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Beware of Oak Wilt

by Barbara Porsch, Permian Basin Master Gardener, Texas Master Gardener Oak Wilt Specialist

Drive around in some areas of Midland and Odessa and mourn the dead and dying oak trees. There is Oak Wilt in the Permian Basin, and it is a serious problem despite thoughts to the contrary.

Oak Wilt is one of the most destructive tree diseases in the United States. It is caused by a fungus which invades and disables the water conducting system of oak trees and greatly reduces the flow of water up the stem of the tree. Eventually, as more of the vessels become clogged, the trees will begin to wilt and most often die.

Red oaks are extremely susceptible and typically die very quickly, often within two to four weeks following the initial appearance of symptoms. Red oaks also play a strong role in the establishment of new oak wilt infections.

Infected red oaks can have fungal spore mats which produce a sweet fruity odor which attracts insects (Netidulid or bark beetles) and as they crawl across these mats, the sticky spores adhere to them and can be transported over long distances to healthy trees. The combination of this fungus, an abundance of active insect vectors and wounded trees due to improper pruning promotes the spread of oak wilt. This danger is the reason the Texas Forest Service and Texas A&M University Research strongly recommend that all fresh pruning cuts on oak trees be painted immediately after being cut.

Live oaks may defoliate and die in a one to six month period. However, approximately 10 percent of the infected live oak population may survive for many years in various states of decline and never fully recover. In live oaks the disease may also be transmitted by interconnected roots. It is for this reason that in central Texas entire neighborhoods and rural landscapes have been devastated. It is not unreasonable to assume that every live oak tree in a city block has interconnected roots.

White oaks (bur oak, post oak, Lacey oak and Chinkapin oak for instance) are resistant to the fungus and rarely die from oak wilt.

During the summer months, diseased red oaks often can be spotted because of their bright autumn like coloration. The most reliable diagnosis for live oak is a symptom called veinal necrosis. The leaves develop yellow veins that eventually turn brown.

To manage oak wilt we must prevent new infection centers by eliminating diseased red oaks and handling the resulting cut wood properly. We must prune properly and paint all wounds on healthy oak trees. The least hazardous times for pruning are during the coldest days of the winter and the hottest days of the mid to late summer. Pruning should be avoided from February through June. Regardless of timing, all pruning cuts or other damage to oak trees should be treated immediately with a pruning paint or latex paint. It takes only minutes for an open wound to attract insects, so waiting to paint until all pruning is accomplished is unacceptable. One must cut and paint, cut and paint immediately.

Trenching can be done to disrupt transmission of the disease by root connections. Mostly used in a rural landscape the most common technique is to trench at least four feet deep with trenching machines or rock saws.

There is a fungicide available for injection as a preventative treatment. Only limited success is reported in this procedure which should be done only by trained applicators.

Loss of our valuable trees is not only aesthetic, but is a great economical loss as far as property value is concerned. We all need to take great care with our oak trees to prevent this disease from spreading.

Come learn more about oak wilt and the selection and care of trees in the Permian Basin by attending either of two identical educational seminars presented by Horticulture Agent Jeff Floyd and the Permian Basin Master Gardeners. The classes will be held on Tuesday January 19 at 6:30 p.m. and Wednesday January 20 at 9:30 a.m. Both will be at the George W Bush Commemorative Hangar at the CAF. For more information call Texas A&M AgriLife Extension Service at 498-4071.



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