Collections of the plant pathogenic fungus Mycosphaerella graminicola were made from the same field of wheat over a 3-year period. The field was planted with small plots containing four varieties of wheat grown in pure stand and in all possible two-, three- and four-way mixtures. In each year, the wheat field was recolonized by a local source of inoculum of unknown origin. Allele frequencies at 10 RFLP loci were compared at two different times within a growing season and over the 3-year period. No significant differences in allele frequencies were found for any of the RFLP loci over any of the time periods. DNA fingerprints were used to identify clones produced by asexual reproduction. Genotypic diversity based on the frequency of each clone was compared for each collection. No significant changes in genotypic diversity were found within a year or between years. Identical genotypes were found in the field at different times within a season, but no clones were conserved between years. No clone existed in a high frequency in any year, suggesting that selection for particular asexual lineages was weak. The founding population each year probably originated from wind-borne ascospores of the teleomorph, which may exist as an indigenous population on alternative hosts, such as Poa annua (annual bluegrass).
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