Diversity of the Seedborne Fungi and Pathogenicity of Fusarium Species Associated with Intercropped Soybean.

Maize/soybean relay strip intercropping has been widely practiced in Southwest China due to its high productivity and effective application of agricultural resources; however, several seedborne diseases such as seedling blight, pod and seed decay are frequently observed causing severe yield loss and low seed quality. So far, the population and pathogenicity of the seedborne fungi associated with intercropped soybean remain unexplored. In this study, seeds of 12 soybean cultivars screened for intercropping were collected from three growing regions in Sichuan Province of Southwest China, and the seedborne fungi were isolated from the surface-sterilized seeds. Based on sequence analysis of ribosomal DNA internal transcribed spacer (rDNA ITS),148 isolates were identified into 13 fungal genera, among whichFusariumcovered 55.0% as the biggest population followed byColletotrichum. Furthermore,Fusariumisolates were classified into five distinct species comprisingF. fujikuroi,F. proliferatum,F. verticillioides,F. asiaticumandF. incarnatumthrough sequence analysis of translation elongation factor 1 alpha (EF-1 alpha) and DNA-directed RNA ploymerase II second largest subunit (RPB2). Among them,F. fujikuroiaccounted for 51.22% (42/82) and was isolated from 91.7% (11/12) soybean varieties. Pathogenicity assay showed that fiveFusariumspecies were able to infect the seeds of soybean cultivar "Nandou12" and caused water-soaked or rot symptoms, whileF. fujikuroiandF. asiaticumhad much higher aggressiveness than other species with significant reductions of seed fresh weight and germination percentage. Accordingly, this study indicates thatFusariumspecies are the dominant seedborne fungi in the intercropped soybean in Sichuan, China, and this provides some useful references for the effective management of seedborne fungal diseases as well as soybean resistance breeding in maize/soybean relay strip intercropping.