The Plant Clinic has received an unusually high num-ber of oak samples over the last month. Most have been sent for oak wilt culturing. Although a few have been positive cases of oak wilt, the majority has been free of the oak wilt fungus. I do not believe there is a problem in sampling. Many of the negative cases have been sent on disposable ice packs as we request in hot months. Most are alive and thumb thick with at least questionable vascular streaking. In other words, sam-ples have been excellent. It appears that the negative oak wilt diagnosis has been reliable in most cases.
Symptoms reported on these oaks include branch tip dieback, flagging of entire branches, general tree decline, and an overall lack of vigor. Leaves are often brown, but in no particular pattern. We have had complaints on red, white, bur, and pin oaks.
Anthracnose has been found on some of these cases, but anthracnose is a wet-season disease that could not possibly continue to be a problem in the drought we have experienced. This oak problem did not start until midsummer. Anthracnose does not cause this much dieback.
It is possible that bacterial scorch could be involved in a few of these cases. Bacterial scorch causes symptoms in mid- to late summer and can cause scorching and tree decline. Usually bacterial scorch increases in severity over 2 or 3 years until the tree is dead. If you suspect bacterial scorch on your oak, read through the details about that disease in issue no. 13 of this newsletter.
We suspect that many of the stressed oaks may be suffering from weather stress related to root injury. The long period of wet weather early in the season, followed by high temperatures and drought stress of 6 weeks, has undoubtedly caused some root injury. Because such stress is not easily confirmed, be certain to inspect your oaks carefully and consider some of the problems listed here. Anthracnose is discussed in issue no. 5. Oak wilt is discussed in issue no. 11.