Leaf-blight disease caused by the Fusarium oxysporum is an emerging problem in Dendrobium chrysotoxum production in China. Symptoms of leaf blight were observed on seedlings of D. chrysotoxum cultivated in a nursery in Ruili City, Yunnan Province, China. In this study, we isolated the Fusarium sp. associated with leaf-blight disease of D. chrysotoxum from the diseased seedlings. A pathogenicity test was performed to fulfill Koch's postulates to confirm the pathogenicity of isolated strains and identified using morphological and molecular techniques. The results revealed that all four isolated Fusarium sp. isolates (DHRL-01~04) produced typical blight symptoms followed by marginal necrosis of leaves on the D. chrysotoxum plants. On the PDA medium, the fungal colony appeared as a white to purplish color with cottony mycelium growth. Microconidia are oval-shaped, whereas macroconidia are sickle-shaped, tapering at both ends with 2-4 septations. The phylogenetic trees were construed based on internal transcribed spacer (ITS), translation elongation factor (EF-1α), and RNA polymerase subunit genes RPB1 and RPB2 genes, respectively, and blasted against the NCBI database for species confirmation. Based on the NCBI database's blast results, the isolates showed that more than 99% identify with Fusarium oxysporum. To our knowledge, this is the first comprehensive report on the identification of Fusarium oxysporum as the causal agent of Dendrobium chrysotoxum leaf blight in Yunnan Province, China, based on morphological and molecular characteristics.
Keywords: Koch’s postulates; internal transcribed spacer (ITS); morphological characteristics; pathogenicity; phylogenetic analysis.