Aggressiveness Diversity of Sclerotinia sclerotiorum and S. minor Isolates in West Azarbaijan Province and Specific Interaction of Sunflower Lines with the Isolates of these Pathogens

Sclerotinia basal stem and stem rot, caused by Sclerotinia sclerotiorum and S. minor, is one of the most devastating diseases of sunflower. The use of resistant cultivars is considered as the most important method to control this disease; however, the employment and development of these cultivars require information on the aggressiveness of the pathogen in the region and interaction of fungal isolates with host genotypes. In this research, aggressiveness of 15 S. sclerotiorum and 14 S. minor isolates collected from sunflower fields in different regions of West Azarbaijan province was initially studied on the cultivar Farrokh. Aggressiveness of S. sclerotiorum isolates ranged from low to high, however, low aggressiveness diversity was observed among S. minor isolates and most of them were highly aggressive. Interactions between three isolates of each fungal species and 40 sunflower lines were then evaluated in controlled condition. Isolate-specific resistances to the pathogens were identified in some studied lines and the two lines, 15031 and ENSAT-699, were resistant to the three isolates of S. minor. Among the resistant lines identified in this study, the Iranian line ‘110’ with resistance to two isolates of each of the pathogens and with low mean infection (79.44%) was among the most resistant lines. The new sources of resistance identified in this study could be used in sunflower breeding programs to develop cultivars with broad spectrum resistance to both pathogens.