Isolation, Identification, and In Vitro Fungicide Screening of the Pathogen Causing Southern Blight of Pepper in Zunyi, Guizhou, China

Pepper southern blight is a destructive soil-borne fungal disease that severely restricts pepper yield and quality in continuous cropping systems. To clarify the causal pathogen of pepper southern blight in Zunyi, Guizhou, and screen efficient fungicides for scientific disease prevention and control, the pathogenic fungus was isolated and identified via morphological observation, multi-locus molecular phylogeny, and pathogenicity verification. Indoor fungicide toxicity assays were performed using the toxic medium method. The results showed that the pathogen was confirmed as Agroathelia rolfsii (A. rolfsii). Pathogenicity tests verified that the strain exhibited strong pathogenicity to pepper seedlings, with a 90% disease incidence at 3 days post-inoculation and a disease severity index of 43.33 ± 2.89. The pathogen was successfully re-isolated from diseased pepper tissues and verified through morphological characteristics and ITS/LSU/TEF1-α multi-gene sequencing, fulfilling Koch’s postulates. The results of in vitro fungicide screening showed that thifluzamide, fludioxonil and pyraclostrobin exhibited strong antifungal efficacy on the tested strains, and the median inhibitory concentration (EC50) was 0.031–0.358 mg/L after 3 days of treatment. Followed by triadimefon and prochloraz, EC50 was 9.731~18.593 mg/L. Triadimefon and prochloraz showed moderate antifungal activity, with EC values of 9.731–18.593 mg/L. This study systematically confirms A. rolfsii as the causal agent of pepper southern blight in Zunyi, China. Thifluzamide, fludioxonil and pyraclostrobin exhibited good inhibitory effects against this pathogen and can be recommended for disease control.