Fusarium solani and Fusarium oxysporum Associated with Root Rot of Glycyrrhiza uralensis in China

Abstract

Liquorice (root of Glycyrrhiza uralensis Fisch.) is an important Chinese traditional medicine for many ailments (4). From 2002, severe outbreaks of root rot occurring on cultivated G. uralensis plants in Ningxia, China, have affected the yield and quality of liquorice and been considered as a major threat to commercial production of liquorice. Approximately 30% of the plants die from this disease in Ningxia every year. The disease, mainly affecting 2- to 4-year-old G. uralensis seedlings, begins with brown rot of root tips or lateral roots followed by internal decay of taproots during June to August every year. The infected plants are wilted with chlorotic foliage and easily pulled up from the soil. Root rot is clearly visible as a severe brown discoloration of vascular tissue along taproots. In severe cases, white mycelia are clearly visible on the surface of diseased roots and roots are decomposed. Isolations from diseased roots were made on potato dextrose agar (PDA) amended with streptomycin sulfate. Isolates (n = 78) were recovered from symptomatic roots (n = 105) and pure cultures were established by the single spore method. The two most frequently isolated fungi were transferred to potato sucrose agar and identified as Fusarium solani (61.5%) and F. oxysporum (30.8%) (1). The monophialides bearing microconidia of F. solani are long when compared to those of F. oxysporum. Genomic DNA of strains F. solani G013 and F. oxysporum FLR were extracted from mycelia with the cetyltrimethylammonium bromide (CTAB) method. Primers EF1-728F and EF1-986R were used to amplify the translation elongation factor-1α (TEF-1α) gene (2). The TEF-1α sequences of F. solani G013 (GenBank Accession No. AB777258) and F. oxysporum FLR (AB777257) shared 99 and 100% similarity with F. solani isolate NRRL52790 and F. oxysporum isolate NRRL 38328, respectively. Pathogenicity tests with one representative isolate of each species were conducted in the greenhouse on 1-month-old potted G. uralensis seedlings (12 plants per treatment). Isolates of the tested fungi were transferred to PDA and incubated in darkness at 24 ± 1°C for 7 days. Plant taproots about 5 cm below the soil surface were wounded with a sterile needle and five 5-mm-diameter fungal disks on the margin of colony were taken and firmly placed on the wounded location of each taproot with tinfoil; wounded taproots of seedlings inoculated with sterile PDA disks were used as controls (3). Root rot was assessed 2 months after inoculation. F. solani G013 and F. oxysporum FLR produced root rot symptoms on inoculated plants that were the same as those observed in field plants, and the fungi were reisolated from roots with typical symptoms. Control plants inoculated with sterile PDA disks remained asymptomatic, and no pathogen was isolated from them. To our knowledge, this is the first report of root rot caused by F. solani and F. oxysporum on G. uralensis in China. Effective control strategies are needed to minimize losses.