Isolation, Molecular Characterization, and Biological Control of Fusarium solani Causing Stem Rot of Durian Tree in China

Durian (Durio zibethinus L.) is a high-value tropical fruit crop that has recently been cultivated on a commercial scale in Hainan Province, China. However, emerging diseases increasingly threaten sustainable production and fruit quality. Field surveys conducted in Sanya, Hainan, identified stem rot as the predominant disease affecting durian trees, with a disease incidence of approximately 26.6%. Affected trees exhibited necrotic lesions at stem-branch junctions, followed by leaf abscission and progressive decline. The causal agent was identified using morphological characteristics, phylogenetic analyses of the internal transcribed spacer (ITS) and translation elongation factor 1-alpha (TEF-1α) and RNA polymerase II second largest subunit (RPB2) gene regions, and pathogenicity assays. Inoculated plants developed symptoms consistent with those observed under field conditions, thereby fulfilling Koch’s postulates. Both morphological and multilocus molecular analyses consistently identified the pathogen as Fusarium solani. A total of 20 Fusarium isolates were obtained from diseased durian stem samples. To our knowledge, this is the first report of F. solani causing stem rot of durian in Hainan Province, China. In addition, Bacillus subtilis SYST2 exhibited strong antagonistic activity against F. solani isolates FS-1, FS-2, and FS-3 in vitro, with inhibition rates of 55.7%, 54.6%, and 53.3%, respectively. Field trials further demonstrated that application of SYST2 significantly suppressed disease development, achieving a biocontrol efficacy of 53.51%. These findings establish F. solani as a causal agent of durian stem rot in Hainan and highlight the potential of Bacillus-based biological control as a sustainable strategy for disease management in durian production systems.