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First Report of Sunn Hemp Fusarium Wilt Caused by Fusarium udum f. sp. crotalariae in Taiwan

Wang, C. L., Dai, Y. L.
Plant disease 2018 v.102 no.5 pp. 1031
Cajanus cajan, Crotalaria juncea, Fusarium udum, Fusarium wilt, Glycine max, Phaseolus vulgaris, Pisum sativum, Sesbania cannabina, Vigna angularis, chlamydospores, conidia, cover crops, culture media, dipping, discoloration, disease transmission, farms, forage, fungi, green manures, host specificity, inoculum, leaves, pathogenicity, pathogens, phylogeny, roots, seedlings, tissues, tubulin, vascular bundles, wilting, China, India, Taiwan, Zimbabwe
Sunn hemp (Crotalaria juncea L.) is cultivated for fiber, green manure, and fodder in tropical and subtropical countries. It is commonly used as a green manure and a cover crop in Taiwan. In May 2016, wilting of sunn hemp was observed in six farms in Tainan, southern Taiwan. About 13% of plants were symptomatic on the most affected farm. External symptoms varied with severity, ranging from general yellowing of the lower leaves to wilting of the plant. Discoloration of vascular bundles first appeared at the crown and continued upward on one side of the stem. Often, white to pinkish fungal spore masses formed on the stem surface at a later stage. To cultivate the presumed fungal pathogens, the internal freshly discolored stem tissues were excised and surface sterilized before plating on potato dextrose agar (PDA). Several individual Fusarium isolates with similar morphological characteristics were obtained from diseased tissues. Single spore isolates (F-1 and F-2) were used for further study. To determine pathogenicity of the fungal isolates, 2-week old sunn hemp seedlings were inoculated by dipping roots in conidial suspensions (5 × 10⁵ conidia/ml) of the two isolates for 30 min before transplanting. Inoculated seedlings wilted at 11 days post inoculation (dpi). Crown discoloration was observed after plants completely wilted. Control plants dipped in sterile water remained symptomless 1 month post inoculation. Pathogens reisolated from the diseased plants were morphologically identical to the original inocula. Pathogenicity assays were performed three times with similar results. The two isolates formed chlamydospores and produced macroconidia and microconidia from monophialide conidiogenous cells. Macroconidia were falcate, 21 to 41 × 3 to 4.5 μm, straight to slightly curved, 1 to 4 (mostly 3) septa, with indistinct foot cells and curved to hooked apical cells. Microconidia were oval to reniform, 6 to 10 × 2 to 4 μm, and 0 to 1 (mostly 0) septa. Chlamydospores were produced singly, in pairs, or in short chains. These morphological characteristics indicated that the two isolates were closed to Fusarium udum (Leslie and Summerell 2006). The partial TEF1α sequences (GenBank accession nos. KY706083 and KY706084) and β-tubulin sequences (MF893323 and MF893324) of the two isolates were amplified and sequenced. Phylogenetic analysis inferred from concatenate sequences of TEF1α and β-tubulin was performed to reveal the relationships between the two isolates and six reference isolates referred to species closely related to F. udum and three distant species (O’Donnell et al. 1998). The result indicated that F. udum (NRRL22949) was the closest species to the two isolates in a highly reliable clade with few nucleotide differences. Host specificity of the pathogens was assayed with six additional leguminous plants (Vigna angularis, Phaseolus vulgaris, Pisum sativum, Sesbania cannabina, Glycine max, Cajanus cajan), but only sunn hemp wilted at 30 dpi. Based on morphological and molecular characteristics and pathogenicity, the two pathogens were identified as F. udum f. sp. crotalariae (G.S. Kulkarni) Subram. Living cultures of isolates F-1 (FU30897) and F-2 (FU30898) were deposited in the Bioresource Collection and Research Center (BCRC), Taiwan. The disease has been reported in India, Zimbabwe, and China (Farr and Rossman 2017; Zhang et al. 1986). Other species of Crotalaria with agricultural and medicinal uses were also reported to be susceptible (Holiday 1995). Given that the disease will limit the utilization of Crotalaria crops and sunn hemp is widely planted in many countries, precaution should be taken to prevent international spread of the disease. To our knowledge, this is the first report of the Fusarium wilt of sunn hemp caused by F. udum f. sp. crotalariae in Taiwan.