EFFECT OF CROPPING HISTORY ON THE POPULATION-STRUCTURE OF TYPHULA-INCARNATA AND TYPHULA-ISHIKARIENSIS

Population structure of Typhula incarnata and Typhula ishikariensis, snow mold fungi, was determined by vegetative incompatibility on agar plates using isolates from 10 sites that differed in the extent of disturbance. Typhula incarnata populations were always diverse, with many different vegetative compatibility groups regardless of habitat differences, whereas diversity of T. ishikariensis populations differed according to habitat. Typhula ishikariensis populations in uncultivated lands were diverse; however, those from agricultural fields planted with perennial grasses became simpler with increasing number of years after the establishment of the host plant community. As a population, isolates from a new habitat were fast growers, less aggressive, and less competitive, and their sclerotia tended to germinate more rapidly than those from an old habitat. Where host plants were annual and the habitat was disturbed annually by tillage, population structure of T. ishikariensis was very simple. These results are discussed in terms of epidemiological differences between the pathogens and the effect of cultivation on population structure change.