Morphological, physiological and genetic diversity within a small population of Cercospora beticola Sacc.

A wide variability was observed among 10 Cercospora beticola Sacc. field isolates collected from a single lesion. The level of genetic diversity in this small population was surprisingly high in relation to the homogeneous origin of the isolates and to the reproduction of this fungus, which is reported to be strictly asexual. Significant differences in growth, morphology, phytotoxin production, pathogenicity, RAPD, and DAMD patterns were found but no grouping based on these characteristics was possible. The likelihood of tetraconazole resistance development was examined in this population, with or without u.v. mutagenesis, and by adaptation. Strains with high level of resistance were easily obtained from the wild type (WT) population by mutagenesis while the simple screening gave rise to a group of less resistant strains, all coming from the same WT parental strain. Differences among strains have also been observed in their adaptation to increasing doses of tetraconazole. Positively correlated cross-resistance was demonstrated in all the strains between tetraconazole and some other demethylation inhibiting fungicides (DMIs). A negative correlation was found between resistance and growth rate. No correlation occurred between resistance and pathogenicity but the average pathogenicity of mutagenised strains was significantly lower than the averages of the other two sensitivity groups.