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.
