INVASION OF WOOD BY BASIDIOMYCETOUS FUNGI .6. QUANTITATIVE BUT NOT QUALITATIVE DIFFERENCES IN THE PATHOVIRULENCE OF PATHOGENIC AND SAPROPHYTIC DECAY FUNGI

Wood dowels sterilely overgrown with test mycelia of pathogenic and saprophytic basidiomycetous wood-decay fungi, respectively, were inserted into drill holes in the stems of 3 hardwood (Fagus sylvatica, Betula verrucosa, Sorbus aucuparia) and 1 softwood tree species (Picea abies). The host trees 40 - 120 mm in diameter were part of the light-starved understory of a timber forest. Eighteen to 52 months after inoculation the trees were dissected and the colonized stemwood was microbiologically examined. Extension growth and survival of the test mycelia in trees of known vitality were taken as a basis for fungal pathovirulence rating. Basidiomycetous wood-decay fungi reach at best the status of physiologically facultative pathogens. They only colonized trees of the lowest vitality classes in the understory. For the expected range of pathovirulence, an appropriate host tree to be chosen had a proper range of vitality which expressed itself in a crown volume of 5 to 70% that of the crown of a free-standing tree and a width of the current annual growth ring from 0.5 to 2.5 mm in dependence of the tree species. Increasingly moribund trees with low crown volumes and narrow growth rings favoured an unlimited mycelial expansion and survival, while in vital trees fungal expansion gradually came to a standstill within 18 to 52 months, frequently combined with an active killing of both the pathogenic and saprophytic mycelia. Development of pathogenic test mycelia much more depended on the choice of an appropriate host tree species than did development of saprophytic test mycelia. The stem colonization by several late saprophytes was nearly as extensive and durable as the stem colonization by notorious tree pathogens. Pathosism in wood-decay basidiomycetes appeared thus to be a quantitative rather than a qualitative feature. For the dieback of the basidiomycetous test mycelia in the live stem reasons such as wood substrate depletion or antagonistic microbial activities could be excluded. The dieback rate of the test mycelia however was clearly enhanced in trees of increasing vitality, and it was enhanced with prolonged host-fungus interaction times. In comparison with sound dominant trees, the endophyte incidence in the sapwood of moribund P. abies trees was not increased. At the instant of tree death or crown decapitation however a severe colonization with blue stain and soft rot fungi was observed that exerted considerable antagonism on test mycelia low in kratovirulence. The implication of this antagonistic action for the colonization of dying trees by pathogenic decay fungi are discussed.