First Report of Fusarium proliferatum Causing Root and Bulb Rot on Tiller Onion (Allium cepa var. aggregatum) in China

Tiller onion is a biennial herb and a fascicular variety of onion. Tiller onion has strong tillering ability and can produce up to ten bulbs per plant. It is widely cultivated due to nutrition and special flavor. In July 2020, we observed a disease that seriously affected the normal growth of tiller onion in Halahai Town, Nongan County, Jilin Province, China. At least 70% of tiller onions in the field were affected by this disease. Aboveground parts of the symptomatic plants showed stunted growth, wilting and drying. Underground parts of infected plants were shown that onion increase tiller number but did not grow and expand. Root appeared red lesions and rot in severe cases. The bulb disc appeared brown to dark brown rot. Symptomatic roots were cut into 0.5 cm pieces and surface-sterilized by dipping in 75% ethanol for 60 s, 3% NaOCl for 3 min, and rinsing three times with sterile distilled water. Pieces were placed on potato dextrose agar (PDA) plates and incubated at 25±1℃ for 4 days. Fifteen isolates were obtained and pure-cultured through single-sporing. On PDA plates, the colonies initially had white aerial mycelia that then turned pale purple. The color of the colonies on the back of the plates was purple. Macroconidia were hyaline, falcate and 14.4 to 38.7 × 1.2 to 3.0 µm. Microconidia were hyaline, reniform or elliptic, unicellular or bicellular and were 7.62 to 19.61 µm in length, and 3.23 to 8.41 µm in width. Based on these morphological and culture characteristics, the causal agent was tentatively identified as F. proliferatum. To confirm the pathogen identity, segments of the internal transcribed spacer region of the rRNA gene ( ITS, primers ITS4 and ITS5, White et al., 1990), β-tubulin gene (TUB2, primers T1 and T2, O'Donnell and Cigelnik, 1997), and translation elongation factor 1-alpha gene (TEF-1α, primers EF1 and EF2 from O'Donnell et al., 1998) were amplified by PCR. Per the BLASTN search, TEF-1α (Accession No. OL355013), TUB2 (Accession No. OL355012), and ITS (Accession No. OL355011) queries showed 99.26%, 100%, and 99.82% homology to F. proliferatum GenBank accessions KU872098, MH398224, and MH997878, respectively. Pathogenicity of fifteen isolates of F. proliferatum from tiller onion was confirmed by inoculating healthy tiller onion roots and bulb disc with a spore suspension (1 × 106 spores/ml) produced on PDA. For each treatment, five plants were injected with 5 ml of spore suspension. Control plants (n=5) were injected with sterilized water. All plants were enclosed in plastic bags for 48 h in a greenhouse at 28℃ and 12 h/d light cycle. After 10 days, inoculated plants showed similar symptoms to those on the original diseased plants, while control plants remained symptomless. F. proliferatum was successfully re-isolated from symptomatic plants to fulfill Koch's postulates. Diseases caused by F. proliferatum are only reported in A. cepa. To our knowledge, this is the first report of F. proliferatum in Allium cepa L. var. agrogatum Don in China. Our findings are important for informed surveillance of the disease in China as F. proliferatum infection can not only reduce the quality and yield of tiller onion but also can contaminate the bulbs with harmful mycotoxins.