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First Report of Pythium aphanidermatum on Basella rubra in Japan

Basella rubra L. (Indian spinach, Japanese name Tsurumurasaki) is cultivated worldwide as an ornamental and its aerial parts are consumed as a vegetable and health food. A severe rot of leaves, stems, and roots was found on B. rubra cv. Midori grown in a commercial field at Naruto-City (34°11′N, 134°36(E) in Tokushima Prefecture, Japan from May to September 2004. More than 50% of approximately 2,100 plants in the field were destroyed by the disease. Soft, black lesions appeared initially at the base of the stems in 2- to 5-month-old plants and enlarged gradually upward and downward within 2 days after plant injury caused by heavy rainfall. A fluffy, white mass of mycelium appeared on the surface of lesions under moist conditions. A Pythium species was routinely isolated from rotted stems and roots and identified as Pythium aphanidermatum (Edson) Fitzpatrick on the basis of its morphology on a grass leaf water culture (2). Characteristics of isolate OPU743 (NBRC No. 101556, MAFF No. 239847) included hyphae as much as 10 μm wide, terminal oogonia 17.8 to 28.8 μm in diameter, and monoclinous or diclinous antheridia 8.8 to 10.9 μm wide, either terminal or intercalary, with one or rarely two per oogonium. Oospores were aplerotic, 13.5 to 22.6 μm in diameter. Sporangia were terminal or occasionally intercalary, and either inflated filamentous hyphae or complexes of swollen hyphal branches were present in cultures. Cardinal temperatures for growth on potato carrot agar were 10°C minimum, 37°C optimum, and 40°C maximum with a daily radial growth rate of 32.9 mm at 25°C. Pathogenicity tests were conducted on potted 3-month-old B. rubra (cv. Midori). A wound (1 mm deep and 5 mm long made by a razor) on the surface of the stem of the plant was inoculated with an 8-mm-diameter agar disk of isolate OPU743 (grown at 25°C for 48 h on potato dextrose agar) attached to a stem of the plant using a paraffin film. The inoculated plants were placed in transparent plastic bags and kept in a growth chamber at 24 to 26°C with continuous light (82 to 126 μmol·m^-2·s^-1). The experiment was done four times with three plants in each experiment. The same number of plants was used for the noninoculated control. Dark brown rot of stems and leaves developed on 66.7% of inoculated plants within 2 days after inoculation. P. aphanidermatum reisolated from diseased tissues was morphologically identical to the original isolate OPU743. Noninoculated control plants showed no symptoms. P. aphanidermatum has been described on B. rubra in Brazil (1), but has not been reported from other regions of the world. To our knowledge, this is the first report of P. aphanidermatum on B. rubra in Japan.

References: (1) C. F. Robbs. A Lavoura 74:43, 1972. (2) A. J. Van Der Plaats-Niterink. Stud. Mycol. 21:1, 1981.