CONSEQUENCES OF ENVIRONMENTAL-STRESS ON OAK - PREDISPOSITION TO PATHOGENS

Stress alone, if severe and prolonged, can result in tree mortality. H owever, stress events usually are neither severe nor frequent enough t o cause mortality directly. Mortality of stressed trees results usuall y from lethal attacks by opportunistic pathogenic organisms that succe ssfully invade and colonize stress-weakened trees. Oak trees are predi sposed to these organisms by defoliation, primarily from insects, but also by fungi and late spring frosts, and by drought. There is some ev idence that injury from extreme winter temperature fluctuations also c an act as a predisposing stress. Stress causes physical, physiological , and chemical changes that reduce energy available for trees to defen d themselves, provide energy to pathogens for rapid growth, or make th e tree more attractive to organisms that, through multiple attacks, ov erwhelm the ability of a tree to defend itself from attack. Fungal org anisms, such as Armillaria spp in the root system, Hypoxylon spp on th e bole, and a number of fungi that invade branch systems, and insect b orers, such as Agrilus spp, take advantage of changes induced by stres s and successfully attack and kill trees. These organisms may be secon dary in the sequence of events, but are of primary importance in causi ng mortality.