In early 2002, somewhere near the meandering Savannah River, Georgia, a tiny female ambrosia beetle emerged from a wooden packing crate and flew inland towards a small stand of native redbay trees and began to drill. Two decades later and that tiny beetle introduction has caused us to lose well over half a billion native trees and over 150,000 avocado trees to the devastating disease laurel wilt.
Laurel wilt kills through a combination of an introduced pest, the redbay ambrosia beetle, an introduced fungus, Raffaelea lauricola, and a severe overreaction to the presence of the fungus by host plants. Host plants such as the avocado, a beloved south Florida fixture, and the previously indestructible native plants swampbay and redbay are all members of the laurel family. The $54 million dollar Miami-Dade avocado industry has already lost over 150,000 commercial avocado trees due to laurel wilt or disease related removals. Native trees have suffered even greater losses.
The fungus itself is not deadly to the infected trees, nor is the vector. Trees die by overreacting to the presence of the fungus once a female ambrosia beetle drills into a suitable tree and begins to grow the fungus to feed herself and her young. The tree overreacts and tries to wall off the fungus to protect itself. The tree does this so quickly and efficiently, that it effectively cuts off its own water and nutrient supply causing its leaves to rapidly wilt, turn brown, and hang on the tree before dying.
The redbay ambrosia beetle brought laurel wilt to south Florida, but once it got here, other ambrosia beetles that were already here also began to farm the same fungus. A few of those beetles are the ones thought to be causing avocado damage, not the redbay ambrosia beetle which prefers natural areas. The disease can be spread by ambrosia beetles, but also through root grafting. In an avocado grove, the disease will often travel right down a row tree by tree because the roots of older trees have fused together and can pass the fungus from one tree to the next.
Signs of Laurel Wilt
-Leaves that stay on your avocado, turn brown, and do not drop
-Beetle holes in the trunk, or sawdust tubes on the trunk (sawdust tubes will be washed away after a heavy rain)
-The tree dies in sections
-The wood under the bark has a dark coloration
Not Laurel Wilt
-Avocado trees that drop a lot of leaves (this is a normal process that usually happens when the trees bloom and is not a cause for concern)
-If a tree is in general decline from poor horticulture or circumstances it will be weak and slowly die, but will not hold its leaves
-A section of a tree turns brown and holds its leaves, but the disease does not progress to another area (this is typically a lightning strike)
-Other tropical fruit trees will not be affected by laurel wilt
How to Manage Laurel Wilt
If you are a commercial avocado grove owner, there is a protocol developed by UF/IFAS that can be found here. The most important thing grove owners can do is to consistently scout their groves and to quickly rogue any laurel wilt affected tree before the disease can spread through the roots to its neighbor. Lateral root transmission is much more efficient and quicker than beetle transfer.
If you are a homeowner, your best bet is to keep your tree healthy through proper horticulture and to make sure not to bring in any wood to your property that may be in the laurel family as that wood may harbor beetles. Remember that any large, dead tree on your property is a hazard, so a dead tree would need to be safely removed by a certified arborist.
Commercial growers and homeowners should keep their trees pruned to let in maximum light. Studies have shown that the ambrosia beetles that vector the disease prefer shade to sunlight.
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