Confirmation of Fusarium oxysporum as a causal agent of mung bean wilt in China

Abstract

Fusarium wilt is becoming an increasingly severe disease affecting mung bean in China. However, the pathogen has not thoroughly been identified. We therefore carried out a multi-year disease investigation and gathered an extensive collection of diseased mung bean plants, aiming to confirm the causal agent through morphological characterization, pathogenicity tests, molecular detection, and phylogenetic analysis. Field surveys were conducted for five consecutive years, covering the main mung bean-growing areas and spanning seven provinces of China. In total, 170 isolates with Fusarium oxysporum-like colonies were isolated from the collected samples, which were subsequently confirmed as F. oxysporum by morphological observation and PCR-based molecular detection. Then, 70 isolates (10/province) were subjected to pathogenicity tests and molecular detection of pathogenicity-related secreted in xylem (SIX) genes. Sixty-eight of the 70 isolates were pathogenic to mung bean and were verified to carry the pathogenicity-related genes SIX6 and SIX11, while the remaining two nonpathogenic isolates did not contain any SIX genes. Additionally, in a host range test of seven representative pathogenic isolates (1/province) on eight other crops, only two isolates caused mild symptoms on one of two adzuki bean cultivars, and no visual symptoms were observed on chickpea, common bean, cowpea, pea, rice bean, soybean, or cotton, indicating the isolates were host-specific to mung bean. Finally, phylogenetic analysis using multiple genes, TEF1-ɑ, IGS, mtSSU, SIX6 and SIX11, revealed that the seven mung bean isolates formed a unique clade distinct from other formae speciales of F. oxysporum. Here, the F. oxysporum isolates causing mung bean wilt were confirmed as an independent forma specialis distinguished from those affecting other crops.