Oak Decline and the Future of Missouri’s Forests

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Published on: Jul. 2, 2002

Last revision: Nov. 12, 2010

tree growth is reduced. Various diseases and insects attack these weakened trees. In time, the combination of stressors so weakens a tree that it dies.

In 1999, the USDA Forest Service estimated severe red oak decline and mortality was occurring on approximately 19,000 acres of the Ozark National Forest in northwest Arkansas. By June 2001, the estimated area of severe damage was about 300,000 acres. The numbers of red oak borer attacks, an indicator of the level of stress of trees, had increased to unprecedented levels of 300 to 500 attacks per tree, compared to a typical rate of less than 10 for healthy trees .

The level of oak decline and mortality in Missouri was not as severe in 2001 as that observed in Arkansas, but deteriorating conditions made oaks increasingly vulnerable here, too. In December 2001, the Forest Service estimated that more than 100,000 acres on the Salem and Potosi Ranger Districts were seriously affected by decline and wood borer damage.

Many red oak stands in southern Missouri and northern Arkansas were already stressed to their limit before the late 1990s, when two to three years of severe drought hastened their decline by making them more vulnerable to disease-causing fungi, such as Armillaria, and wood-boring insects, such as the red oak borer.

Disease and Insect Factors

The most common disease agents involved in oak decline are armillaria and hypoxylon fungi. The most common insects involved include the red oak borer, carpenterworm, and two-lined chestnut borer. All of these organisms are native to Missouri.

Armillaria is a fungus that causes white rot of woody roots. It is found wherever trees grow. Most of the time, Armillaria acts as a decomposer, decaying coarse, woody debris that has fallen to the forest floor. When trees become stressed or wounded, this fungus can also act as an aggressive parasite, attacking their root system. Infected roots no longer effectively take up water, particularly in drought-stressed soils. This causes a progressive branch die-back throughout the tree crown.

The red oak borer is a reddish brown beetle that is 7/8-inch to 1 3/8- inches long and has antennae as long as, or longer, than the rest of its body. It has a two-year life cycle. After mating, the adult female deposits her eggs in crevices in the bark of host trees, usually red oaks. The eggs hatch, and young larvae chew through the bark and into the tree where they will spend

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