Fusarium oxysporum KB-3 from Bletilla striata: an orchid mycorrhizal fungus.

Orchid mycorrhizal fungi are essential for the seed germination and vegetative growth of orchids. The orchid Bletilla striata has great medical value in China because its tuber is rich in mannan. Some endophytic fungi were isolated from the roots of B. striata. The isolate KB-3 was selected for experiments because it could promote the germination of B. striata seeds. Based on morphological characters and phylogenetic analysis, the isolate KB-3 was identified as Fusarium oxysporum. Co-cultivation experiments of KB-3 with B. striata and Dendrobium candidum were performed to demonstrate orchid mycorrhizal structures. Microscopic examination showed that KB-3 established colonization and produced coiled hyphal structures known as pelotons within the cortical cells of both orchid roots. The results confirm that F. oxysporum KB-3 can behave as an orchid mycorrhizal fungus.