Induction of disease resistance in Norway spruce (Picea abies) by necrotizing fungi

Norway spruce trees (Picea abies) preinoculated with the root rot fungus He terobasidion annosum, Nectria fuckeliana or a pathogenic strain of the blue -stain fungus Ceratocystis polonica were more efficiently protected against a subsequent massive inoculation with pathogenic C. polonica than trees pr etreated with nonpathogenic C. polonica or sterile malt agar. Control trees that received no pretreatment were extensively colonized by the mass inocu lation. There was a strong negative correlation between the length of the p hloem necroses induced by the pretreatment inoculations and the extent of h ost symptoms caused by mass inoculation with pathogenic C. polonica. The de gree of induced resistance in Norway spruce thus depended on the amount of host tissue destroyed by the pretreatment.