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First Report of Guinea Grass Smut Caused by Conidiosporomyces ayresii in Japan

Guinea grass (Panicum maximum Jacq.) is an important C-4 perennial plant that grows in southern Japan. In March 2010, a smut disease was found in grass that is cultivated in the Ishigaki Islands, Okinawa, in southernmost Japan. Spikelets of susceptible cultivars were swollen and filled with gray spore masses and seed production was substantially reduced. Two single-spore isolates of a smut fungus were obtained from infected spikelets and deposited at the NIAS Genebank, Japan as MAFF511519 and 511520. The 28S rDNA sequences of the isolates were analyzed as described by Boekhout et al. (1). The 28S rDNA sequence (GenBank Accession No. AB647346) of isolate MAFF511519 matched that of Conidiosporomyces ayresii (GenBank Accession No. AY819017) isolated from P. maximum with 99.8% similarity. Spores were pale brown to brown, globose to subglobose, verrucose, and 14 to 16 × 15 to 18 μm in diameter with relatively thick walls of 2 to 3 μm. With scanning electron microscopy, warts appeared dense and short with pointed tips. Spores germinated under wet conditions and produced masses of basidiospores. Basidiospores were aseptate, long, cylindrical, straight to slightly curved, 20 to 37 × 2 to 3 μm, and often germinated into Y-shaped conidia. This description matches previous descriptions (3) of C. ayresii (Berk.) Vánky (Tilletia ayresii Berk.) of the smut pathogen of guinea grass (2). The smut fungus was identified as C. ayresii on the basis of morphology and molecular phylogenetic analysis. To produce inoculum, the isolates were grown on potato dextrose agar at 25°C in the dark for 7 days. Two plants of cv. Ryukyu 5-gou with half-flowering heads were grown in a greenhouse for approximately 1 month and then inoculated by atomizing them with conidial suspensions of each isolate (10⁶ conidia/ml). A plant sprayed with sterilized distilled water served as the control. Inoculated heads were covered with plastic bags for 48 h at 25°C. After 30 days, all inoculated plants were symptomatic with swollen spikelets releasing gray spores. Controls remained asymptomatic. The smut fungus was reisolated from released gray spores, confirming Koch's postulates. To our knowledge, this is the first report of smut caused by C. ayresii on guinea grass in Japan.

References: (1) T. Boekhout et al. Stud. Mycol. 38:175, 1995. (2) J. M. Lenné and P. Trutmann. Diseases of Tropical Pasture Plants. CAB International, Wallingford, UK, 1994. (3) K. Vánky and R. Bauer. Mycotaxon 43:427, 1992.