Occurrence, distribution, race identification and variability in metalaxyl sensitivity of Plasmopara helianthi isolates in central Italy

A four-year investigation was carried out from 2001 to 2004 in sunflower fields in central Italy to determine the incidence, distribution and race variability of Plasmopara helianthi isolates. Race characterization of isolates using the triplet coding system resulted in races 700 and 703 and does not reveal a great variability of P.helianthi population. Race 700 was recovered frequently (54% of isolates) from the Italian regions except Emilia-Romagna. Race 703 was not detected in Tuscany whereas both races were recovered in Umbria and The Marche. Our results confirmed previous investigations on the presence of restricted downy mildew attacks, generally with a low disease incidence (1-12%). Greenhouse trials showed a reduction sensitivity to metalaxyl of P.helianthi isolates artificially inoculated on the treated seeds of the cv. Ala. An insufficient seed dressing was also revealed by laboratory analyses in most of commercial hybrids treated seeds. The causes of P.helianthi infections (1-3 years), lack of resistance genes to these races in commercial hybrids, insufficient seed dressing and decreased metalaxyl sensitivity of pathogen isolates. It should also not be excluded that subletally metalaxyl tares, used in seed treatment, selected and increased naturally insensitive sub-populations of P.helianthi.