Occurrence of Verticillium Wilt Caused by Verticillium dahliae on Mung Bean in Northern China

During 2014-2015, wilted and stunted mung bean plants (Vigna radiata(L.) R. Wilczek) were observed in fields located in Inner Mongolia, Shanxi, and Hebei provinces, northern China. The affected plants showed typical Verticillium wilt symptoms, including stunting, gradual leaf yellowing, wilting, and shortened internodes. After dissecting the basal stems and roots, the vascular tissue showed brown discoloration in a ring pattern. Plants that were initially wilted later died as disease progressed. To isolate the causal organism, roots and basal stems of diseased plants were washed under tap water, and then the discolored vascular tissue was cut into 0.5-cm pieces, which were surface-disinfested with 2% NaClO solution for 2 min, rinsed with sterile distilled water three times, dried on sterile filter paper for 3 min, and then placed on potato dextrose agar (PDA) medium with lactic acid, and incubated at 25[degrees]C in the dark. Verticillium- like isolates were consistently obtained from plants taken from the three locations. Colonies of all isolates grew slowly, and were white initially, becoming darker with age. Masses of black microsclerotia were produced on the colony after 2 weeks of incubation. Conidiophores were erect or slanted and were arranged in whorls. Conidia were hyaline, smooth-walled, aseptate, and cylindrical to oval, with size range of 5.1 to 6.5 [mu]m length x 2.9 to 3.5 [mu]m width. These morphological characteristics were consistent with those of Verticillium dahliae(Inderbitzin et al. 2011). The internal transcribed spacer (ITS) region of three representative isolates were sequenced using primers ITS1 and ITS4. The resulting sequences of the three isolates showed 100% identity with reported V. dahliae strains (GenBank Accession Nos. HQ703411.1, JQ070078.1, and KJ696553.1) by BLAST analysis in NCBI. Moreover, all the isolates amplified about 490 bp fragments specific for V. dahliae, using the species specific primers Df/Dr (Inderbitzin et al. 2013). Thus, the pathogen was confirmed as V. dahliae based on morphological and molecular data. Pathogenicity tests of three representative isolates were performed on 7-day-old seedlings of mung bean cv. Yulv2 using the root-dip inoculation method with a conidia suspension of 5.0 x 10 super(6) conidia/ml (Jabnoun-Khiareddine et al. 2006). The inoculated seedlings were maintained in a greenhouse at 25[degrees]C. Seedlings treated with sterile distilled water were used as control plants. At 14 days po