Pests in the Spotlight

Integration of Control Strategies for Management of Phytophthora Root Rot and Ambrosia Beetles in Nursery Trees

By Madhav Parajuli* and Dr. Cansu Oksel**, Dr. Jason B. Oliver, Dr. Karla M. Addesso, and Dr. Fulya Baysal-Gurel, Tennessee State University, Otis L. Floyd Nursery Research Center

*PhD student & **Postdoctoral Researcher

How do Phytophthora root rot and ambrosia beetles interact to make nursery problems worse (and which host plants are most likely to be affected)?

Ambrosia beetles (Xylosandrus spp.), woodboring insects, and Phytophthora spp., soilborne pathogens, are economically important and destructive challenges to both susceptible trees during nursery production and trees being established in landscapes. Root and crown injury caused by Phytophthora may block the intake and movement of nutrients and water, leading to root mortality and plant death. Infection with Phytophthora is more prevalent and made worse when environmental conditions adversely impact the plant like high temperature, excessive moisture, oxygen deficiency, ethylene, and carbon dioxide production in the root zone. Ambrosia beetles use host plant cues to locate and then colonize the main trunk and branches of highly stressed or recently killed trees, primarily by seeking airborne plant stress-related volatile compounds like ethanol. Phytophthora may increase the production of stress-related ethanol from infected host plants; and thus, increase the attraction of ambrosia beetles. Some of the ornamental trees affected by Phytophthora include cherry (Prunus spp.), dogwood (Cornus spp.), maple (Acer spp.), oak (Quercus spp.), and rhododendron (Rhododendron spp.). Common host plant species attacked by ambrosia beetles include cherry ( Prunus spp.), crapemyrtle ( Lagerstroemia spp.), dogwood (Cornus spp.), golden rain tree (Koelreuteria spp.), magnolia (Magnolia spp.), maple (Acer spp.), oak (Quercus spp.), redbud (Cercis spp.), and styrax (Styrax spp.). Many tree species that are susceptible to ambrosia beetles and Phytophthora are also flood intolerant. Working to address all of the pest and environmental challenges simultaneously will provide more effective control than dealing them individually.

How do we monitor symptoms of Phytophthora root rot infection and ambrosia beetle attacks?

Phytophthora infection may result in dark brown to black discoloration in infected host plant roots and the crown region that can lead to complete root decay ( Fig. 1 ).

Figure 1. Dark brown to black discoloration can be seen on roots and the crown of a tree infected by Phytophthora.

Leaves lose their luster and may show signs of premature leaf aging, changing color from green to light green and then pale yellow or purplish brown. As the infection progresses, yellowing and wilting symptoms can be visible in foliage and stems, which may be followed by stunted growth and tree mortality. Ambrosia beetles excavate tunnels into the wood (i.e., xylem) forming ‘toothpick’-like extrusions of excavated wood, sawdust and frass (debris or excrement produced by insects). These toothpicks are often the very first indication of beetle attacks (Fig. 2).

Figure 2. Adult ambrosia beetles are attracted to the trunk of a flooded and infected tree. Toothpick-like frass and sawdust structures emerge from the tree following beetle attack.

Other visible symptom of ambrosia beetle attack includes small, round exit holes that may exude sap (Fig. 3). Severe ambrosia beetle infestations interrupt movement of nutrients and water in attacked host plants, causing branch dieback and wilting, and eventually tree mortality. Overwintering female ambrosia beetles exit galleries as early as March in Tennessee, generally following several consecutive days in which temperatures reach about 70ºF. Phytophthora is also severe during the summer when warm temperatures are favorable for infection.

Figure 3. An ambrosia beetle exit hole and sap drainage are visible on the smooth bark of flowering dogwood.

How can we reduce severity and limit damage caused by Phytophthora root rot and ambrosia beetles?

Because root rot disease caused by Phytophthora appears to increase host plant attractiveness to ambrosia beetles, perhaps through release of ethanol, several management actions can help protect crop plants from ambrosia beetle attacks.

I. Use good sanitary measures

Growing healthy trees, thereby reducing stress-related ethanol production, is a best management strategy for ambrosia beetles. A first step is to make sure only disease-free trees are being planted. After planting disease-free trees, root rot diseases caused by Phytophthora can be managed by implementing good sanitary measures. It is important to prevent introducing pathogens into a nursery by monitoring newly received plants, including their root systems. Ideally, new plants are held in an area quarantined from main production blocks while being screened and observed. If diseased trees are observed in the field, limit spread by immediately removing infected plants, plant debris, and fallen leaves from the growing area. Sterilize pruners and use proper pruning practices when removing plant tissue that are exhibiting initial signs of ambrosia beetle attack or plant pathogen infection. It is essential to sanitize any equipment before moving from one nursery plot to another.

II. Consider cover cropping for field production systems

Mixed cropping, intercropping, and crop rotation are important practices that can reduce the inoculum buildup of Phytophthora. Cover crops that belong to the Brassicaceae family such as mustard, turnip, arugula can be incorporated into the soil to control Phytophthora root rot and this process is widely known as biofumigation. Winter cover crop usage (crimson clover or triticale or mixed crimson clover and triticale) can provide improved management of Phytophthora root rot.

III. Practice good water management through proper irrigation and enhanced drainage

Flooding of nursery plant root zones is the main factor contributing to both ambrosia beetle attacks and Phytophthora infection in susceptible host plants. Soil flooding causes direct stress to the plant by depriving oxygen to the roots, and it provides a mechanism for spread of soil- and water-borne oomycete pathogens, including Phytophthora, from host to host. Flooded trees (especially flood intolerant species like dogwood) produce greater amounts of ethanol, a volatile cue used by ambrosia beetles to locate susceptible hosts. Moreover, environmental stress factors predispose trees to pathogen infection by reducing host plant defenses. Monitoring soil moisture levels in locations of fields where potential host plants are planted could allow risk forecasting for ambrosia beetle and pathogen infestations and provide a possible preventative tool.

IV. Select effective chemical management options

The best control strategy against ambrosia beetles and Phytophthora is to avoid plant stress factors that enhance these destructive issues. Use good sanitation practices and avoid planting disease-infected plants. When infestations are likely or actively occurring, however, several classes of fungicides and insecticides will provide preventative and curative effectiveness against ambrosia beetle and Phytophthora. In a recent field study in McMinnville, TN, dogwood trees exposed to Phytophthora cinnamomi and flooding had a large number of ambrosia beetle attacks. The combination of fungicide mefenoxam + insecticide permethrin was the most effective treatment in reducing Phytophthora root rot severity and ambrosia beetle attacks in dogwood trees that were exposed to the pathogen and flooding stress. Mefenoxam was drenchapplied 18 days before Phytophthora inoculation and 21 days before intentional flooding. The same trees also were sprayed with permethrin two days before flood initiation. The combination of mefenoxam + activated charcoal + kaolin also significantly reduced the number of ambrosia beetle attacks. Charcoal + kaolin was sprayed two days after flooding. The application of activated charcoal and kaolin may block or absorb ethanol emissions from stressed trees. In another study, fungicide active ingredients applied preventively or curatively (i.e., fluxapyroxad + pyraclostrobin), and preventatively (i.e. mefenoxam) reduced ambrosia beetle attacks and Phytophthora in flooded dogwoods. The preventative application was performed seven days before flooding and the curative application was performed 24 hours after flooding using a spray-to-drench (sprench) application to the lower trunk and potting substrate surrounding the base of the plant. Because ambrosia beetles feed on symbiont ambrosia fungi, the application of systemic fungicides like fluxapyroxad, pyraclostrobin, and mefenoxam also may provide protection against symbiont fungi. When the ambrosia beetles are not able to farm fungi in the tree, the adult beetles will not make galleries. Other treatment options showing promise for repelling ambrosia beetles include methyl salicylate (MeSA), a host defense and signaling compound, combined with verbenone (a bark beetle anti-aggregation pheromone). Research is on-going with regard to the best formulation, rate, and application methods for MeSA and verbenone.

To improve application timing of chemical-based ambrosia beetle treatments, ethanol-baited traps can be used to monitor seasonal beetle flight activity near ornamental nurseries. Several companies sell slowrelease ethanol lures, which can be placed in various commercial traps or homemade soda bottle traps. Generally, a beetle-killing agent like soapy water or low toxicity antifreeze (propylene glycol) is placed in the bottom of the trap. Be aware that large numbers of non-pest ambrosia beetles also may respond to ethanol-baited traps, so a good approach is to monitor traps for large numbers of beetles when temperatures are near 68ºF and to take captured specimens to local Extension offices for assistance with beetle identifications.

For more information about ambrosia beetle and Phytophthora, please visit our recent publication at: https://academic.oup.com/jee/article-abstract/115/4/1213/6619515 and https://www.sciencedirect.com/science/article/abs/pii/S0261219419301747 For soda bottle traps, visit: https://stopab.org/wp-content/uploads/2022/08/JC-Clemson-Ambrosia-beetle-brochure.pdf