Synergistic Fungicide Mixtures Control Multiple-Fungicide-Resistant Fusarium Complexes Causing Cowpea Root Rot in Hainan, China

Cowpea root rot severely constrains the development of the cowpea industry in Hainan Province, China. However, effective control is hindered by the unclear population structure of the causal pathogens and prominent field fungicide resistance in this region. In this study, 282 isolates were obtained from diseased plants collected across various production areas in Hainan. ITS sequence analysis revealed that Fusarium spp. was the dominant genus, accounting for 78.7% of the isolates. Based on colony morphology, representative isolates were selected for multilocus phylogenetic analysis, which classified them into four species complexes: Fusarium oxysporum, F. fujikuroi, F. solani, and F. incarnatum-equiseti, with the F. oxysporum species complex being the predominant group. Pathogenicity tests confirmed that all four species complexes are causal agents of cowpea root rot. Fungicide sensitivity assays indicated that these isolates exhibited widespread resistance to fludioxonil, azoxystrobin, carbendazim, phenamacril, and pydiflumetofen. Notably, isolate FIEHN13 displayed resistance to all tested fungicides, posing a severe risk of multiple-fungicide resistance. Given that tebuconazole retained high activity against most isolates, this study evaluated its potential for combination with phenamacril or pydiflumetofen. The optimized combinations exhibited synergistic or additive effects in vitro. Greenhouse pot trials further demonstrated that seed coating with phenamacril + tebuconazole (1.74:1) or pydiflumetofen + tebuconazole (2.76:1) reduced disease incidence to 0–50%, achieving control efficacies of 75–100%. These treatments were significantly superior to single-agent applications, indicating considerable application potential. Collectively, these results indicate that cowpea root rot in Hainan Province is caused by multiple-fungicide-resistant Fusarium species complexes, and rational tebuconazole-based combination strategies can provide an effective approach for managing fungicide resistance.