IX WORLD AVOCADO CONGRESS

CONTROL AND MITIGATION STRATEGIES FOR LAUREL WILT AND AMBROSIA

BEETLE VECTORS

Jonathan H. Crane1, Daniel Carrillo1, Romina Gazis1, Jeff Wasielewski2, Edward A. Evans1,

Fredy Ballen1, and Bruce Schaffer1

1. University of Florida/IFAS, Tropical Research and Education Center, Homestead, Florida 2. University of Florida/IFAS, Miami-Dade County Extension, Homestead, Florida

Abstract. Laurel wilt (LW) was first detected in the Florida avocado production area in 2012. To

date, the death of an estimated 120,000 avocado trees may be attributed to LW. The pathogen is

disseminated by several ambrosia beetle (AB) species and through root grafts among adjacent

trees. Recommendations for LW management include early detection and rogueing of infected

trees followed by the application of contact insecticides to chipped wood and the trunks and

major limbs of healthy trees within a 1 acre (0.4 ha). Since 2012, surveys indicated >90% of the

production managers have attempted to control the epidemic. Approximately 85% of producer’s

or grove managers scout for LW affected trees and most attempt to remove LW affected trees

promptly. However, due to personnel and equipment shortages rogueing may be delayed by

several days to weeks reducing the efficacy of the practice. Currently, AB are controlled by

chipping diseased trees and directed applications of insecticides to chipped wood. Outside of

recommended strategy some producers prophylactically inject fungicide into avocado trees,

others prune LW affected and/or adjacent non-affected trees in an attempt to stop the spread of

LW, whereas others heat treat (solarize) pruned trees to disinfest them of the pathogen and AB.

These pruning and solarization methods have not been experimentally verified as effective. New

evidence demonstrates that AB activity is reduced by high light levels within the canopy,

indicating that pruning may be important to limit spread of LW by AB. Mitigation of the

epidemic includes replanting avocado or alternative fruit crops, instituting a pruning program to

increase canopy light levels, or top-working of existing orchards. Some producers have exited

the industry.

Key words: Xyleborus glabratus, Xyleborus volvulus, and X. bispinatus, Raffaelea lauricola,

Lauraceae

Introduction. The ambrosia beetle, Xyleborus glabratus (Xg) and its fungal symbiont, Raffaelea

lauricola (Rf) were introduced into the Port Wentworth, Georgia (U.S.) through infested wood

packing material from Asia during 2002 (Mayfield and Thomas, 2006). This insect-disease

complex affects trees in the Lauraceae and spreads through natural areas by Xg movement and

anthropogenic movement of infested wood products. Plant hosts of this Xg-Rf complex include at

least ten native lauraceous woody species [e.g., redbay (Persea borbonia), swampbay (P.

palustris)] potentially California bay (Umbellularia californica) and non-native species such as

camphor (Cinnamomum camphora) and avocado (P. americana) (Campbell et al., 2017; Dreaden

et al., 2016; Fraedrich, 2008; Fraedrich et al., 2008; Fraedrich et al., 2011; Fraedrich et al., 2015;

Hughes et al., 2013; Mayfield et al., 2013; Ploetz and Konkol, 2013). By 2005 and 2006 this Xg-

Rf complex was detected in north Florida and central Florida, respectively, due to movement of

Xg-Rf infested wood. During February 2010 Xg was detected in a natural area 21 miles (33.7 km)

north of the south Florida avocado production area (125 sq. miles; 324 sq. km) in Miami-Dade

County (Thomas, 2010; Ploetz et al., 2011). By 2011 the first confirmed swampbay tree to

succumb to laurel wilt (LW) was documented in this natural area and by 2012 LW was detected

in a commercial avocado orchard (J. Crane, personal communication). At present this lethal

insect-disease complex has been detected in 11 southeastern U.S. states including North

Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Arkansas, Texas,

Kentucky and Tennessee (US-Forest Service, 2019). LW has been detected in all of Florida’s 67

counties (FDACS, 2016). LW is now endemic in the avocado production area of Miami-Dade

County.

As the disease epidemic ensued, research determined that Xg did not colonize avocado

orchards but the Rf pathogen had contaminated at least 14 native and non-native ambrosia beetles

documented to reside in commercial avocado orchards (Carrillo et al., 2012 Carrillo, et al., 2014;

Ploetz et al., 2017). Two of these, Xyleborus volvulus and X. bispinatus have been documented

capable of transmitting Rf pathogen to avocado trees.

There is potential for spread of LW to other commercial avocado production areas

through movement across natural areas by Xg, potentially other Rf-contaminated ambrosia beetle

species, and by anthropogenic means including trade, tourism, and movement of firewood,

wood-turned items (e.g., bowls) and wood used for barbeque smoke (Crane et al., 2015). Of

immediate risk is California, Mexico, and Central and South America.

Sixteen avocado cultivars exposed to Xg under controlled conditions, were attacked

(Mayfield et al., 2008; Peña et al., 2012). However, Xg infestation of container-grown ‘Hass’

avocado trees under controlled conditions did not result in LW symptoms (e.g., wilted and

discolored sapwood) and Rf was not recovered from the sapwood (Peña et al., 2012). In another

study, the Rf susceptibility of 24 avocado cultivars of varied genetic background was tested and

those of Guatemalan x Mexican background (e.g., ‘Hass’, ‘Winter Mexican’) were less affected

by Rf inoculation than avocado cultivars of Guatemalan x West Indian (e.g., ‘Miguel’)

background which were more tolerant than West Indian (e.g., ‘Simmonds and ‘Donnie’)

cultivars (Ploetz et al., 2011). This suggests there may be some variability in LW tolerance

dependent upon genetic background. In addition, symptom severity was positively correlated

with plant size, meaning larger plants react more rapidly to the presence of Rf than smaller

plants. However, mature trees of 32 different avocado cultivars of various genetic backgrounds

have been documented to succumb to LW under orchard conditions (Table 1) (A. Palmateer, R.

Gazis and J. Crane, personal communication). Recent, greenhouse studies have shown that rate

of LW disease progression is related more to the scion than the rootstock, but the rootstock does

have some influence (B. Schaffer, personal communication). No tolerant scion or rootstock has

been confirmed to date, but work is underway to try to identify LW-tolerant scions, rootstocks

and scion/rootstock combinations.

In mature avocado orchards, LW disease spreads by root grafts among adjacent trees and

by ambrosia beetle (AB) infestations. The pathogen may move at a rate of three to six new trees

per month; this can result in the loss of 90 or more trees in a six-month period (Crane et al.,

2016; Ploetz et al., 2017). The visible external symptoms of laurel wilt disease begin as green

leaf wilting, typically in one section of the tree (Fig. 1). Frass straws resulting from AB boring

may be evident and the sapwood often has blackish-blue stained streaks. As the disease

progresses, leaves desiccate, turn brown and remain attached to the branches (for up to about 12

months) and progressively dieback takes over more sections of the tree. Eventually the tree dies

although sometimes if the tree is left in-place or is cut to a stump it may regrow from the stump

or large roots (Fig. 2). Interestingly, sometimes trees cut to a stump (~1.2 m height) regrow

canopy and appear disease-free, but this regrowth may dieback repeatedly until the tree is dead

(Fig. 3). In other instances, stumped trees regrow their canopy and resume fruit production after

two to three years. How frequently regrowth and fruit production resume is not known and under

evaluation.

Research findings related to current control strategies.

1. Rf is highly virulent and trees inoculated with as little as 39 colony forming units (CFUs)

may result in mature tree death (R.C. Ploetz, unpublished data). The higher the number CFUs

the more quickly disease progression commences. The pathogen moves rapidly within the

xylem to new locations causing rapid blockage of the xylem water conducting tissue and

decline of the tree (Inch and Ploetz, 2012; Inch et al., 2012).

2. Previous recommendations based on research suggested prophylactic systemic propiconazole

(Tilt®) fungicide infusion could protect avocado trees from LW for 12 to 18 months (Crane et

al., 2016; Ploetz et al., 2011). However, the economics and long-term sustainability of

propiconazole fungicide infusion is not sustainable because of the high cost of

implementation and the physical damage caused to the trees by repeated applications. More

recently, the sustainability of long-term, repeated injections has been questioned due to the

physical damage caused at the injection sites by repeated applications, the damage caused by

high concentrations of fungicide at injection points, the long time it takes for injected

fungicide to distribute within the trees, and the uneven distribution of the fungicide within

the tree (Crane et al., 2016; Ploetz et al., 2017). Therefore, a cost-benefit analysis is

warranted to ascertain injections long-term viability.

3. A number of conventional contact insecticides have shown efficacy at controlling ambrosia

beetles including Malathion, Danitol®, Agri-Mek®SC, Talstar®S, and Hero® (non-bearing

trees only) (Carrillo et al., 2013; Peña et al., 2011). In addition, several entomopathogenic

insecticides (e.g., BotaniGard® and PRF 97®) have been demonstrated to control ambrosia

beetles (Carrillo et al., 2015). However, these insecticides are only effective when the AB is

outside the tree and AB spend most of their lifecycle inside the tree. Antidotal evidence

suggests orchard-wide applications of entomopathogenic insecticide applications (e.g.,

BotaniGard® and Mycotrol®) that coincide with increased AB activity are effective at

suppressing AB activity. Effective systemic insecticides have been investigated and is on-

going.

4. Light levels are significantly higher and AB activity significantly reduced in orchard areas of

newly planted and recently stumped [trees cut to 1.2 m (4 ft)] and recently top-worked areas

of the orchard compared to highly shaded, dense canopied avocado orchards (Fig. 2) (D.

Carrillo and J. Crane, unpublished data). Thus, implementing or re-establishing a pruning

program to enhance light penetration within orchard trees is recommended to reduce AB

activity and new LW outbreaks.

Current control and mitigation recommendations:

1. Optimize fertilizer and irrigation practices as well as Phytophthora root rot and other diseases management to maintain tree health. Healthy trees are less susceptible to AB

attack than nutrient deficient and/or drought-stressed trees. Any stress (e.g., flooding,

drought, freezing, other pathogens etc.) causes avocado trees to become attractive to ABs.

2. Scout orchards frequently to detect trees with early LW symptoms (i.e., green-leaf wilting).

3. Rogue LW affected trees immediately: uproot and chip/shred the entire tree (roots-trunk, limbs, etc.).

4. Spray chipped/shredded wood twice with contact insecticide. Insecticides registered for use on avocado include Malathion, Danitol®, Agri-Mek®SC, Talstar®S, Hero® (non-

bearing trees only), Botanigard® and Mycotrol® (

5. Make two trunk and major limb directed applications [up to about 3 m (10 ft) height from the ground] at a 14-day interval of a registered contact insecticide to healthy trees within

a 0.4 ha (1 acre) area.

6. During the annual period when AB populations increase (late winter-early spring) apply two orchard-wide applications of the mycoinsecticides BotaniGard®ES or Mycotrol®

(Beauveria bassiana) to suppress AB populations.

7. Maintain a pruning regime to increase sunlight penetration and hours of high sunlight in

avocado orchard trees to suppress ambrosia beetle activity. This may be accomplished

with mechanical pruning (i.e., hedging and topping equipment) or by selective pruning.

8. Replace avocado trees lost to LW to increase future fruit production and maintain economic viability (Evans et al., 2010; Mosquera et al., 2015). Young trees will take

many years to root graft to adjacent trees and therefore movement of the LW pathogen by

tree-to-tree root grafting is not an issue and young trees are not a preferred host of AB.

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Acknowledgements: The authors thank the following supporting entities: USDA-NIFA grant

(2015-51181-24257): Laurel Wilt of Avocado: Management of an Unusual and Lethal Disease),

University of Florida-Institute of Food and Agricultural Sciences, the Florida Avocado

Administrative Committee, UF/IFAS Extension Miami-Dade County, and Florida Department of

Agriculture and Consumer Services.

(u//extension/AB-LW/2019/Avo Congress/Crane et al production control and mitigation strategies paper.docx)

Table 1. Mature avocado tree

cultivars and their genetic

background that have succumb

to the laurel wilt pathogen. Cultivar name Background

Arue WI

Bernecker WI

Day WI

Donnie WI

Dupuis WI

Hardee WI

Peterson WI

Pollock WI

Russell WI

Simmonds WI

Waldin WI

Beta G-WI

Booth 7 G-WI

Booth 8 G-WI

Brooks Late G-WI

Buck II G-WI

Choquette G-WI

Hall G-WI

Loretta G-WI

Lula G-WI

Miguel G-WI

Monroe G-WI

Nadir G-WI

Nesbitt G-WI

Tonnage G-WI

Tower-2 G-WI

Wheeling G-WI

Brogdon G-M-WI

Marcus Pumpkin G

Winter Mexican G-M

Toni Nd

Jim Lapeck Nd

Fig. 1. The visible external symptoms of laurel wilt disease begin as green leaf wilting (A),

typically in one section of the tree (B). Fras straws from AB boring may be evident (C) and the

sapwood has blackish-brown-blue streaks (D).

Figure 2. Light exposure of highly shaded (A),

top-worked (B) and newly planted (C) areas of

an orchard.

Fig. 3. (A) regrowth and some branch die-back of tree cut

to a stump (~1.2 m height) and (B) tree regrowth dieback

repeatedly until the tree is dead.