Resistance against Sclerotinia basal stem rot pathogens in sunflower

Basal stem rot (BSR), caused by Sclerotinia sclerotiorum and Sclerotinia minor, is one of the most devastating diseases of sunflower worldwide. The use of resistant cultivars is the most economical strategy to manage BSR. However, development of cultivars with effective resistance requires adequate information on the interactions of host genotypes with pathogen isolates. To this aim, the present study was conducted to assess the reactions of 100 oilseed sunflower genotypes of diverse origin to three isolates of each of S. sclerotiorum and S. minor at the seedling stage in growth room conditions. Sunflower genotypes exhibited isolate-specific resistance (differential interactions) to both Sclerotinia species. Among interactions (n = 600), 24 isolate-specific resistances were found to S. sclerotiorum and six to S. minor in 24 genotypes. None of the genotypes was resistant to all fungal isolates. Among the resistant genotypes, “H543R/H543R” and “110” were resistant to two isolates of S. sclerotiorum, whereas others were resistant to one isolate of each of the pathogens. Furthermore, 21 genotypes were specifically resistant to only isolates of S. sclerotiorum or only isolates of S. minor, while, “8A*/LC1064C,” “H156A/H543R,” and “110” showed resistance to one or two isolates of both fungal species. Comparison of mean disease severity scores revealed that sunflower genotype “110” had the greatest general resistance to S. sclerotiorum and genotype “ENSAT 699” to S. minor. Cluster analysis effectively identified the existence of genetic variability among sunflower genotypes and classified them into three or four separate classes based on disease severity scores after infection by S. sclerotiorum and S. minor, respectively. The expression level of pathogenesis-related 5 (PR5) protein was investigated in a resistant (8A*/LC1064C) and a susceptible genotype (SDR19) inoculated with S. sclerotiorum isolates. Mean comparison revealed that the expression of PR5 in 8A*/LC1064C was fivefold higher than SDR19 at 3 h after inoculation indicating the possible involvement of this protein in resistance. The resistance sources identified in the present study have the potential for use in breeding programs to develop sunflower hybrids with BSR resistance.