The commonly occurring shoestring root rot fungus (Armillaria mellea) has been associated with oak mortality species (Wargo 1977) and has been implicated as the primary causal agent (D. J. Lodge, pers. comm.) in mortality observed in the southeast during the 1980s. Nonetheless, there is considerable speculation about whether primary or secondary causes of mortality can be assigned to a single vector (Wargo 1977). The effectiveness of the fungus in causing or contributing to mortality is related to an individual tree's condition, its degree of stress because of low moisture availability (Staley 1965), defoliation (Wargo 1977), or the presence of stem borers (Agrilus bilineatus Weber; Dunbar and Stephens 1975). To more efficiently support respiration and other metabolic processes during prolonged periods of severe moisture stress, carbohydrates stored in the root systems as starch are converted into simple sugars (Wargo 1977 and 1996). The fungus is better able to use simple sugars than complex starches and therefore depletes stored energy in roots much more quickly. The added stress on tree physiology often results in mortality or predisposes the individual to mortality during subsequent periods of stress.
By contrast, other species are susceptible to vectors whose optimum influence comes under much different conditions. Fungal pathogens tend to be more virulent under the cool, moist conditions characteristic of periods of abundant rainfall. For example, Cornus florida (flowering dogwood) has been under attack by the an-thracnose fungus (Discula destructiva) since it was first observed in the northeastern United States in 1977 (Daughtrey and Hibben 1983), and at some sites the species is in serious decline. In 1992, the fungus was found in 144 southeastern U.S. counties, particularly in the southern Appalachians and the foothills of the Carolinas and Georgia. Some areas above 900 m in elevation have 100% mortality (Hofacker et al. 1992). The fungus attacks the dogwood's leaves, effectively severing communication between the leaf and the branch and disrupting the exchange of essential metabolites. Mortality results when the fungus moves from the leaves to the shoots and into the main stem, where stem cankers coalesce and girdle the tree. Chellemi et al. (1992) found that the disease was more prevalent and active on cooler, moister northeast-facing slopes than on drier southwest-facing slopes. Similarly, B. D. Clinton et al. (unpub. data, 1999) found that dogwood mortality rates were highest on north-facing slopes but were also higher during a wet period (1989-1998) than during a dry period (1983-1988), whereas oak mortality was higher during the dry period. Hence, in the dogwood example, drought has the effect of mitigating against mortality.
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