First Report of Fusarium oxysporum and F. proliferatum Causing Postharvest Corm Rot on Taro in China

Abstract

Taro (Colocasia esculenta), a perennial tuberous herb of the family Araceae, is cultivated widely in southern China. In December 2020, postharvest corm rot occurred on taro of 5 tons with approximately 70% incidence in a 18 square meter cellar in the Qingshanhu District (115°83’E, 28°76’N) of Nanchang City, Jiangxi Province, China. Infected corms had round, soft and slightly sunken lesions covered with white mycelia. The lesions gradually expanded, causing part or whole corm to become soft and shrink, and the inner corm tissue turned brown and rotten. To isolate the pathogen, a total of 30 diseased corm samples were collected. The corms were surface-disinfected by wiping them with 70% ethanol and then passing them over flame back and forth for 5 s. After epidermal tissue of the corms was removed using a sterilized scalpel, small portions of the inner tissue were transferred onto potato dextrose agar (PDA) and incubated at 25°C in the dark. A total of 27 isolates forming Fusarium-like colonies were obtained using monosporic isolation, of which 11 isolates were identified as F. oxysporum and 16 isolates were identified as F. proliferatum based on the colony characteristics and conidial morphology (Leslie and Summerell, 2006). Colonies of F. oxysporum isolates produced dense whitish to light purple mycelia with dark red pigments. Macroconidia were sickle-shaped, straight to slightly curved, 3-5 septa, measuring 25.6 to 45.8 × 3.3 to 6.1 µm. Microconidia were hyaline, oval or ellipsoid, aseptate, and measured 5.2 to 11.8 × 2.2 to 3.5 µm. Chlamydospores were round, 3.5 to 7.6 µm in diameter. Colonies of F. proliferatum isolates were whitish with abundant aerial mycelia and orange pigments. Numerous oval unicellular microconidia were 4.5 to 11.8 × 1.9 to 4.2 µm, and sparse falcate macroconidia with 3-4 septa were 19.4 to 39.2 × 1.9 to 5.2 µm in size. No chlamydospores were observed. Genomic DNA of two representative isolates (F. oxysporum isolate YTU1 and F. proliferatum isolate YTH1) was extracted, and the internal transcribed spacer (ITS) region and translation elongation factor 1-α (TEF1-α) gene were amplified and sequenced using primers ITS1/ITS4 and EF-1H/EF-2T (White et al., 1990; Zhang et al., 2014) respectively. Using BLAST analysis, the ITS sequences of isolates YTU1 (506 bp) and YTH1 (508 bp) exhibited 100% homology with F. oxysporum (MN633363) and F. proliferatum (MT534188), respectively, and the TEF1-α sequences of YTU1 (712 bp) and YTH1 (703 bp) shared 100% homology with F. oxysporum (MN507110) and F. proliferatum (MK952799), respectively. Sequences were deposited in GenBank with the Accession Nos. MZ157124 and MZ310443 for ITS, and MZ383814 and MZ383815 for TEF1-α. The pathogenicity of each isolate was determined on six healthy taro corms. All the taro corms were surface-disinfected with 70% alcohol and two locations from each corm were inoculated. One location was inoculated with 20 μl of conidial suspension (1×105 conidia/ml) and the other was inoculated with sterilized water as a control. All corms were incubated in a growth chamber at 25℃ and 95% relative humidity in the dark. After 15 days, all inoculated corms developed brown rot symptoms, while the non-inoculated control corms remained symptomless. The original isolates were successfully reisolated from all symptomatic corms and identified by sequencing, fulfilling Koch's postulates. F. oxysporum has been reported causing postharvest corm rot of taro in Bogor, Japan, and British Solomon Islands (Widodo et al., 2011). However, to our knowledge, this is the first report of F. oxysporum causing postharvest corm rot of taro in China and F. proliferatum causing postharvest corm rot of taro in the world. The disease poses a potential threat to taro production and should be timely assessed and properly managed.