Fungicide sensitivity of isolates of Sclerotinia sclerotiorum from different hosts and regions in Brazil and phenotypic instability of thiophanate-methyl resistant isolates

Fungicide application is the most widely adopted method to control white mold epidemics in several crops. The objectives of this study were to assess the sensitivity to thiophanate-methyl (TM), fluazinam and procymidone of 238 Sclerotinia sclerotiorum isolates collected during 2014 to 2017 from soybean (n = 77 isolates), dry bean (49), cotton (28), tomato (22), sunflower (17), and other (45) hosts, in different regions in Brazil; to investigate any mutation in the genome of resistant isolates; and to assess the phenotypic stability of resistant isolates. Fungicide sensitivity was assessed using discriminatory doses for TM (5 µg/mL), fluazinam (0.05 µg/mL), and procymidone (0.5 µg/mL). Compared to the controls, the mycelial growth inhibition of the sensitive isolates by fluazinam, procymidone, and TM varied from 76 to 94%; 68 to 96%; and 67 to 98%, respectively. There was no evidence of resistance to fluazinam or procymidone, but 13 isolates from dry bean fields were resistant to TM, all had a mutation at codon 240 (L240F) of the β-tubulin gene. The phenotypic stability of TM resistant isolates was evaluated during and after 10 transfers in culture medium without fungicide. The mycelial growth rate of six TM-resistant isolates reduced with successive transfers, but there was no reversion to a sensitive phenotype. Resistant isolates to TM were more frequently observed in the northwestern region of Minas Gerais state; and there is evidence for altered growth pattern in vitro for resistant isolates. Management of resistant populations must employ fungicides with different modes of actions.