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Ann. For. Sci.
Volume 53, Number 2-3, 1996
Page(s) 359 - 368
Ann. For. Sci. 53 (1996) 359-368
DOI: 10.1051/forest:19960218

Consequences of environmental stress on oak: predisposition to pathogens

PM Wargo

USDA Forest Service, Northeastern Center for Forest Health Research, 51 Mill Pond Road, Hamden, CT 06514, USA

Abstract - Stress alone, if severe and prolonged, can result in tree mortality. However, stress events usually are neither severe nor frequent enough to cause mortality directly. Mortality of stressed trees results usually from lethal attacks by opportunistic pathogenic organisms that successfully invade and colonize stress-weakened trees. Oak trees are predisposed to these organisms by defoliation, primarily from insects, but also by fungi and late spring frosts, and by drought. There is some evidence that injury from extreme winter temperature fluctuations also can act as a predisposing stress. Stress causes physical, physiological, and chemical changes that reduce energy available for trees to defend themselves, provide energy to pathogens for rapid growth, or make the tree more attractive to organisms that, through multiple attacks, overwhelm the ability of a tree to defend itself from attack. Fungal organisms, such as Armillaria spp in the root system, Hypoxylon spp on the bole, and a number of fungi that invade branch systems, and insect borers, such as Agrilus spp, take advantage of changes induced by stress and successfully attack and kill trees. These organisms may be secondary in the sequence of events, but are of primary importance in causing mortality.

Résumé - Conséquences de contraintes de l'environnement sur le chêne : prédisposition aux pathogènes. Un stress seul, s'il est suffisamment intense et prolongé, peut induire la mort d'un arbre. Cependant, dans la plupart des cas, les périodes de contraintes ne sont ni assez sévères, ni assez fréquentes pour causer directement une mortalité. Cette dernière résulte généralement d'attaques létales par des organismes pathogènes opportunistes, qui envahissent et colonisent avec succès des arbres affaiblis. Les chênes sont prédisposés à de telles attaques par des défoliations, dues primairement à des insectes, mais aussi à des champignons ou des gelées tardives, et par la sécheresse. Les contraintes provoquent des modifications physiques, physiologiques et chimiques qui réduisent la disponibilité en énergie pour assurer une défense efficace, mettent à la disposition des pathogènes des ressources permettant une croissance rapide, ou rendent l'arbre plus attractif pour des organismes qui, par le biais d'attaques multiples, submergent ses possibilités de défense. Des champignons comme les armillaires dans le système racinaire, Hypoxylon spp dans l'écorce, un certain nombre de champignons qui envahissent les branches, d'insectes mineurs comme Agrilus spp peuvent profiter de ces modifications induites par les contraintes et envahir, voire tuer, les arbres. Ces organismes sont sans doute secondaires dans la chronologie des événements, mais probablement de première importance comme cause de mortalité.

Key words: decline / stress / secondary pathogen / Quercus

Mots clés : dépérissement / stress / pathogènes secondaires / Quercus

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