Major Southern Forest Diseases

A variety of forest health issues, concerns, and events have rapidly changed southern forests and plantations in the past two decades. These factors have strongly impacted the ways we manage forest pests in the Southern United States. This trend will no doubt continue to shape forest pest management in the future. The major issues and events of concern include changing forest conditions, urbanization, multiresource issues, increased harvesting, forest fragmentation, expanding human populations, pesticide bans, expansions of native and nonnative pests into new regions, emergence of new damaging insect-disease complexes, and reduced resources to manage these problems. The effects of some of these factors on forest health priorities and specific pest-suppression practices are discussed in relation to some major hardwood and conifer diseases in southern forests. The ways in which these pests are influencing southern forest management priorities and practices and the progress that past and present pest-suppression research has made toward solving some of these pest-suppression problems also are discussed. INTRODUCTION The preceding two decades have brought a barrage of new developments that are shaping the evolution of forest management practices with regard to forest health issues and disease suppression in southern forests. Some of the more important developments impacting forest health management in the South include: (1) legislative bans on the use of many pesticides and chemical controls formerly used to manage forest pests; (2) continued introductions of nonnative pests to which many of our endemic tree species have little resistance; (3) expansion in distributions and outbreaks of important native pests into previously unaffected areas; (4) occurrence of new synergistic forest pest complexes previously unrecognized as important to forest management decisions; (5) nationwide reductions in the research work force (forest pathologists) available to study and develop new pest suppression technologies; (6) drastic reductions in forest management and pest-suppression activities on Federal lands; (7) inadequate approaches to regional pest problems as a result of overemphasis of theoretical research approaches, e.g., modeling systems and disease forecasting, instead of improvements in direct, applied approaches to disease suppression; and (8) the existence of new emerging endemic diseases such as bacterial leaf scorch (Xylella fastidiosa Wells and others) that are causing widespread damage previously unrecorded in commercially important fiberand lumberproducing tree species (Billings 2000, Britton and others 1998). The impact of these issues and events on disease suppression and forest management decisions in general will be treated in the following discussions relevant to individual major hardwood and conifer diseases that occur in the southern region. 1 Principal Research Pathologist, Research Plant Pathologist, and Principal Research Entomologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Stoneville, MS, 38776; and Supervisory Research Pathologist and Principal Research Pathologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Athens, GA 30602, respectively. So ut he rn F or es t Sc ie nc e: Pa st , P re se nt , a nd F ut ur e Fo re st H ea lt h 162 MAJOR HARDWOOD DISEASES Oak Wilt in Urban Forests U rban forests are becoming increasingly important components to be considered in the development of forest management objectives as cities and municipalities continue to encroach on natural forest stands. Protection of tree resources in urban areas is becoming more important, not only because urban trees have commercial lumber value or provide habitat and food for wildlife, but because their aesthetic value contributes significantly to property values. A good example of this trend has been demonstrated by the impact of oak wilt, caused by Ceratocystis fagacearum (T.W. Bretz) J. Hunt, on urban forestry. Within the last 20 years, oak wilt has caused increasingly devastating losses to valuable urban and suburban trees within and near metropolitan areas of Texas in the South and within major cities in Minnesota, Wisconsin, Iowa, and Illinois in the Midwest (Wilson 2001). 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 (fig. 16.1). 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. novoulmi 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, Figure 16.1—Live oak injured by heavy tree-clearing equipment at a residential building site in Austin, TX, providing entry points (infection courts) for introduction of the oak wilt fungus into the living sapwood by insect vectors. Photo by A. Dan Wilson.