Quantitative resistance to downy mildew (Plasmopara halstedii) in sunflower (Helianthus annuus)

Abstract

Downy mildew of sunflower, caused by the Oomycete, Plasmopara halstedii is at present controlled by major resistance genes. However, the pathogen has shown a considerable capacity for changes in virulence and these resistance genes are overcome only a few years after they have been introduced into new sunflower varieties. This paper presents research for quantitative, non-race-specific resistance independent of major genes. The reaction of cultivated sunflower genotypes to field attack by downy mildew was studied over 4 years in several environments and in the presence of the two most common races in France: 703 and 710. An experimental protocol with pre-emergence irrigation was developed, making it possible to observe downy mildew reaction whatever the weather conditions. Significant levels of partial resistance were observed in about 50 inbred sunflower lines among the 800 observed. These results suggest that it should be possible to select for non-race-specific downy mildew resistance and to include it in modern varieties. However, since this non-specific resistance is partial, it may be necessary to combine it with major gene resistance. Possible strategies are discussed to obtain durable resistance to downy mildew.