First Report of Spot Blotch and Common Root Rot Caused by Bipolaris sorokiniana on Switchgrass in Tennessee

Light-to-dark brown, irregular-shaped leaf spots, chlorosis, necrotic roots, and severe stunting were observed on 'Alamo' switchgrass (Panicum virgatum L.) grown on the campus of the University of Tennessee in December 2007. Symptomatic leaf and root samples were surface sterilized, air dried on sterile filter paper, and plated on 2% water agar amended with 10 mg/liter of rifampicin (Sigma-Aldrich, St. Louis, MO) and 10 μl/liter of 2,4 EC Danitol miticide (Valent Chemical, Walnut Creek, CA). Plates were incubated at 25°C in darkness for 4 days. A sporulating, dematiaceous mitosporic fungus was noted and transferred to potato dextrose agar (PDA). Conidia were ovate, oblong, mostly straight, and olive to brown with three to nine septa. Conidial dimensions were 12.5 × 27.5 (17.5) to 20 × 77.5 (57) μm. Conidia were produced on single, light brown, multiseptate conidiophores that were polytretic, geniculate, and sympodial. Morphological features were as described for Bipolaris sorokiniana (Sacc.) Shoemaker (teleomorph = Cochliobolus sativus) (2,3). Disease assays were conducted with 5-week-old 'Alamo' switchgrass grown from surface-sterilized seed. Ten 9 × 9-cm with ~20 switchgrass seedlings were sprayed with 2.4 × 10 spores/ml of sterile water. Plants were subjected to high humidity created by enclosure in a plastic bag for 45 h. The bag was removed and plants were incubated at 25/20°C with 50 to 60% relative humidity. During the incubation, plants were maintained in growth chamber with a 12-h photoperiod of fluorescent and incandescent lighting. Foliar leaf spot symptoms appeared 6 to 10 days postinoculation for plants in all 10 replicates and necrotic lesions were observed on roots. Foliar lesions and diseased roots were surface sterilized, plated on water agar, and resultant fungal colonies were identified as B. sorokiniana. The internal transcribed spacer (ITS) and mitochondrial small subunit (SSU) regions of ribosomal DNA from the original isolate, and the isolate recovered from plants in the pathogenicity assay, were amplified with PCR, with primer pairs ITS4 and ITS5 and NMS1 and NMS2. PCR amplicons of ~551 and 571 bp were obtained with the two primer pairs, respectively. Both amplicons were obtained from both isolates and sequenced. Amplicon sequences from the original isolate and re-isolate were identical and the sequences were submitted to GenBank (Accession Nos. HQ611957 and HQ611958). The ITS sequences had 98% homology to 23 B. sorokiniana isolates