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

Analyzing the Influence of Qualitative Resistance Selection Pressure on Variation of Aggressiveness in Plasmopara Halstedii

“…To characterize aggressiveness criteria of P. halstedii strains, two INRA inbred lines carrying no Pl gene and known to have different levels of quantitative resistance (Vear et al 2007; Tourvieille de Labrouhe et al 2008) were studied: FU, which has greater resistance in the field and BT, rather susceptible in the field. However, these criteria were used by Sakr (2009a, b) to analyze aggressiveness of P. halstedii strains.…”

“…The first is qualitative resistance which is conferred by the major Pl genes and tends to produce a disease-free plant (Tourvieille de Labrouhe et al 2000). The second is quantitative resistance which is controlled by minor genes and tends to impact the rate of disease development (rate reducing) rather than producing a disease-free plant (Tour) -vieille de Labrouhe et al 2008). For qualitative resistance selection pressure, Tourvieille de Labrouhe et al (2010) showed that whatever the method of management (mixture, alternation, and monoculture) of Pl genes, their selection pressure led to appearance of new virulence.…”

Evolution of Two Components of Pathogenicity in Plasmopara halstedii (Downy Mildew) under Sunflower Qualitative Resistance Selection Pressure

“…It carried no Pl gene, but is known to a have high level of quantitative resistance (Tourvieille de Labrouhe et al, 2008). The index of aggressiveness of P. halstedii single zoosporangium isolate was calculated as the ration of (percentage infection × sporulation density) / (latent period×dwarfing).…”

“…Indeed, it has been possible to identify up to 35 races, with different virulence patterns (Ahmed et al, 2012). Disease resistance in sunflowers to P. halstedii can be placed in one of two categories, the first is qualitative resistance (Tourvieille de Labrouhe et al, 2000), and the second is quantitative resistance (Tourvieille de Labrouhe et al, 2008).…”

Advances in sunflower downy mildew research

“…Partial resistance to P. halstedii in high oleic sunflower hybrids has been reported as well by Baldini et al (2006) in Italy. Quite recently, in summarizing their 4-year-study, Tourvieille de Laboruhe et al (2009) suggested that it should be possible to select for non-race-specific downy mildew resistance and to include it in modern varieties. However, since this type of resistance is partial, it may be necessary to combine it with major gene resistance.…”