Other tree pests
In addition to all of the non-native, invasive tree pests challenging the health of Missouri’s trees, there are also several native insects and diseases that can cause serious harm. Although we cannot eradicate these pests from our natural ecosystems, it is good to know something about them and the best way to control them to limit their damage.
Oak wilt is a lethal disease of oaks, especially species in the red oak group. A fungus invades the tree, which causes it to die. In Missouri, the oak wilt fungus is spread primarily when sap-feeding beetles carry oak wilt spores to fresh wounds during the early part of the growing season. Once established in a tree, oak wilt can move though root grafts connecting nearby oaks.
The first symptom of oak wilt in red oaks is usually browning and wilting of leaves in the upper crown in early summer. (Photo by Joseph O’Brien, USDA Forest Service, bugwood.org.)
Wilted leaves display olive drab or light tan to bronze tissue starting at the margins and progressing toward the leaf base. (Photo by C.E. Seliskar, unknown affiliation, bugwood.org.)
Rapid defoliation of red oaks can occur within two to six weeks of initial infection, and death occurs within a year. White oaks might take years to die from the infection. Under ideal conditions, oak wilt fungal mats form under the bark of dead red oaks the spring following the tree's death. This causes cracks in the bark and emits a sweet, fermenting odor, which attracts sap-feeding insects that spread the fungus. (Photo by John N. Gibbs, Forestry Commission, bugwood.org.)
Hypoxylon canker is a common disease of hardwoods, especially species in the red oak group. It is often one of several factors ultimately responsible for tree death. A disease of the inner bark and sapwood, Hypoxylon damages tissues used by the tree to conduct water from soil to the leaves. Vigorous, healthy trees are colonized by the fungus, but only damaged or stressed trees develop cankers and die. Outbreaks of this disease tend to follow severe drought.
Initial symptoms of the disease include a noticeable thinning of the crown, small patches of brown leaves and branch dieback.
As the disease advances, large areas of bark fall off the branches and trunk, revealing a thick, dusty fungal mat with tan spores. (Photo by Ronald F. Billings, Texas Forest Service, bugwood.org.)
Eventually, the fungus turns silver or gray as it matures, giving the appearance of thick paint on the side of the tree. (Photo by Robert L. Anderson, USDA Forest Service, bugwood.org.)
Trees are weakened by environmental stresses such as drought, waterlogging, frost or by pests such as defoliating or sucking insects. Weakened trees are then invaded and killed by insects and diseases that cannot successfully attack healthy trees. Usually the progression of decline is slow, and it occurs over several years.
The initiating stress factors associated most frequently with oak decline are drought, frost injury or insect defoliation. Trees on ridge tops and in wet areas suffer most severely from drought. Frost often affects trees growing in valleys and frost pockets. Defoliated trees that refoliate the same season might exhibit dieback symptoms the next year. Other factors such as leaf diseases and soils that are waterlogged, compacted or shallow have occasionally been implicated in oak decline.
These stress factors often weaken trees so much that they succumb, sometimes suddenly, to the root killing and girdling actions of insects and diseases. The two major pests associated with oak decline are armillaria root rot and the two-lined chestnut borer. Periodic occurrences of decline and death of oaks over widespread areas have been recorded since 1900. These outbreaks, variously named oak decline, oak dieback or oak mortality, are caused by a complex interaction of environmental stresses and pests.
Trees affected by oak decline show a general and progressive dying back from the tips of the branches. Other symptoms include production of chlorotic, dwarfed and sparse foliage; development of sprouts on main branches and stem; and premature autumn leaf color and leaf drop. Often, growth is reduced before the appearance of symptoms. (Photo by Hank Stelzer, University of Missouri.)
Additional information on oak decline can be found at:
Rapid white oak mortality
Beginning in August 2011 and continuing through fall 2013, the Missouri Department of Conservation’s Forest Health staff received many reports of rapid white oak decline and mortality, often occurring within one growing season. In 2013, a majority of reports came from east central and southeast Missouri.
Seemingly healthy white oaks are experiencing rapid mortality and forest health experts have yet to identify the cause. (Photo by Robbie Doerhoff, Missouri Department of Conservation.)
Missouri counties where rapid white oak mortality has been reported.
This syndrome is different from other oak decline patterns reported in Missouri, which typically include red oak species on ridge tops and upper slopes of southwest-facing aspects and is often attributed to the combined effects of advanced tree age, high stem density and drought. In many recent white oak reports, pockets of white oak decline in mixed oak stands have been located on lower slopes. Other oak species are rarely affected.
Research is currently underway to better understand this mortality and begin to predict where and when it is likely to occur. Reports of locations with significant numbers of dying white oaks will help this research.
If you observe seemingly healthy white oaks dying and would like to fill out a survey, you can complete this survey and submit it to MU.