First Report of Pythium myriotylum Causing Root Rot of Yam in China

Abstract

Yams (Dioscorea spp.) are widely grown in China as vegetables and for herbal medicine. From 2016 to 2018, water-soaked lesions appeared on the fibrous roots of yam seedlings, and some plants were wilted, up to 15% in Heze city, Shandong Province. Fragments of 1 mm in size were excised from water-soaked roots of diseased plants, dipped in a 0.2% calcium hypochlorite solution for 10 min, placed on a V8 medium, and incubated in dark at 28°C for 5 days. A Pythium sp. was consistently isolated from water-soaked roots of 10 yam plants. The Pythium sp. produced sporangia, oogonia, antheridia, and oospores. Sporangia were terminal or intercalary, filamentous, inflated lobulate. Oogonia were globose, terminal or intercalary, ranging from 20 to 40 µm (average 29.3 µm) in diameter mating with one to five antheridia. Most oospores were globose, smooth, and aplerotic, ranging from 17.8 to 33.5 µm (average 25.3 µm). The antheridia were diclinous and shaped like clavates and crooknecks. Five of the isolates were molecularly identified further. Genomic DNA was extracted from the five isolates of the Pythium sp. using a DNA extraction kit (Omega, U.S.A.). The cytochrome c oxidase subunit I (COI) gene was amplified and sequenced using primers FM55/FM52R (Long et al. 2012). The five COI sequences were determined and identical. BLASTn analysis of COI with a 539-bp sequence (accession no. MH177892) resulted in 100% homology with that of Pythium myriotylum isolate AB292 (accession no. MF374747). Isolates were identified as P. myriotylum based on morphological characteristics (van der Plaats-Niterink 1981) and COI sequences. Koch’s postulates were conducted by first producing inoculum for one of the isolates (SDHZSY-1). Inoculum was produced by first soaking corn kernels for 24 h in water, followed by autoclaving for 2 h at 121°C. Isolate SDHZSY-1 was grown on corn meal agar medium for 10 days, after which agar plugs were transferred to the sterilized corn kernels, which were incubated at 28°C for approximately 15 days, until the corn kernels were covered in white hypha. Ten healthy yam plants were transplanted into a sterilized sandy loam potting soil that was artificially infested with the corn inoculum (3 g of inoculum per 100 g of sandy loam mixture). Inoculated and noninoculated control plants were maintained in a greenhouse at 28°C and 80% relative humidity with a 12-h photoperiod, and the experiment was repeated. All 20 inoculated plants appeared water-soaked within 4 weeks, and P. myriotylum was recovered only from the water-soaked roots of inoculated plants, whereas all 20 control plants remained symptomless in both experiments. To our knowledge, this is the first report of root rot on yams caused by P. myriotylum in China and worldwide. Identification of the pathogen will assist in devising strategies to protect this important plant from the pathogen and to prevent yield losses.