Disease name: Laurel wilt
Pathogen name: Raffaela lauricola (fungus)
Hosts: Laurel wilt is a disease of plants in the laurel family (Lauraceae). In the southeastern United States, where it is common, the hosts are redbay (Persea borbonia), swamp bay (P. palustris), and avocado trees (P. americana). In the upper midwest, the hosts are sassafras (Sassafras albidum) and spicebush (Lindera benzoin). It is generally believed that all plants in the laurel family are susceptible, but not all have been tested.
Laurel wilt is a lethal vascular wilt disease which is moved (vectored) by the red bay ambrosia beetle (Xyleborus glabratus). Neither the fungus, nor the beetle are native to the United States. Both were first discovered in the United States on the coast of Georgia in 2002. They have since been found from North Carolina to Florida to eastern Texas, and north to Kentucky.
Symptoms
Symptoms and signs associated with laurel wilt are related to both the activity of the beetle and the infection caused by the fungus.
Laurel wilt is a vascular disease and the tree’s main symptom is wilting, similar to other vascular diseases.
This is followed by death of individual branches, and eventually the entire plant.
Disease Cycle
The red bay ambrosia beetle and the laurel wilt fungus have a symbiotic relationship. The adult female beetle carries the fungus in a specialized pouch called a mycangium. The female will bore into the host plant to create galleries for egg laying. To clear the gallery of the wood debris, she pushes the wood shavings out of the hole. Signs of her boring activity include toothpick-like wood shavings sticking out from the stem. These are often called ‘noodles’.
Once the female has bored into the host she deposits the fungus in the gallery and lays her eggs nearby. The eggs hatch into larvae, which will feed on the fungus until they are grown and emerge from the plant as adult beetles. They will keep some of the fungus to take with them when they emerge. This movement of fungal spores by the beetles is the main method of spreading the disease.
While some of the fungal spores are used for food by the beetle, the rest can infect the host plant and lead to wilting. The wilting occurs because the plant’s immune system tries to compartmentalize, or isolate, the fungus that has infected the plant’s vascular (water-conducting) system. The plant does this by clogging its own vascular system with gums (chemicals) and tyloses (physical structures) to prevent the fungus from spreading.
This isolation process also causes a loss in water and nutrient flow, which kills the plant. This is what causes the wilting symptoms that are seen in the canopy. The wilting symptom is associated with the activity of the beetles, so it is common to see plants with only a few wilting branches during the early stages of an infestation. The visible symptom of the tree’s isolation process is dark streaking in the vascular tissues of the infected branches. The vascular streaking can be seen by peeling the bark off of infected branches.
Management
Cultural Management
Monitoring and reporting suspected infections is the best way to culturally manage the spread of laurel wilt. For accurate diagnosis, samples need to be grown in culture. Please contact your state’s Cooperative Extension Plant Diagnostic Clinic for guidance. You can find your state’s clinic through the National Plant Diagnostic Network website www.npdn.org. Do not move infected tree limbs for use as firewood.
Resistant Cultivars
If you live in a region where laurel wilt is present, planting any members of the laurel family should be avoided. Researchers are working on finding resistant redbay and swamp bay, but it will be a few years before they are available.
Chemical Management
There are no curative treatments available for trees already infected. There is a fungicide that can be injected into the stem to protect the tree from the disease. The treatment has been found to be quite successful if the treatment is done at the right time. This is a restricted use pesticide available only to professionals with a pesticide license.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.