Invasion of wood by basidiomycetous fungi I. Pathosism ) and saprophytism as determined by certain experimentally accessible virulence properties

Representative strains of 41 wood‐destroying basidiomycetous fungi (WBF) indigenous to living trees, fresh wood pieces, decayed wood and straw‐sawdust materials were followed up during their colonization of freshly cut trunk sections, twin panels cut from fresh and pre‐degraded wood blocks, and sawdust samples of fresh and pre‐degraded wood under nonsterile conditions. Transfer of any WBF mycelium to nonsterile substrates is controlled by the antagonism of the substrate's resident microflora. Increasing susceptibility to microbial contaminants goes along with a rise in pathosism of the fungus concerned. The conspicuous failure of the pathogenic as well as the virulent potency of saprophytic WBF to utilize contaminated substrates justified the delimitation of 7 homogenous fungal categories whose physiological characters are based on experimental results. They may cover the entire range of fungal habits that occurs in WBF. The classification was as follows: (i) Low‐activity pathogens, (ii) low‐competition pathogens, (iii) transferable pathogens, (iv) high‐competition pathogens, (v) fresh‐wood saprophytes, (vi) successive saprophytes, and (vii) waste‐substrate saprophytes. Rising with their row of figures, the members of the 7 fungal categories display a declining pathogenic potency, a growing resistance to succession of the inoculum as placed on nonsterile substrates, an increasingly undisturbed transmission of the mycelium from the inoculum to the subjacent wood surface, uninhibited growth of mycelial mats over unbarked wood surfaces, more uniform but not compartmentalized intrusion of mycelia into the substrate wood, decreasing host specificity, more lasting infections, utilization of pre‐degraded substrates, and the formation of soil mycelium. While most of these vital characters are absent in the lower fungal categories that comprise pathogens, they increasingly appear in fungi with saprophytic abilities. This major property that confers the “competitive ability of fungal mycelia to overcome, on entering a natural substrate, the fungistatic effect of the resident/microflora” is proposed to be termed kratovirulence (Greek: kratein, prevail, vorherrschen). The physiological basis of kratovirulence remains obscure. It is concluded that both kratovirulence as linked with saprophytism, and fungistasis exerted by the microbial substrate contamination are the major control mechanisms of mycelial invasion and durability in natural substrates. A threshold amount of mycelial kratovirulence is indispensable for the uniform invasion of harvested wood substrates that have lost their biological defence mechanisms thus being open to random contamination by xylophilous microorganisms. But when it is the kratovirulence which makes wood conveniently accessible to saprophytic WBF it cannot be explained yet that saprophytes are not the dominant organisms on living trees, too. Pathosism in WBF is then necessarily not only a simple evasion of fungi weak in kratovirulence onto near‐sterile substrates such as living trees, pathosism must have a substantial physiological basis that is lacking in saprophytes. Nevertheless, maintenance of infection courts of pathogenic WBF depends on the tree's ability to keep the multitude of deleterious microbial competitors off the discoloured trunk zone in which the WBF intends to advance. The effect of host specificity may at least in part be determined by the harmfulness to the WBF of microbial invaders the tree admits in the discoloured zone of advance but less by certain wood protectives or favourable wood constituents. The pathogenic members of WBF are assessed to be physiologically facultative due to their convenient growth in vitro but sometimes, when lacking kratovirulence, ecologically obligate by their binding to standing timber that actively keeps the competitors down.