THE TREE DOCTOR (aka The Plant Doctor)

Field Diagnosis of Oak Wilt

There have been a substantial number of misconceptions and misinterpretations of oak wilt. Many oaks have suddenly died and individuals are perplexed with regard to the cause of death of their oaks. Submissions to diagnostic labs often result in negative tests for oak wilt and the interpretation is that oak wilt is not the cause of death. In several instances, the lack of confirmation of oak wilt resulted in extensive but unnecessary deaths of oak trees due to continued advance of oak wilt. On a good day, with highly expert plant diagnosticians analyzing excellent sample submissions, the oak wilt fungus may only be confirmed about 40% of the time from actual oak wilt affected trees. Unreliable lab tests are why I developed the following criteria for helping us to diagnose oak wilt. 

  Be Suspicious of Oak Wilt If All or Most of the Following Apply:

 1) Identity of the oak species as a member of the red oak family

2) Presently, wilting symptoms are evident in live trees

 3) Sudden death, generally on the order of months

 4) Recent pruning or storm damage (months to a year)

 5) Proximity to other oaks (root graft transmission from nearby dead oaks)

 6) Elimination of other causes of oak death: gas leaks, root injury, other diseases, etc.

If laboratory diagnosis is desired, consider the following recommendations: collect samples during the growing season from live symptomatic trees (dead trees are practically valueless for lab diagnosis unless the above mentioned pressure pads are evident); branch segments should be at least one inch in diameter; bole (trunk) samples, preferably with vascular staining, can be submitted-do not include bark; all samples should be kept cool and clean-the oak wilt fungus does not compete well with other fungi and bacteria, and submitted as soon as possible to a reputable diagnostic lab. Remember, a negative lab report for oak wilt does not guarantee that oak wilt is not present in the tree from which the sample was collected.