Department of Plant Protection, Huazhong Agricultural University, Wuhan, Hubei, 430070, China
Department of Environmental Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada
Curvularia leaf spot affects many species of grasses worldwide but has not been previously reported from central China. In October 2003, symptoms were observed on Festuca arundinacea in a city park in Yichang, Hubei. Fungicides were not used at this site. Leaf spots formed on the foliage after a few days of rainfall. In the most heavily infected areas, the incidence of disease was one leaf in five with spots. Overall, there was an average of 1 leaf in 50 with spots. Infected leaves most commonly bore a single spot, but some leaves had as many as 10 small spots. The spots were round, 5 to 10 mm across, and gray brown in the center with a dark margin. The samples were brought to the lab, and isolations were made on acidified potato dextrose agar (PDA; 3% acetic acid). After 3 days at room temperature, 10-mm-diameter white cultures with appressed mycelium were observed. Two days later, the colonies acquired a greenish cast particularly at the centers, and these yielded spores were identified as Curvularia affinis according to Ellis (1). The key characteristics were a scarcely protuberant hilum, spore size of 25 × 11 μm, and four-septate, curved conidia. DNA sequencing of the internal transcribed spacer region of ribosomal DNA was used to confirm the identity, and aside from sequence length differences, the match with a C. affinis sequence in GenBank was 100%. Inoculating the fungus onto field-grown F. arundinacea that had been washed, cut, and placed in 15-cm-diameter glass petri plates tested pathogenicity. The stems were at least 10 cm long and bore multiple attached leaves. The plates were layered with tissue paper that had been wetted and autoclaved. Sterile water was added to ensure sufficient moisture, and the tissue paper was folded over the bottom 1 cm of stems. Growing the fungus on PDA produced inoculum. Of the eight stems in the plate, one-half were used as a noninoculated check, while the remaining four stems were inoculated with three 5-mm-diameter agar plugs per stem. The plates were incubated at 25°C under continuous fluorescent light, and the inoculum plugs were removed after 48 h. Within 24 h after plug removal, lesions began to appear on the leaves. Circular brown spots were initially less than 5 mm wide, but within 2 days they darkened and grew to 10 mm wide and some of the lesions had a bright yellow margin. No lesions were found on noninoculated stems, which remained turgid and green throughout the trial. C. affinis was successfully reisolated from symptomatic tissue, completing Koch's postulates. To our knowledge, this is the first report of C. affinis on a turfgrass species, although it has been previously found on rice (1).
Reference: (1) M. B. Ellis. Dematiaceous hyphomycetes VII: Curvularia, Brachysporium, etc. Mycol. Pap. 106:1, 1966.