Differential aggressiveness of Sclerotinia sclerotiorum isolates from North and South America and partial host resistance in Brazilian soybean and dry bean cultivars
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White mold (or Sclerotinia stem rot), caused by Sclerotinia sclerotiorum, is a yield-reducing disease of great importance to both dry bean and soybean crops in the Americas. Characterization of both the physiological resistance in commercial cultivars to white mold disease and the range of aggressiveness among S. sclerotiorum isolates collected from locations where these cultivars are to be deployed provides useful information for breeding and management. In this study, aggressiveness was characterized for a collection of 82 isolates originating North and South America and the reaction of 23 dry bean and 11 soybean cultivars from Brazil was evaluated. The detached leaf bioassay (DLB) was used to assess aggressiveness of 63 isolates on U.S. soybean cultivar Dassel and 25 isolates on Brazilian dry bean cultivar IAC Alvorada. The straw test (ST) was used to evaluate the aggressiveness of 32 isolates on U.S. dry bean cultivar G122 and 26 isolates on Brazilian cultivar IAC Alvorada. Results of the DLB test showed that the isolates were in general more aggressive towards bean IAC Alvorada (x¯ = 14.26 cm 2 ) than on soybean Dassel (x¯ = 9.20 cm 2 ). ST ratings classified 24 26 isolates inoculated on dry bean IAC Alvorada as highly aggressive (x¯ = 8.0), whereas no isolates were classified as highly aggressive on dry bean G122 (x¯ = 5.1). When the isolates were compared for aggressiveness, some of them were consistently rated in the top 10, regardless of the evaluation method or plant host used. Based on results of both inoculation methods, IAC Diplomata and IPR Tangará were the most resistant to S. sclerotiorum among 23 Brazilian dry bean cultivars. In soybean, M5410 and M6410 were classified as the most resistant. Collectively, results of this study contribute new knowledge of variation in aggressiveness in the pathogen population and of cultivars that may have partial resistance, warranting further investigation.