First report of Pythium ultimum var. ultimum causing damping off on okra in Japan

In February 1993, pre-emergence damping off was observed in a field of okra (Abelmoschus esculentus) Moench) in Nankoku, Kochi, Japan. The disease has subsequently been detected in this region every winter since. In 2005, almost a quarter of okra grown in this region was affected. The disease occurs under cool conditions and affects okra by significantly reducing seedling emergence. It is successfully controlled by soil treatment with metalaxyl at 112 mg a.i. per m² (Fig. 1). A Pythium-like organism was consistently isolated from decayed seeds and identified as P. ultimum Trow var. ultimum, based on the species description of van der Plaats-Niterink (1981). Characteristics of the representative isolate OPU774 included hyphae ca. 10 µm wide, terminal or intercalary oogonia 17-25 µm diameter (Fig. 2), and monoclinous or diclinous antheridia. Oospores were aplerotic, 15-20 µm in diameter. Hyphal swellings were globose or subglobose, terminal or intercalary, 15-30 µm (Fig. 3). Cardinal temperatures for growth on potato-carrot agar were 10˚C minimum, 28°C optimum and 34°C maximum, with a daily radial growth rate of 36 mm at 25˚C. Isolate OPU774 was deposited in NBRC (National Institute of Technology and Evaluation, Biological Resource Center, Japan) and MAFF (The Ministry of Agriculture, Forestry and Fisheries of Japan) (accession references : NBRC102096 and MAFF240023 respectively). The rDNA-ITS sequence data of this isolate was identical to P. ultimum var. ultimum (Lèvesque & Cock, 2004) and deposited in the DDBJ/EMBL/GenBank (accession no. AB272754.)

Pathogenicity testing was conducted using isolate OPU774, in a pot experiment. P. ultimum var. ultimum was grown on moistened, autoclaved bentgrass seeds at 25˚C for 4 days and was used as inoculum. This was mixed with nursery soil in a plastic pot (6 cm diameter) and one seed of okra (cv. Early Five) was sown per pot. The test was repeated 32 times with an equal number of control plants, inoculated with autoclaved inoculum. Plants were maintained in a glasshouse at 7-20˚C. Seedling establishment and disease development were recorded 14 days after sowing. Seed rot was observed on all replicates but no evidence of disease was observed on non-inoculated plants. The pathogen was consistently re-isolated from diseased plants. This confirmed that P. ultimum var. ultimum is the causal agent of the disease.

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Damping-off on okra caused by P. ultimum var. ultimum has only previously been recorded in the USA (Dobbs et al. 1986). The disease caused by an unidentified Pythium sp. has been reported in Japan (Saito 1977). To our knowledge, this is the first report of occurrence of P. ultimum var. ultimum on okra in Japan.