Indiangrass or yellow indiangrass (Sorghastrum nutans L.) is a warm-season, perennial grass grown for livestock forage, erosion control, wildlife food and cover, landscaping, and more recently, as a biofuel crop. In August of 2007, foliar lesions were observed on plants within mature stands of a number of cultivars and populations of indiangrass at the USDA-NRCS Plant Materials Center in Big Flats (Chemung County), NY. In subsequent years, similar lesions were observed in both mature and immature (less than 3 years old) stands of indiangrass in Chemung and Tompkins counties. Lesions were elliptical to irregular with distinct or diffuse purple margins often surrounded by tan-to-maroon halos and were sometimes observed on the leaf sheath and stem. Lesions were generally less than 2 cm long, approximately 2 mm wide, and often coalesced when disease was severe. Centers became necrotic and often developed numerous acervuli with black setae. After 2 to 5 days of incubation in moist chambers, symptomatic leaf tissue developed acervuli containing masses of cream-colored spores. Spores streaked onto potato dextrose agar containing streptomycin gave rise to cultures with gray mycelium often accompanied by sporulating avervuli. The fungus was identified as Colletotrichum caudatum (Peck ex Sacc.) Peck on the basis of cultural characteristics and conidial morphology (2). Conidia were one celled, hyaline, fusiform, and falcate with a filiform, caudate appendage. Conidial length averaged 28 μm (21 to 45 μm), width averaged 5 μm (4 to 6 μm), and the appendage averaged 15 μm (5 to 29 μm) long. The sequence of the rDNA internal transcribed spacer (ITS) regions of an isolate from ‘Rumsey’ indiangrass in Chemung County, NY (Cc004NY07, GenBank Accession No. JF437056) exhibited 98% nucleotide identity to C. caudatum isolates (GenBank Accession Nos. AB042304 and AB042305) collected from bentgrass (Agrostis sp. L.) and cogongrass (Imperata cylindrica L.) in Japan (1). Colletotrichum species from grasses are not discriminated solely on ITS sequence, but the unique caudate appendage is diagnostic of C. caudatum. Pathogenicity of the sequenced isolate plus a second isolate from ‘Rumsey’ indiangrass (Cc006NY07) was evaluated in greenhouse experiments. Eight-week-old plants of indiangrass population ‘PA Ecotype’ (Ernst Conservation Seeds, Meadville, PA) were inoculated with conidial suspensions (2 × 106 conidia/ml) of C. caudatum. Twelve plants were sprayed with either inoculum or sterile water (as the control treatment) until runoff with a spray bottle. After inoculum had dried, plants were placed in a mist chamber for 48 h. Plants were then returned to the greenhouse and observed for disease development, which occurred within 1 week of inoculation. No symptoms developed on the control plants. Foliar lesions closely resembled those observed in the field. C. caudatum was reisolated consistently from symptomatic tissue collected from greenhouse experiments. To our knowledge, this is the first report of C. caudatum causing anthracnose on indiangrass in New York, though it has been reported in the adjoining states of New Jersey (2) and Pennsylvania (3). Indiangrass cultivars should be assessed for susceptibility to regional isolates of C. caudatum prior to expanded regional production of indiangrass as a biofuel crop.
References: (1) J. Moriwaki et al. J. Gen. Plant Pathol. 68:307, 2002. (2) T. R. Nag Raj. Can. J. Bot. 51:2463, 1973. (3) K. E. Zeiders. Plant Dis. 71:348, 1987.