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 1C, 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.
