Oak wilt was discussed in issue no. 9 of this newsletter. Recently, the Plant Clinic was asked to test some wood for the presence of oak wilt on a tree that had been injected with Alamo. The cultures were negative, as we suspected, but this brings up several points about working with oak wilt.
One option for oak wilt control is the use of fungicide injections to prevent disease development. Alamo (propiconizole) is registered for use on oaks. The label states that it is registered as a preventive or therapeutic treatment for oak wilt and that oaks exhibiting less than 20% crown loss from oak wilt have the best chance of responding to treatment by Alamo. Often, the oaks around an infected tree are the ones targeted for these injections because they have 20% or less crown loss.
This decision to use an injection is probably one that is made quickly because injections need to be made before the disease can spread. Cultures to prove the presence of the oak wilt fungus require 10 to 14 days of incubation in a lab, and applicators often do not have time to wait for culture results before injecting a tree. On the other hand, sampling a tree after it is injected is probably a useless task. Propiconizole is a sterol biosynthesis inhibitor (SBI). This means it inhibits the production of sterols in the oak wilt fungus. Sterols are necessary for growth of this fungus, so simply put, propiconizole inhibits fungal growth. If a wood sample submitted for oak wilt testing contains Alamo, then the fungus in that wood will not grow. The label suggests considering possible retreatment 12 to 36 months after the initial injection, so culturing from an injected tree is probably of little value for at least a year after the last injection.
If you want to know whether a tree is infected with the oak wilt fungus, sample the tree before it is injected. That is the bottom line.