Presence of sunflower downy mildew in an integrated weed control field trial

Surveys were carried out during May–June 2004 to investigate the presence of sunflower plants (cv. Sanbro) showing systemic downy mildew infections in an experimental field located in central Italy (Umbria) where chemical and mechanical weed control measures were being compared. Race characterization of Plasmopara helianthi isolates, using the triplet coding system, resulted in race 703. Furthermore, greenhouse and laboratory experiments on the sensitivity of pathogen isolates to metalaxyl revealed an insufficient seed dressing of the commercial hybrid compared with the registered rate and a decreased sensitivity of P. helianthi isolates to metalaxyl. Besides, an increase of the disease incidence following some mechanical weed control measures was also noted. Plasmopara helianthi attacks could probably be attributed to agronomic (short crop rotation), phytosanitary (insufficient seed dressing) and genetic (lack of resistance genes) factors influenced by favourable climatic conditions.