CLONAL DISPERSAL AND SPATIAL MIXING IN POPULATIONS OF THE PLANT-PATHOGENIC FUNGUS, SCLEROTINIA-SCLEROTIORUM

Two thousand seven hundred and forty-seven isolates of Sclerotinia scl erotiorum were sampled from four field populations of canola in wester n Canada. Each field was sampled in a grid of 128 50-m x 50-m quadrats plus four intensive quadrats each sampled in a diagonal transect. Sam pling was done at two phases of the disease cycle: (I) from ascospore inoculum on petals and (2) from disease lesions in stems. A total of 5 94 unique genotypes was identified by DNA fingerprinting. In each fiel d, a small group of clones represented the majority of the sample, wit h a large group of clones or genotypes sampled once or twice. Clone fr equencies were compared by chi(2) tests. The difference in profiles of clone frequencies for the two fields sampled in 1991 was not signific ant, but in 1992 the difference in profiles was marginally significant , indicating some local population substructure. The difference in pro files of clone frequencies for petals and lesions was not significant in each of the two fields sampled in 1991. In each of the two fields s ampled in 1992, however, the difference was highly significant, consis tent either with selection for some clones or with waves of immigratio n during the disease cycle. Nine of the 30 most frequently sampled clo nes from this study were previously recovered in a macrogeographical s ample from western Canada in 1990. For spatial analyses, randomization tests indicated no significant spatial aggregation of either clones o n petals or clones from lesions. Also, isolates of a clone on petals w ere not closer to isolates of the same clone from lesions than could b e predicted by chance. Both observations suggest spatial mixing of asc ospore inoculum from resident or immigrant sources.