For 7 cultures of basidiomycetous wood-decay fungi the decline of the following vitality marks were recorded over a 6-year period: (i) Consumption of sterilized beech wood substrate within a time unit as an indicator of the overall vigorousness of the fungus, (ii) decline of the competitive saprophytic ability (kratovirulence), which enables the fungus to colonize natural nonsterile substrates, (iii) capacity for sexual reproduction, (iv) several morphological features of the fungal colony. The reference strains kept growing at + 1 ... 2 "C on fertilized wood dust substrate retained in 5 of 7 cases all of their juvenility marks, although the wood-decay capacity gradually dropped back to 65 to 34% that of the initial value. The substrains maintained on liquid still cultures at 23 "C lost their vitality marks after a period of continuous decline rather abruptly. With the loss of kratovirulence and fruiting capacity the strains were no longer viable under field conditions, and with the drop of the sterile wood-decay capacity to less than 3% by dry weight the formation of aerial mycelium and melanin pigments virtually ceased. In several fungal species the colonies segregated into coexisting sectors of extreme senescence and comparative juvenility, and a spread of senescence to the juvenile sectors did not occur. It is concluded that stock cultures of wood-decay fungi are subject to progressive senescence even under optimum conditions of strain preservation. Their current vitality should be examined in a sterile wood decay test before they are used in biotechnology or research.In expensive screening programmes the most efficient microbial isolates for fermentative processes, plant resistance breeding, plant mycorrhiza establishment, biological control of plant pathogens, environmental pollution control, mushroom growing, and mycological timber refinement are elected. The creation of the most efficient isolate includes activities such as wild-type screenings, hybridizations, mutagenic treatment and even gene transfer techniques.The desired properties of certain microbial strains are nevertheless sometimes lost during long-term storage in stock culture as well as in service. In strains of the commercial mushroom, Agaricus bisporus (LGE.) IMBACH, a reduced fruiting capacity can occur (FRITSCHE 1969) which has no pathogenic roots. In other fungal species the capacity for both sexual and asexual reproduction terminate after several years of propagation in monoxenic culture (ASCHAN-ABERG 1960, ESSER 1973). In addition, the microbial cell can abate in its capacity to form antibiotics (PRAEVE 1959), enzymes (MASSOW and SCHMIDT 1973) and fungal cell pigments (ASCHAN- ABERG 1960, MOSER 1958 al. 1975) as well as in the potential to form mycorrhizas with plant roots ( MOSER 1958, MARX and DANIEL 1976, KROPP et al. 1987. Surprisingly, a common root of these effects of debilitation has never been established. Moreover, the mere possibility of the presence of debilitation symptoms in strains obtained from stock culture collection...
