INOCULATION METHODS OF COMMON BEAN SEEDLINGS FOR EVALUATION OF GERMOPLASM RESISTANCE TO Sclerotinia sclerotiorum (Lib.) De Bary

Abstract

The development of a reliable method of inoculation of dry bean (Phaseolus spp.) with Sclerotinia sclerotiorum is of great importance for evaluation of genotypes for resistance to white mold. The objective of this study was to test three inoculation methods in different parts of 11-day-old Phaseolus spp seedlings. The plant was inoculated with a suspension of ascospores at the blossom stage as a reference for comparing the efficiency of the methods tested, since it simulates field infections. The methods consisted of the inoculation of leaves or axils with disks of PDA containing mycelia; and inoculation of the stems with a toothpick colonized by the pathogen. Nine dry bean genotypes of Phaseolus spp. and two isolates of S. sclerotiorum (UnB 1541 and UnB 1547) were used to test the methods. The inoculated plants were incubated in a fog room, with 100% humidity, a temperature of 21±2oC and a photoperiod of 12 hours. The incubating period varied from two days for the PDA disk leaf inoculation, four days for the axil inoculation and ten days for stems inoculation with toothpicks colonized by the pathogen and flowers inoculation with ascospores. Disease assessment was accomplished by using a key ranging from 1 to 9 (1= no symptoms; 9= plant dead). Inoculation of seedlings in the axils using PDA disks containing mycelia was the best method to differentiate the genotypes, it also presented similar results to the inoculation of flowers with ascospores. The isolate UnB 1541 was more virulent than UnB 1547, and discriminated better the genotypes for resistance or susceptibility to S. sclerotiorum.

KEY-WORDS: White mold; genetic resistance; Phaseolus spp; inoculation method.