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5.2 Native Diseases of Hardwoods

5.2.1 Oak Wilt

Oak wilt is a vascular wilt disease of oaks that currently is found only in North America. The causal fungus (Ceratocystis fagacearum) was first identified in Wisconsin in 1942, but scientists believe the disease is native to North America and was present long before its discovery (MacDonald 1995, Tainter and Baker 1996). Oak wilt is known to occur in 21 States in the Central and Eastern United States (Rexrode and Brown 1983); 9 of the 13 Southern States are known to harbor the disease, but severe mortality is occurring only in central Texas (Figure 6).

Oak wilt causes affected trees to wilt and usually to die. All species of oak are susceptible, but species in the red oak group (northern red, scarlet, and black oak) are most readily killed. Oaks in the white oak group (white, post, and chestnut oaks) are infected but mortality occurs much less frequently and more slowly. Live oaks die at a rate generally intermediate between red and white oaks.

Infection centers develop when the fungus spreads to adjacent, susceptible trees via root grafts. Sap feeding beetles can carry spores to nearby healthy trees. Control strategies consist of cutting or killing infected trees and others nearby to prevent tree-to-tree spread (MacDonald 1995, Rexrode and Brown 1983, Tainter and Baker 1996).

Oak wilt control programs were implemented in a number of Eastern States in the 1960s and 1970s, but devastation of oaks never developed as originally feared. Evaluations of control programs seem to indicate that efforts had little effect on the number of infection centers or the number of oaks that died, and most control programs have been discontinued.

In central Texas, however, catastrophic losses, primarily in live oaks with lesser loss of Texas red oak, have generated much interest and concern since the 1980s (Appel and Billings 1995). Oaks in this area have little commercial value, but they are highly prized for shade, aesthetics, wildlife, and their contribution to watershed health. Both rural and urban trees are affected. An active control program has been in operation since 1988 (Cameron and Billings 1995). Control treatments successfully implemented in central Texas include trenching to sever root connections and fungicide injections to prevent mortality of individual, high-value trees.

Concern over the importation of oak wilt to Europe has resulted in an import quarantine being imposed by the European Economic Community countries on oak logs from United States counties where oak wilt has been documented. Oak logs exported from such counties must be fumigated and then be certified disease free.

Oak wilt will continue to affect the oak resource in its current range. Of greater concern is the possibility that the oak wilt fungus, having adapted to Texas oaks and their environment, may now spread throughout the Southern range of oak.

5.2.2 Oak Decline

Because of the history of woods grazing, widespread wildfire, exploitive logging for wood products, and the loss of American chestnut to chestnut blight, oaks probably represent a larger component of the southern forest ecosystem today than at any time in the past (Millers and others 1990).

Oak decline in upland hardwood and mixed oak-pine forests is a disease complex involving environmental stressors, often drought, root diseases such as are caused by Armillaria spp., insect pests of opportunity such as the two-lined chestnut borer, introduced pests such as the Japanese beetle and Asiatic oak weevil, and physiological maturity of the trees (Staley 1965, Wargo 1977, Wargo and others 1983). Bottomland oak forests are also subject to oak decline but at a lower incidence. Stress agents of bottomland hardwoods also include seasonal, sometimes prolonged flooding.

Decline progression is measured in decades rather than months or years. Introduction of the gypsy moth into northern parts of the region has worsened oak decline because oaks are preferred hosts, and spring defoliation contributes to the chain of events that increase susceptibility. While decline development may take decades from inception to visible symptom expression, susceptible trees die within a few years after dieback exceeds one-third of the crown volume. Not all affected trees reach this point. Species in the red oak group (particularly black and scarlet oaks) are most susceptible. Hickories are the only non-oak species group commonly observed with symptoms in decline areas (Starkey and others 1989).

Forest workers have reported oak decline occurrences since the mid 1800s (Beal 1926, Balch 1927) and in every decade since the 1950s (Millers and others 1990). A severe drought in the 1950s may have led to the current cohort of trees being highly susceptible to oak decline (Tainter and others 1990, Dwyer and others 1995). Significant oak decline episodes continue to occur in the region (primarily in Arkansas and Virginia) where predisposing conditions, inciting events, and contributing factors are coincident (Starkey and others 2000).

Not all oak forests are equally affected (Figure 5); Virginia, North Carolina, and Tennessee have the highest incidence. Among physiographic subregions, the Southern Appalachian and Ozark-Ouachita Mountains are most affected. Species in the red oak group suffer greater impacts than those in the white oak group (Gysel 1957, Oak and others 1988).

Although most of the decline-affected area is on privately owned land, national forests have by far the highest incidence of this problem because they have a higher frequency of stands with the attributes that favor this disease (older aged oaks predominate, oak species composition favoring susceptible species, and average to low site productivity [Oak and others 1991, 1996]). Among national forests, the George Washington and Jefferson have the highest incidence of oak decline.

The relative importance of oak is both a biological and a social question, but the cumulative impacts of the loss of American chestnut, continued oak decline, and ongoing defoliation by the gypsy moth indicate that special efforts must be made if the oaks are to maintain their prominence in the forest. Risk rating models have been developed to aid in this process (Oak and Croll 1995, Oak and others 1996, Oak and Courter 2000).

Oak decline will continue to be a forest health problem, particularly on national forest land. Oaks will not be eliminated from affected areas, but their numbers and diversity will be reduced. Red maple, blackgum, and other relatively shade tolerant species are likely to replace the oaks. As this change occurs, forest structure becomes more complex, the quantity of standing trees and woody debris increases, and overall susceptibility to oak decline and gypsy moth is reduced.

Subsequent decline in hard mast production is another serious impact of this process