The occurrence and mechanism of field resistance to boscalid and pyraclostrobin in Stemphylium solani, the causal agent of tomato gray leaf spot in China

The destructive disease gray leaf spot, caused by Stemphylium solani, is prevalent in tomato plants in China. A variety of fungicides have been extensively used for controlling the disease, with a particular focus on succinate dehydrogenase inhibitors (SDHIs) and quinone outside inhibitors (QoIs). However, there was a lack of information regarding the resistance of S. solani to boscalid (SDHI) and pyraclostrobin (QoI) in China. In this study, the sensitivity of S. solani to boscalid and pyraclostrobin was monitored. The EC₅₀ values for boscalid ranged from 0.02 to 3.0 μg∙mL⁻¹, with an average value of 0.62 μg∙mL⁻¹, while the EC₅₀ values for pyraclostrobin ranged from 0.21 to 14.71 μg∙mL⁻¹, with an average value of 6.03 μg∙mL⁻¹. Based on these findings, the frequencies of observed resistance were as follows: 36.7% for boscalid and 50% for pyraclostrobin; while the resistance frequency to both boscalid and pyraclostrobin in S. solani was 19.4%. The mutation associated with boscalid resistance in S. solani within tomato fields was identified as SdhB-H277Y, while the mutation related to pyraclostrobin resistance was found in cytochrome b, specifically Cytb-G143A. The resistant mutants displayed diminished fitness in terms of mycelial growth, yet their pathogenicity exhibited no significant disparities. To delay the development of resistance, it is advisable to employ a rotation strategy using alternative fungicides with different modes of action or mix with fungicides with multi-site-contact activity for disease management.