Characterization and Chemical Management of Phytophthora palmivora and Fusarium incarnatum-equiseti Species Complex Associated with Durian Root Rot in China

Durian cultivation has become a key development industry in China in recent years, and durian root rot (DRR) constitutes a major threat to production. In this study, symptomatic root and stem base tissues and leaves were systematically collected from the main durian-producing regions of China, yielding 202 fungal and oomycete isolates. Isolates were identified based on morphological characterization and multi-locus phylogenetic analysis. Fusarium spp. and Phytophthora spp. were the most frequently recovered taxa, accounting for 39.11% and 16.34% of the isolates, respectively. Koch’s postulates confirmed Phytophthora palmivora and the Fusarium incarnatum-equiseti species complex (FIESC) as the causal agents. In controlled co-inoculation experiments, simultaneous inoculation with both pathogens resulted in a shorter latent period and increased disease severity compared with single inoculations, indicating a potential interaction that warrants further investigation. Fungicide sensitivity assays showed that P. palmivora was sensitive to oxathiapiprolin, mandipropamid, fluopicolide, and chlorothalonil, while metalaxyl-M, mancozeb, cymoxanil, and propamocarb hydrochloride showed poor inhibitory activity. FIESC exhibited sensitivity to tebuconazole, fludioxonil, and pydiflumetofen but was insensitive to difenoconazole, mefentrifluconazole, and phenamacril. Oxathiapiprolin and azoxystrobin mixed at ratios of 1:13.8 and 1:10.5 produced pronounced synergistic effects against P. palmivora, with synergy ratios of 3.57 and 2.91 and co-toxicity coefficients of 357.98 and 291.08. This study compared three application methods (root drenching, trunk painting, and trunk injection) of fungicides and identified root drenching as the most effective strategy. Subsequent pairwise screening of eight combinations, composed of four anti-oomycete fungicides (oxathiapiprolin, mandipropamid, fluopicolide, azoxystrobin) paired with two anti-fungal fungicides (tebuconazole, fludioxonil), was conducted via 0.2 L root drenching. Among them, the tebuconazole-azoxystrobin combination offered the highest control efficacy (98.95%), whereas tebuconazole-mandipropamid exhibited the lowest. Further ratio optimization of the tebuconazole-azoxystrobin mixture, performed using a reduced application volume of 0.1 L per plant, identified a 1:2.36 ratio as the most effective formulation, providing 94.65% control. Overall, this work clarifies the pathogen complex associated with DRR in China, provides preliminary evidence for enhanced disease severity during co-inoculation of P. palmivora and FIESC, and identifies optimized fungicide combinations and application methods under greenhouse conditions. These results provide a foundation for further field validation and for the development of integrated management strategies.