaf distortion, and plant stunting.

Biology and epidemiology. TMV belong to the Virgaviridae family. The virushas a ss + RNA genome. Symptoms attributed to TMV are often caused by otherTobamoviruses, and many Tobamovirus species were once classified as strains ofTMV. TMV is occasionally transmitted by chewing insects, but most commonly it ismechanically spread (Harris and Bradley 1973; Lojek and Orlob 1969), in fact TMVvirions are extremely stable. TMV can persist in the soil, probably on plant debris,and can infect roots. It can penetrate wounded embryos from infected seed coats(Broadbent 1965). TMV is a special concern in greenhouses, where it can be veryhard to eradicate (Broadbent and Fletcher 1963).

Fig. 4 Impatiens necroticspot virus on begonia (Photocourtesy of The PennsylvaniaState University Dept. of PlantPathology and EnvironmentalMicrobiology)

Diseases of Begonia 11

Management. Sanitation is the best TMV control method.

4.5 Tobacco Necrosis Necrovirus (TNV) and Carnation MottleCarmovirus (CarMV)

Geographic occurrence and impact. TNV (Price 1940) can experimentally infect88 species in 37 plant families and CarMV can infect members in more than9 families of plants worldwide.

Symptoms. On most hosts TNV causes necrotic lesions and rarely systemicsymptoms. TNV can support coinfection with satellite viruses. Carnation mottlevirus can infect Begonia elatior and Begonia x cheimantha where it is associatedwith symptoms of vein clearing, leaf curling, and flower breaking.

Biology and epidemiology. These viruses are in the Family Tombusviridae.Members of this family have a ss + RNA monopartite genome. TNV (describedfirst by Smith and Bald 1935 and more recently by Fraenkel-Conrat 1988) istransmitted by the zoospores of the fungus-like microorganism Olpidium brassicae(Kassanis and MacFarlane 1964; Teakle 1962; Teakle and Gold 1963), is nottransmitted by seed or by pollen, and can be transmitted mechanically.

CarMV is not known to be transmitted by a vector or by seed, but it is probablytransmitted by plant to plant contact and mechanically during cultural practices. Thevirus is also stable in water and can be transmitted through irrigation. Anunconfirmed report from India indicates that aphids could be vectors of this virus.

Management. To control TNV, care should be taken in using clean irrigationwater and potting soil, and in avoiding diseased propagative material. Control ofCarMV consists of using certified virus-free material, strict sanitation, and care inhandling infected plants.

4.6 Tobacco Ringspot Virus (TRSV) and Arabis Mosaic Nepovirus(ArMV)

Geographic occurrence and impact. ArMV (Smith and Markham 1944) has beendetected in 13 States in the USAwhere it has been the object of quarantine, but it isnot widespread or established outside Europe. ArMV has a wide host range. TRSV(Fromme et al. 1927) geographic distribution is mainly in northern USA and China,but it has also been found in Europe and Australia. More than 17 plant families aresusceptible to TRSV (Price 1940).

Symptoms/signs. Begonia yellow spot, caused by TRSV, shows symptoms ofchlorotic local lesions and yellow mottle patterns, sometimes resembling naturalvariegation.

Biology and epidemiology. ArMV and TRSV belong to the family Secoviridae.Their genomes consist of two linear ss + RNA segments. ARMV is nematode andseed transmitted and can be transmitted by Cuscuta sp. Nematodes lose the virusduring molting and do not transmit it to their progeny. TRSV is transmitted by

12 C. Rosa and G.W. Moorman

nematodes (McGuire 1964), pollen, and seeds as well as by insects (Dunleavy 1957;Komuro and Iwaki 1968; Messieha 1969; Schuster 1963) and mites (Thomas 1969)in a nonspecific manner. TRSV does not replicate in its nematode vector, is lostduring molting, and is not transmitted to the nematode progeny.

Management. TRSV is controlled by using clean potting soil, discarding infectedpropagative material, and by using clean seed.

4.7 Broad Bean Wilt Virus (BBWV) Described by Stubbs (1947)

Geographic occurrence and impact. The virus has been reported worldwide (Lisaand Boccardo 1996). The virus was reported in the USA in New York state, SouthCarolina and Minnesota in the early 1980s. BBWV was subdivided in two species,BBWV-1 and BBWV-2, in 2000. The two species are serologically distinct, withBBWV-1 more prevalent in Europe, while BBWV-2 is more prevalent in NorthAmerica, Asia, and Australia. BBWV has a broad host range (Edwardson andChristie 1991).

Symptoms/signs. Symptoms associated with BBWV on begonia are leaf mot-tling, faint ringspots, color-break in flowers, and generalized stunting. Symptomstend to be more severe in winter.

Biology and epidemiology. As ArMV and TRSV, BBWV is in the familySecoviridae, and its genome consists of two ss + RNA segments. In contrast toArMV and TRSV, BBWV is aphid transmitted in a noncirculative (nonpersistent)manner (Lisa and Boccardo 1996).

Management. Control of BBWV is difficult because its host range is broad and itis transmitted in a nonpersistent manner; thus, exclusion of aphids is advised, as wellas of weeds that serve as virus reservoirs.

4.8 Zucchini Yellow Mosaic Virus (ZYMV) Described by Lisaet al. (1981)

Geographic occurrence and impact. The virus was detected on begonia in Taipeiin 2008. As for CMV, another nonpersistently aphid transmitted virus, several plantfamilies are susceptible to ZYMV and the virus is widely distributed, making itscontrol difficult.

Symptoms. Symptoms in begonia consist of faint ringspots on leaves at the earlystage of infection; the spots become chlorotic and coalesce during diseaseprogression.

Biology and Epidemiology. ZYMV is a potyvirus in the family Potyviridae. Itsgenome consists of a ss + RNA filament and is monopartite. The virus is aphidtransmitted in a noncirculative (nonpersistent) manner.

Management. While the use of insecticides is not recommended to controlnonpersistent viruses and not many experiments have been reported on control ofZYMV in begonia, strategies used to control this virus on other crops would be

Diseases of Begonia 13

probably appropriate. These strategies are: the use of resistant material, the use ofclean propagative material and adoption of good hygiene practices, the use ofmineral oil, and the exclusion of aphids from greenhouses.

5 Nematode Diseases

5.1 Foliar Nematode (Aphelenchoides fragariae)

Geographic occurrence and impact. Aphelenchoides affects a large number ofornamental plants, including begonia. In some cases, the nematode invades thevascular tissue and the begonia remains mostly symptomless. Thus, the nematodecan be inadvertently shipped from place to place in infected plants. Severe damagecan occur in begonias.

Symptoms/signs. Plants may be stunted. In some cases, excessive red pigmen-tation develops in infected leaves. Bronzed or water-soaked areas develop on leavesof some cultivars. Fibrous-rooted cultivars have small brown leaf spots. Somecultivars exhibit no symptoms despite heavy infection. If leaf or stem tissue is placedin a drop of water in a clear dish and teased into small pieces, numerous colorlessnematodes can be seen with some magnification, vigorously moving in a snake-likemotion in the water. The nematodes will tend to settle to the bottom of the dish.

Biology and epidemiology. This nematode can enter stomata or wounds, con-tinue to move in the plant, and feed on cells inside the plant (migratory endoparasiticlife style). Or the nematode can remain on the surface of the plant, continue to moveat will if there is water on the plant tissue, and feed on surface cells (migratoryectoparasitic life style). There is some suggestion that the nematode may have theability to enter intact leaf surfaces directly (Riedel 1985). Peirson (1974) found thatthe “Aphrodite Rose” cultivar of Rieger begonia can harbor 12,000 larvae of thisnematode per gram of fresh weight of leaves, yet exhibit no symptoms.

Management. Purchase nematode-free plants. Irrigate plants in a manner thatkeeps water off the foliage so that movement of the nematodes on the leaf surface isinhibited and localized dispersal among closely spaced plants by splashing isavoided. Discard infected plants. A very high value stock plant can be heat-treatedto eliminate nematodes (Guba and Gilgut 1938). Some experimentation is requiredto determine the best temperature and exposure duration for eliminating the nema-todes while not killing the plant. An effective hot water dip may range from 1 min at49 �C (120 �F) to 5 min at 45 �C (115 �F), depending upon the cultivar. A treatedplant may be severely damaged, but it will recover over time and be free of thenematode.

Additional Diseases with Nematode PathogensRoot-knot (Meloidogyne spp.) (Powell 1985)

14 C. Rosa and G.W. Moorman

6 Abiotic Diseases

Avariety of leaf symptoms develop as a result of nutrient deficiencies in begonia. Ifyellowing is the predominant symptom and the yellowing is between the veins, ironor magnesium may be lacking. Uniformly yellowing leaves indicates nitrogen orcalcium deficiency. Dead tissue at the leaf margin may indicate potassium deficiency,while stunting of an otherwise green plant could be due to lack of phosphorus. A lackof boron causes the foliage to russet and cracks may develop in the leaf petioles andthe plants may be very brittle (Nelson et al. 1977).

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