First Report of Blast of Guinea Grass Caused by Pyricularia sp. LS-Group in Japan.

Abstract

Guinea grass (Panicum maximum Jacq.) is an important C-4 perennial herbage in the southern part of Japan. In February 2002, a blast disease was found on the grass cultivated on the Okinawa Islands, the southern most region of Japan. Early symptoms appeared as small, round or ellipsoid lesions on leaves. Lesions later expanded to 2 to 5 × 1 to 2 mm and were spindle shaped and grayish white in the central area with dark brown margins. We obtained three single-conidia isolates of a Pyricularia-like fungus from the lesions and deposited them in the NIAS Genebank, Japan as MAFF306662, 306671, and 306672. The isolates were grown under near-UV light on V8 juice agar for 7 days to produce conidia, and guinea grass plants of the seven- to eight-leaf stage grown from seeds in a green house, five plants for each isolate, were inoculated by atomizing them with the conidial suspension of 105 conidia/ml. The same number of plants sprayed with sterilized distilled water served as the control. The experiments were repeated twice. All plants were covered with plastic bags for 24 h at 25°C to maintain high relative humidity. After 7 days, all inoculated plants showed symptoms identical to those observed in the field. Controls remained symptom free. The Pyricularia-like fungus was reisolated from lesions on inoculated leaves. The morphologies of the isolates were observed and described from the colonies grown under the condition described above. Conidiophores were pale brown, emerging singly or in small groups, straight or flexuous, geniculate toward the apex, and 36 to 197 × 2 to 5 μm. Conidia were obpyriform, straight, colorless to pale brown, smooth, and 19 to 30 × 5 to 10 μm with two to three septa. The morphologies were the same as those of the description of the genus Pyricularia. Previously, all Pyricularia isolates from Gramineae had been identified as P. grisea, except for those from rice (3,4). However, a new taxonomy of Pyricularia spp. based on DNA analyses was proposed by Couch and Kohn (1). Only the isolates from Digitaria were classified as P. grisea and those from C-3 grasses classified as P. oryzae. However, the species names for the isolates from the other C-4 grasses were not described. We analyzed the sequences of the rDNA-ITS region (ITS1-5.8s-ITS2) and β-tubulin gene of the isolates from guinea grass following Couch and Kohn (1). The sequences of rDNA-ITS (GenBank Accession No. AB512785) and β-tubulin (AB512786) of the isolate MAFF306672 matched the sequences of those of the Pyricularia sp. LS-group (AB274426 and AB274458, respectively) isolated from Leersia oryzoides. Hirata et al. (2) reclassified Pyricularia isolates from Gramineae by multilocus phylogenetic analysis and showed that non-P. oryzae and non-P. grisea isolates could be classified into two groups of the Pyricularia sp., a LS- and a CE-group, corresponding to those isolated from Leersia spp. and Setaria spp. or Cenchrus spp. of grasses, respectively. Since no Magnaporthe teleomorph was produced by the crossing tests using the isolates, we identified the isolates from guinea grass as the Pyricularia sp. LS-group on the basis of their morphology and the molecular phylogenetic analysis. To our knowledge, this is the first report of blast on guinea grass in Japan.