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 (see figure at left).
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.
Tree mortality in urban areas causes economic losses in several ways. Reductions in landscape aesthetics resulting from tree mortality can significantly lower property values. The death of a single large urban live oak in Texas can result in a loss of as much as $20,000 in property value (Dewers 1971). It is not uncommon for landowners in Austin, TX, to sell their property once oak wilt has been diagnosed on their land in order to avoid the investment loss associated with the reduction in property value. Losing valuable shade trees can substantially increase utility bills (cooling costs) for homeowners. Tree removal costs also can be significant when they involve large trees. Finally, replacement costs associated with replanting trees adds to the final expense of losing valuable landscape trees. The consequences of increases in oak wilt incidence in valuable urban trees have resulted in accelerated economic losses now estimated to have exceeded $1 billion over an area of at least 61 of 254 counties in Texas alone (Wilson 2001).
The rise in oak wilt incidence in urban areas has been attributed in part to increases in home construction and landscape improvement activities associated with urban development. Austin, TX, with over 10,000 live oaks (Quercus fusiformis Small and Q. virginiana Miller) killed by oak wilt in the last 20 years, may be the most heavily affected city in the United States. Residual trees often sustain considerable damage during initial tree clearing of land prior to home construction. Heavy equipment frequently scrapes and removes bark from trees, creating infection courts for the introduction of oak wilt inoculum by insect vectors.
Tree wounding also occurs when trees are pruned by landowners during times when insect vector activity is high. When such trees become infected, they initiate infection foci from which new oak wilt infection centers develop and spread by root-graft transmission. The storage of oak wilt and bark beetle-infested firewood in piles near residences provides both inoculum and insect vectors by which wounded trees may become infected. An increase in incidence of oak wilt in natural stands has also contributed to a higher incidence in urban areas. Oak wilt incidence increased in many natural oak stands during the first half of the 20th century in the Eastern United States when Dutch elm disease, caused by Ophiostoma ulmi (Buisman) Nannf. and O. novo-ulmi Brasier, and chestnut blight {Cryphonectria parasitica (Murrill) Barr [formerly Endothia parasitica (Murrill) Anderson & Anderson]} caused changes in stand composition by removing dominant species that were largely replaced by red oak species susceptible to oak wilt (Wilson 2001). The increased incidence of oak wilt in natural stands has since been closely linked to changes in forest management practices such as high-grade harvesting, preferential thinning, overgrazing, and fire suppression that favor reduced species diversity and increase the number of susceptible red oaks in stands.
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.
The combined effect of using the improved trenching methods (cultural control) with trench inserts to prevent root transmission of oak wilt, together with the increased effectiveness of soil-applied fungicide treatments, should significantly advance efforts to suppress oak wilt disease in semievergreen live oaks in Texas and in deciduous oak species affected by this malady in other States. If these controls are implemented, they could potentially save landowners hundreds of millions of dollars in tree removal costs and property value depreciations in Texas, and substantially greater savings in other areas of the United States affected by this disease.
Encyclopedia ID: p986
