In vitro sensitivity to thiophanate-methyl of Sclerotinia sclerotiorum isolates causing white mold in common bean (Phaseolus vulgaris L.) in Sinaloa: Sensitivity of Sclerotinia sclerotiorum to thiophanate-methyl

Abstract

Common bean (Phaseolus vulgaris L.) is a crop of major nutritional and economic importance in Mexico, particularly in the state of Sinaloa, where an annual production of approximately 172,238.91 t is reported. However, production is constrained by diseases such as white mold, caused by Sclerotinia sclerotiorum, a pathogen that can cause yield losses of 30-50 % under favorable environmental conditions. This study aimed to evaluate the in vitro sensitivity of 16 S. sclerotiorum isolates to the fungicide thiophanate-methyl. Isolates were collected from bean-producing areas in the municipalities of Ahome and Guasave, Sinaloa, and evaluated using mycelial growth bioassays on PDA medium amended with 0.05-7.5 µg·mL^-1 of the active ingredient. Data revealed differential responses among isolates; at 2.5 µg·mL⁻¹, mean relative mycelial growth decreased to approximately 49 %. Some isolates exhibited complete inhibition of mycelial growth, whereas others continued to grow even at the highest concentrations evaluated. Statistical analysis revealed significant differences between the untreated control and the highest fungicide concentrations (p ≤ 0.05). These findings indicate variation in fungicide sensitivity within the evaluated population. In conclusion, although thiophanate-methyl demonstrated in vitro efficacy, the emergence of less-sensitive isolates underscores the need to implement integrated disease management strategies, including rotating fungicide modes of action and continuously monitoring sensitivity, to enhance the sustainability of bean production in major growing regions.