Abstract
Pathogenic, morphological and genetic variation was studied in 35 Plasmopara halstedii (sunflower downy mildew) isolates of different races. Virulence spectrum was analyzed in sunflower hybrids carrying effective Plgenes. Aggressiveness was analyzed in one sunflower inbred line showing a high level of quantitative resistance. There were differences in virulence spectrum for pathogen isolates. Index of aggressiveness was calculated for each isolate and two groups were revealed as more aggressive isolates of 100 and 3xx races, and less aggressive isolates of 7xx races. Significant morphological differences were found in zoosporangia and sporangiophores morphology. Genetic relationships were detected between the pathogen isolates using 12 EST-derived markers. Five multilocus genotypes (MLG) were identified among 35 P. halstedii isolates. Our results did not show a correlation between pathogen variation and both morphological and genetic characteristics.
Highlights
Sunflower (Helianthus annuus L.) is one of the major oilseed crops cultivated worldwide
Based on the reaction for the P. halstedii isolates to four sunflower hybrids H1 to H4 varying only in their Pl genes, there were differences in virulence spectrum in P. halstedii isolates (Table 2)
The phenotypic properties expressed in abiotrophic stages can be factors of selection of zoospores or cystospores, but this kind of selection is not necessarily related directly to pathogenicity of P. halstedii (SPRING; THINES, 2004; VIRANYI; SPRING, 2011)
Summary
Sunflower (Helianthus annuus L.) is one of the major oilseed crops cultivated worldwide. Downy mildew is a common sunflower disease responsible for significant yield loss and can be controlled by fungicides and cultivation of resistant hybrids. P. halstedii displays a gene-for-gene interaction with its host plant and shows physiological races (pathotypes) capable of infecting a variable range of sunflower genotypes. The nomenclature of these races is based on the reaction of. 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 (TOURVIEILLE DE LABROUHE et al, 2008)
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