LAUSR

Faba bean (Vicia faba L.) is an important legume crop in China. Soil-borne charcoal rot, caused by Macrophomina phaseolina (Tassi) Goid, is a devastating disease of many crops, including legumes. In late March 2018, wilt symptoms similar to those of charcoal rot were observed in faba bean plants (‘Yundouzao 6’) in a field located in Jianshui County (23.38° N, 102.49° E), Yunnan Province, China. This disease caused serious yield loss of faba bean. Infected plants initially showed leaf chlorosis and wilting, and the plants eventually died with the leaves remaining attached. When affected plants were examined, the basal stem pith was rotted, and a brown discoloration was observed after splitting the roots and stems. Symptomatic plants were sampled to isolate the pathogen. Three fungal isolates were obtained (FMP8-1, FMP8-2, and FMP8-4) and displayed rapid mycelial growth on potato dextrose agar at 28°C with a 12-h light regime. The mycelia were initially hyaline and turned from gray to dark with age. Masses of dark oblong microsclerotia (46 to 96 × 61 to 129 µm) formed on the cultures 3 days after incubation. On the basis of these morphological characteristics, the three isolates were presumptively identified as M. phaseolina (Smith and Wyllie 1999). To confirm this identification, the three isolates were molecularly analyzed using primers specific for M. phaseolina: MpKF1/MpKR1, which yielded a 350-bp target fragment (Babu et al. 2007). Additionally, the internal transcribed spacer (ITS) regions of the three isolates yielded ∼600-bp PCR fragments using the universal primers ITS4/ITS5 (White et al. 1990), and the resulting sequences (accession nos. MH323404, MH323405, and MH323406) showed 99 to 100% identity with several known M. phaseolina strains using a BLAST algorithm-based analysis, including strains CIAT519 (KR012878) and CPC13084 (KF95698) isolated from Phaseolus vulgaris; CPC21464 (KF951750) and CPC21386 (KF951701) from Arachis hypogaea; CPC21514 (KF951775), CPC21517 (KF951778), and CPC21450 (KF951739) from Vigna unguiculatain; CBS231.33 (KF951631) from Saccharum officinarum; CJMR135 (MF480342) from Corchorus olitorius; and CPC13086 (KF951700) from Solanum melongena. Thus, this fungus has a broad host range. Both morphological and molecular characteristics confirmed that the three isolates were M. phaseolina. Pathogenicity tests were performed on 2-week-old faba bean seedlings using three inoculation methods (detached leaf, cut stem, and stem puncture) and conducted in three replications with five seedlings per replication (Keeling 1982). All inoculated leaves, stems, and plants showed necrotic symptoms 4 days after inoculation. No symptoms were observed on the control leaves, stems, or plants. M. phaseolina was reisolated from all inoculated leaves, stems, and plants, fulfilling Koch’s postulates. The three isolates were also pathogenic on other legume crops, including soybean (Glycine max), mungbean (Vigna radiata), pea (Pisum sativum), and grass pea (Lathyrus sativus). Although M. phaseolina has been reported on legume crops including soybean, mungbean, and adzuki bean in China, to our knowledge this is the first report of M. phaseolina causing charcoal rot on faba bean in China.