Main factors influencing downy mildew (Plasmopara halstedii) infection in high-oleic sunflower hybrids in northern Italy

Abstract Downy mildew is a common disease in sunflower in many areas of Europe where cool rainy springs are ideal for this crop. This study evaluated, during 2 years’ experiments, the response of some “high-oleic” sunflower hybrids to the local race of Plasmopara halstedii. The main climatic factors affecting development and spread of the disease on which to base a prediction model of disease incidence were measured and analyzed. The race was determined by artificial infection of differential inbred lines in a growth chamber, and the effects of sunflower genotype, sowing time and seed dressing on the infection percentage of P. halstedii during different growth stages were studied in a field trial. The Pl genes, responsible for resistance to race 700 that was detected in the trial area and named according to the international code, were absent in all the high-oleic hybrids studied. They did, however, manifest a partial resistance to downy mildew, which was probably due to a mixture of races present in the trial soil inoculum determining a non-specific response of the studied genotypes. Seed treatment with metalaxyl helped to limit the severity of P. halstedii attack to 5% of plants infected, with a maximum of 19.6% in the susceptible check. This result may indicate the presence of some metalaxyl-resistant downy mildew strains, but further research, such as specific tests in controlled conditions, is required to confirm this hypothesis and exclude the possibility that rainfall might wash the fungicide active ingredient off the seeds before it can penetrate. The most favorable mean air temperatures during the 5 d after sowing were from 10 to 15 °C, whereas water availability did not appear to be a limiting factor for the infection. The next objective will be to modify and calibrate the infection prediction model to take into account the aggressiveness and spread of the pathogen in a given area and the correlated local climate and edaphic characteristics.