Take steps to protect your avocado trees from laurel wilt | Sally Scalera
Avocado trees are a popular fruit tree grown in Brevard County. There are many cultivars available that range in hardiness, the months when they ripen and the size of their fruit. If you are growing avocados now, or you would like to in the future, you need to be aware of laurel wilt and how to keep your tree healthy.
Laurel wilt is a fungal disease carried by the redbay ambrosia beetle. This disease infects trees in the laurel family (Lauraceae). This non-native pest was first discovered in Georgia in 2002.
After its first appearance in Florida’s Duval County in 2005, it spread to the surrounding counties and then jumped three counties south, where it was discovered In the Sebastian area of Indian River County in 2006.
The logical reason for this large “jump” to an area so far south indicates that it occurred because infested wood, most likely firewood, was brought down from infested areas. The current map of counties with laurel wilt disease, as of 2016, shows 64 of 67 counties have laurel wilt.
The original infestation in Georgia killed native redbay trees (Persea borbonia).
The ambrosia beetle has been found to kill other trees in the laurel family, such as avocado (P. americana), swamp bay (P. palustris), Pondberry (Lindera melissifolia), Pondspice (Litsea aestivalis), sassafras (Sassafras albidum) and even camphor trees (Cinnamomum camphora), which isn’t a bad thing since that tree is an invasive exotic.
Typically, the idea of a beetle boring into a tree would be the expected cause of a tree’s death, but in this case, it isn’t. The Redbay ambrosia beetle carries fungal spores on its body of Laurel wilt (Raffaelea lauricola), which it grows in the plants' sapwood for food. The fungus plugs the flow of water in the xylem, causing leaves to wilt.
We have at least 30 species of ambrosia beetles in Florida, several of which are non-native. All ambrosia beetles carry fungal spores with which they have a mutually beneficial relationship. Therefore, ambrosia beetles can be regarded as “fungus farmers.”
The presence of ambrosia beetles, without the fungus, is unlikely to kill trees. Research suggests that some avocado varieties are less susceptible to laurel wilt than others.
The first visible symptom of laurel wilt is the appearance of drooping leaves that turn off color and, depending on the species affected, could be grayish, reddish, or purplish. The wilted foliage typically occurs on one branch or just a section of the canopy initially.
The foliage on the affected section of the tree eventually turns brown but remains on the tree for a year or more, though avocado trees may shed the brown leaves relatively soon after they wilt.
If laurel wilt is suspected, a section of the bark can be removed to check for black to brown discoloration in the sapwood. The discoloration will appear as streaks that run parallel to the grain of the wood. The longer the tree has been infected, the longer the streaks will be.
In advanced stages and during dry weather, small strings of compacted sawdust may be found protruding from the boreholes along the trunk and limbs. They disintegrate easily, so their absence is not necessarily a good sign if the foliage is wilting in a portion of the canopy.
Since its arrival in the state, it has been recognized that several species of ambrosia beetles transmit laurel wilt to avocado trees, and new research at the University of Florida has discovered that the fungus, Raffaelea lauricola, grows faster in the fall and winter than in the summer. The best growth occurs between 68 and 79 degrees, the fungus barely grows between 86 and 89 degrees.
If you have an avocado tree, follow these suggestions to help your tree stay healthy and not become a susceptible host for the laurel wilt fungus:
Test the soil to make sure the tree isn’t suffering from deficiencies in the soil, especially potassium and magnesium. The soil testing form can be downloaded at edis.ifas.ufl.edu/publication/SS187. Be sure to pay for the $10 Test B so all the nutrient levels are tested, and use crop code 602 for woody ornamentals or trees in the landscape.
Establish the soil food web around the root system of your tree and all plants throughout your yard. This step is easy, so email the UF/IFAS Extension Brevard County Master Gardeners at [email protected] for the information.
To produce nutritious avocados and support a healthy, thriving soil food web, re-mineralize the soil with rock powder (i.e., Azomite, etc.). With the presence of the soil food web, which will get the nutrients the plant requests, rock powder won’t need to be continuously applied.
Spray the foliage with liquid seaweed or kelp. This is a simple way to provide trace elements and growth hormones directly to the foliage of the tree for translocation throughout the entire plant. Spraying weekly will provide the most benefit.
Add aspirin to the liquid seaweed solution and spray them on the foliage together. Research at the University of Rhode Island found that spraying a plant's foliage with salicylic acid from aspirin results in Systemic Acquired Resistance in the plant, which can help the plant protect itself from insects and diseases. In the research, ? of aspirin was dissolved in water and sprayed on the foliage of plants every two to three weeks. It may be easier just to dissolve 3 chewable baby aspirin in the same gallon of water with the liquid seaweed and spray them together.
If any of these suggestions are new, begin implementing them now. They are easy to do, and once the soil food web is established, your yard will do much better with less effort from you. Begin helping your avocado trees now, because the cooler temperatures of winter are when the laurel wilt fungus thrives. The suggestions above will also help other fruit trees and vegetable plants.
Sally Scalera is an urban horticulture agent and master gardener coordinator for the University of Florida’s Institute of Food and Agriculture Sciences. Email her at [email protected].
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This article originally appeared on Florida Today: Here's how to prevent laurel wilt in your Florida avocado trees
