Invasion of wood by basidiomycetous fungi. V. The role of fungal elicitors of plant cell necrobiosis in the expression of pathovirulence and kratovirulence

Within the intact tree the live cell is the exclusive carrier of phytopathological defence reactions against microbial attack. In a comprehensive search for fungal virulence factors capable of weakening the living tissues in timber, 48 species of wood‐destroying basidiomycetous fungi (WBF) 1 ) with known degree of pathosism and the corresponding degree of saprophytism were grown on liquid still cultures. Droplets of the filtered culture fluids were then applied to pin‐prick wounds in the interveinal area of leaves from several tree and one herb species. Within 48 hours of development circular spots of brown necrotic tissue expanded around the pin pricks to indicate and approximately quantify the presence of fungal elicitors of necrobiosis. The resulting necrobiosis index was, with certain exceptions, not correlated with the antimicrobial potency (kratovirulence) of the fungus, its pathosism against standing timber, and the activity of exoenzymes in white‐rot fungi. The necrobiosis index was, in part, positively correlated with the activity of those brown‐rot fungi that dispose of an effective wood‐decay capacity. Characteristic of the fungal species as well as of the fungal strain the necrobiosis index ranked from 0 to 1,421.2 mm 2 necrotic net area per 300 pin pricks but there was no fungus to be unable of eliciting browning in at least a few of the leaf species. The most severe necrosis was permanently observed on leaves of several generally oversusceptible tree species rather than on the notorious host species of the test fungus. It is therefore concluded that the elicitors of plant cell necrobiosis in WBF are predominantly host‐nonspecific stress factors that may sometimes increase fungal pathovirulence. Their purification should help characterize their action as phytotoxic, necrobiotic, enzymatic, or even antimicrobial.