ASSESSMENT BY MEANS OF RAPD MARKERS OF VARIABILITY IN MYCOSPHAERELLA BRASSICICOLA RESPONSIBLE FOR THE RINGSPOT DISEASE OF CRUCIFERS
C. Lesur, C. Monot, D. Silué, F. Guidet, Y. Tirilly
The genetic variability of the fungus Mycosphaerella brassicicola was tested using RAPD markers. We analysed 28 isolates with 17 RAPD primers, generating 176 markers among which we identified 114 polymorphic bands. The cluster analysis revealed the existence of two groups of isolates, one heterogeneous (group II) and the other extremely homogeneous (group I). Most of the isolates in group I exhibited twice as many fragments as the isolates of the heterogeneous group. This difference in fragment number could be due to either diploidy, aneuploidy, heterocaryosis or DNA content. Twenty isolates were in group I thirteen of which had been isolated in Brittany in 1992. Some are probably identical. This suggests that the ringspot epidemic in 1992 was mostly due to group I isolates. Two isolates originating from Brittany behaved as intermediates of both groups. They had the sum of fragments found in both groups. This result suggests that sexual recombinants may occur in the homothallic fungus M. brassicicola. No relationships were observed between the genotypes of the isolates and their pathogenicity, host specificity or geographic origin.