Inter-isolate variation for virulence in Plasmopara halstedii (sunflower downy mildew) from Hungary.
A total of 82 isolates of P. halstedii, collected from all production areas of Hungary between 1976 and 1993, were assessed for their virulence pattern on a standard set of sunflower differentials under greenhouse conditions. The isolates were classified into 6 pathogenic races each representing a particular virulence phenotype. From 1976 until 1988 all the isolates were virulent only on sunflowers possessing no known resistance genes, thus classified as race 1. There was an apparent shift in the virulence of the P. halstedii population collected after 1988. Six races (1, 2, 3, 4, 8 and 9) were identified among the 45 samples collected between 1989 and 1993, with races 1 and 3 predominant, at a frequency of 35% each. While the increase in race virulence is undoubtedly due to selection imposed by resistant hybrids, the origin of the new races is unknown. It is concluded that whether new races have arisen from the indigenous P. halstedii population, or whether they have been imported from abroad, can only be reliably determined by DNA techniques, such as fingerprinting.