Light-to-dark brown leaf spots and general chlorosis were observed on ‘Alamo’ switchgrass (Panicum virgatum L.) grown in ornamental plantings on the campus of the University of Tennessee in Knoxville in December 2007. Disease distribution was patchy, infecting ~10% of plants. Patches had mild to severely infected plants with stunting in areas of severe infection. Symptomatic leaf tissue was 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 26°C in darkness for 5 days. A sporulating, dematiaceous mitosporic fungus was observed and transferred to potato dextrose agar (PDA). Conidiophores were single, light brown, multiseptate, mostly straight, polytretic, geniculate, and sympodial. Conidia were 17.5 × 12 (22) to 30 × 14 (12.5) μm, oval, light brown, and distoseptate, with one to three septa and a flattened hilum on the basal cell. Conidia germinated from both poles. The causal agent was identified as Bipolaris spicifera (Bainier) Subram. Morphological features were as described for B. spicifera (2). Pathogenicity studies were conducted with 5-week-old ‘Alamo’ switchgrass plants grown from surface-sterilized seed in 9 × 9-cm pots containing 50% ProMix Potting and Seeding Mix (Premier Tech Horticulture, Rivière-du-Loup, Québec, Canada) and 50% Turface ProLeague (Profile Products, Buffalo Grove, IL) (vol/vol). Ten replicate pots with ~20 plants each were sprayed with a spore suspension of 4.5 × 106 spores/ml of sterile water prepared from 6-day-old cultures grown on PDA. Plants were subjected to high humidity for 45 h then incubated at 25/20°C with a 12-h photoperiod in a growth chamber. Leaf spot symptoms similar to the original disease appeared on plants in each of the 10 replicate pots 6 days postinoculation. Lesions were excised from leaves, surface sterilized, plated on water agar, and the resulting cultures were again identified as B. spicifera. The internal transcribed spacer (ITS) region of ribosomal DNA from the original isolate used for inoculation and the reisolated culture recovered from plants in the pathogenicity studies were amplified with PCR using primers ITS4 and ITS5 (3). PCR amplicons of ~560 bp were obtained from both isolates and sequenced. Amplicon sequences were identical and the sequence was submitted to GenBank (Accession No. HQ015445). The DNA sequence had 100% homology to the ITS sequence of B. spicifera strain NRRL 47508 (GenBank Accession No. GU183125.1) that had been isolated from sorghum seed. To our knowledge, leaf spot caused by B. spicifera has not been described on switchgrass (1). B. spicifera can be seedborne and has been reported on turfgrass seed exported from the United States to Korea (2). As switchgrass is transitioned from a prairie grass to a biofuels crop planted in large acreages, disease incidences and severities will likely increase, necessitating rapid disease identification and cost effective management strategies.
References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, 4 August 2010. (2) H.-M. Koo et al. Plant Pathol. J. 19:133, 2003. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds, Academic Press, San Diego, 1990.
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