Fungal disease taking out large number of oaks, other hardwoods

Published 12:00 am Sunday, November 9, 2008

We have lost quite a large number of oak trees and other hardwood tree species the last few years. This summer has been no exception. Quite often, we have clients contact the Extension office questioning the demise of their oak, maple or hickory. What is the likely culprit?

I’ve talked with one of our Extension plant pathologists about this phenomenon and she believes the main cause is a fungal disease called Biscogniauxia dieback. That is actually a new name for a disease which used to be called Hypoxylon dieback, although I suspect few folks are familiar with either name.

The disease, caused by either of two fungi, Biscogniauxia atropunctata or Biscogniauxia mediterranea, is found throughout the United States and is especially common in the South. Unfortunately, all oaks are susceptible to this disease, and the most frequently and severely affected oaks include black, blackjack, laurel, live, post, Southern red, water and white oaks. Other tree species such as maple, hickory, beech, sycamore, basswood and hornbeam also can be infected. We have even had some 65-year-old walnut trees die unexpectedly and, although I have not been able to confirm they are infected with Biscogniauxia dieback, I wouldn’t be surprised if walnut is susceptible, as well.

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Perhaps the most important and noteworthy characteristic of Biscogniauxia dieback is that it is a disease of stressed trees. The fungus enters the tree through wounds and natural openings in the bark. In healthy trees, the fungus survives in small colonies in the bark and sapwood and is kept in check by the tree’s natural defenses. Stresses such as drought, heat, wounds and root damage reduce the tree’s defenses and give the fungus an advantage. The fungus is favored by warm, dry conditions. In Mississippi, outbreaks of this disease often are seen a year or two following a significant drought. For us, the last four years have been unusually dry, so that could definitely explain trees dying the last few summers. 

The fungus disrupts the flow of water and nutrients through a tree by destroying the plant’s nutrient-conducting tissues called the sapwood. Symptoms mimic general water stress and include the following:

* Smaller than normal leaves that make the crown of the tree look thin

* Dead branches

* Yellowing or wilting leaves

* Brown sapwood

Because the symptoms are so generic, diagnosis relies on visible signs of the fungus. The fungus forms a cushion-like mat, called a stroma, between the wood and the bark of the tree. Pressure from this cushion causes the bark to peel off in patches or long strips.

No chemical treatment is registered, so management centers on preventing the disease and getting rid of infected trees quickly. Options include the following:

* Avoid wounding trees

* Fertilize trees properly and water during hot, dry weather

* Prevent or relieve compacted soil

* Prune out dead limbs

* Severely infected trees should be cut and burned. Ideally, the stumps should be destroyed, as well.

Reminders:  Ethanol Co-Products Conference for cattle producers, Central Mississippi Research and Extension Center in Raymond, Thursday at 8:30 a.m.

John C. Coccaro is county Extension director. Write to him at 1100-C Grove St., Vicksburg, MS 39180 or call 601-636-5442. E-mail him at