Quelles perspectives pour la lutte génétique vis-à-vis du mildiou du tournesol ?

Abstract

Downy mildew of sunflower exists as physiological races which have changed in France following introductions or selection pressure applied by large-scale use of varieties with single major genes giving resistance to the disease. The duration of efficiency of single or pyramided resistance genes, their alternation over different years or use of mixtures of several isogenic forms of a hybrid with different resistance genes, were studied under six irrigated 60m 2 net cages which made it possible to obtain disease attack whatever the weather conditions. Studies have been made over 4 years of continuous cropping. The susceptible form of the hybrid showed from 37 to 75% attack, always by race 710, present at the beginning of the study. Monoculture of the two forms of the hybrid each carrying one efficient resistance gene showed no significant attack in years 1 and 2, but then 4-5% attack in year 3 and 15-21% attack in year 4, with the appearance of races new to the trial plots but which have appeared elsewhere in France where varieties with similar resistances are grown. In contrast, there were no significant attacks on the form of the hybrid carrying both efficient resistance genes. In the cage in which different forms of the hybrid were alternated, after 75% attack on the susceptible form, cropping of single resistance genes and then of the pyramided form, always gave less than 2% attack. The cage in which a mixture (25% of each hybrid form) was grown always showed one quarter of the attack in the cage containing the susceptible hybrid, and very little change in virulence. Thus, pyramiding, alternation or mixtures of a few major genes all appear to increase the duration of effective control of downy mildew. Their possible use on a large scale, the difficulties that would be encountered by both breeders and farmers and combination with other possible methods to limit downy mildew attacks are discussed.