High-Throughput Sequencing-Assisted Microbial Profiling and Koch’s Postulates Validation Identify <i>Fusarium pernambucanum</i> as the Causal Agent of Fruit Rot in <i>Idesia polycarpa</i>

Idesia polycarpa is an economically important tree species valued for urban greening and oil production owing to its oil-rich fruits and high levels of unsaturated fatty acids. The increasing incidence of fruit diseases has become a major constraint on its cultivation; however, the microbial dynamics during disease progression and the primary causal agent remain unclear. In this study, diseased fruits were collected from Sichuan Province, China, a region with frequent disease outbreaks, and categorized into five disease stages based on symptom severity, including initial infection (S1), advancing infection (S2), severe infection (S3), blackening (S4), and necrosis (S5). High-throughput sequencing (HTS) of bacterial 16S rRNA gene and fungal ITS regions from S1, S3, and S5 samples revealed limited variation in bacterial communities across disease stages, whereas fungal communities exhibited pronounced compositional shifts, including a progressive enrichment of Fusarium spp., a well-known group of plant pathogens. Subsequent fungal isolation, morphological characterization, molecular phylogenetics analyses, and pathogenicity assays identified F. pernambucanum as the causal agent of fruit rot. This study elucidates HTS-based microbial profiling during fruit rot, and the results guided pathogen identification based on Koch’s postulates, providing insights into disease diagnosis and management in I. polycarpa.