Downy Mildew (Plasmopara Halstedii) Infection in High Oleic Sunflower Hybrids in Northern Italy

This study was conducted to evaluate the behavior of some high oleic sunflower hybrids towards the race of Plasmopara halstedii present in the environment and to analyze the main climatic factors affecting development and spread of the disease in order to draw up the basis for a prediction model of disease incidence. The race was determined by means of artificial infection of differential inbred lines in a growth chamber experiment and a field trial was also done to study the effects of the factors genotype, sowing time and seed dressing on the infection percentage of P. halstedii during different sunflower growing stages. The Pl genes responsible for resistance to race 700, detected in the trial area and named according to the international code, were absent in all the high oleic hybrids studied, which, however, manifested a partial resistance to downy mildew, probably due to the partial polygenic, non-qualitative and non-specific resistance to the race detected. Seed treatment with metalaxyl helped to limit the severity of P. halstedii attack to 5% of plants infected, but with a maximum of 19.6% in the susceptible check. Concerning the environmental conditions, water availability did not result as being limiting for the infection, whereas the most favorable mean air temperatures during the 5 days following sowing ranged from 10 to 15°C.