First Report of a Leaf Spot Caused by Bipolaris oryzae on Switchgrass in New York

Switchgrass (Panicum virgatum L.) is a perennial grass with biofuel potential. From 2007 to 2010, foliar lesions were observed on first year and mature stands of switchgrass in various locations in New York. Foliar lesions were purple, elliptical (up to 1 cm) with either distinct or diffuse margins, and occasionally with yellow halos and/or white necrotic centers. After 2 to 5 days of moist chamber incubation, surface-sterilized, symptomatic leaf tissue produced conidia that when streaked onto potato dextrose agar containing 0.3 g of streptomycin per liter gave rise to cultures with gray-to-black mycelium that developed brown conidia. The fungus was identified as Bipolaris oryzae (Breda de Haan) Shoemaker on the basis of conidial morphology (1,2). Conidiophores were brown, straight, cylindrical, and multiseptate. Conidia were brown, curved, ellipsoidal tapering to rounded ends, with 3 to 14 septa. Conidia averaged 105 μm (54 to 160 μm) long and 16 μm (12 to 20 μm) wide. Sequences of the glyceraldehyde-3-phosphate dehydrogenase (GDP) gene of three isolates from Tompkins County (Cornell Accession and corresponding GenBank Nos.: Bo005NY07 [cv. Cave-in-Rock], JF521648; Bo006NY07 [cv. Kanlow], JF521649; and Bo038NY07 [cv. Shawnee], JF521650) exhibited 100% nucleotide identity to B. oryzae isolates (GenBank Nos. AY277282-AY277285) collected from switchgrass in North Dakota (1). Sequences of the rDNA internal transcribed spacer (ITS) regions of the isolates (Cornell Accession and corresponding GenBank Nos.: Bo005NY07, JF693908; Bo006NY07, JF693909; and Bo038NY07, JF693910) exhibited 100% nucleotide identity to B. oryzae isolates (GenBank Nos. GU222690-GU222693) collected from switchgrass in Mississippi (3). Pathogenicity of two of the sequenced isolates (Bo006NY07 and Bo038NY07) along with one other isolate (Bo116NY09 from 'Cave-in-Rock' in Cayuga County) was evaluated in the greenhouse. Six- to eight-week-old switchgrass plants were inoculated with conidial suspensions (40,000 conidia/ml) of B. oryzae. Inoculum or sterilized water was applied until runoff. There were three plants per treatment of each of 'Blackwell', 'Carthage', 'Cave-in-Rock', 'Kanlow', 'Shawnee', 'Shelter', and 'Sunburst'. After inoculum had dried, plants were placed in a mist chamber for 24 h and then returned to the greenhouse. Symptoms developed 2 to 4 days after inoculation for all cultivars. No symptoms developed on the control plants. Foliar lesions closely resembled those observed in the field. B. oryzae was consistently reisolated from symptomatic tissue collected from greenhouse experiments. B. oryzae was first reported as a pathogen of switchgrass in North Dakota (1) and more recently in Mississippi (3). To our knowledge, this is the first report of B. oryzae causing a leaf spot on switchgrass in New York. Observation of severe leaf spot in several field plots suggests that switchgrass populations should be screened for their reaction to regional isolates of B. oryzae prior to expanded production of switchgrass as a biofuel crop. References: (1) J. M. Krupinsky et al. Can. J. Plant Pathol. 26:371 2004. (2) R. A. Shoemaker. Can. J. Bot. 37:883, 1959. (3) M. Tomaso-Peterson and C. J. Balbalian. Plant Dis. 94:643 2010.