VIRULENCE VARIATION AND DNA POLYMORPHISM IN SPHAEROTHECA-FULIGINEA, CAUSAL AGENT OF POWDERY MILDEW OF CUCURBITS

Strains of Sphaerotheca fuliginea, one of the causal agents of powdery mildew of cucurbits, were examined for differences in virulence, mati ng type and DNA polymorphism. The 28 strains were chosen to be diverse according to host and geographic origin. Characterization of virulenc e phenotypes was based on the expression of symptoms on 4 species of c ucurbits and 6 cultivars of melon. Two pathotypes, capable of attackin g either cucumber cv. 'Marketer' and melon cv. 'IranH' and squash cv. 'Diamant' or cucumber cv. 'Marketer' and melon cv. 'IranH' were observ ed. Tests on melon cultivars revealed 3 races. In tests of sexual comp atibility with reference strains, heterothallism was observed for all isolates. Frequency of the two mating types differed significantly in the population. DNA polymorphism was determined both by restriction fr agment length polymorphism (RFLP) of the ribosomal internal transcribe d spacers (ITS) and 5.8S DNA amplified by the polymerase chain reactio n and by random amplified polymorphic DNA (RAPD). For any one of the 1 1 restriction enzymes tested all strains presented an identical patter n of ITS RFLP. RAPD analysis, using 22 primers which provided reproduc ible patterns, revealed a relatively low degree of polymorphism. Furth ermore, cluster analysis based on RAPD data (152 markers) did not sepa rate groups within the species S. fuliginea. No association could be f ound between virulence, mating type, geographical and host origin and RAPD patterns. The lack of association between phenotypic and molecula r markers and the close fit to linkage equilibrium for the characters examined suggest that recombination may play a role in populations of S. fuliginea.