After-effects of Drought Did Not Predispose Young Picea abies to Infection by the Bark Beetle-transmitted Blue-stain Fungus Ophiostoma polonicum

The defence of Norway spruce against a combined attack of the bark beetle Ips typographus and its associated blue-stain fungi is based upon a) constitutive resin stored in ducts of the bark and sapwood, and b) induced resinosis in reaction zones surrounding the point of infection. Empirically, beetle epidemics are associated with external stresses, drought being a particularly prominent factor. An attempt was made to mechanistically explain the apparent link between drought and infestations, through studying after-effects of drought on the induced defence in stressed trees. In the field, 3-5.5 m tall trees were exposed to artificial drought over three growth seasons to investigate whether this treatment would predispose them to fungal infection in a fourth season when drought was absent. Pre-dawn xylem water potentials down to -1.85 MPa and a considerable foliage depletion were recorded. In the fourth season, the trees were inoculated with Ophiostoma polonicum, a pathogenic associate of Ips typographus. No difference in susceptibility could be seen between drought stressed trees and unstressed controls. Carbohydrate concentrations of foliage and branch bark were slightly enhanced and mineral nutrient concentrations strongly reduced in stressed trees.