Sclerotinia homoeocarpa causes dollar spot on many grass species; however, it has not been described on switchgrass (Panicum virgatum L.) as a host. In August 2010, bleached, tan-to-straw-colored leaf spots with dark brown-to-reddish brown margins were found in patchy distribution in small field plots of ‘Alamo’ switchgrass at the East Tennessee Research and Education Center, Knoxville, TN. The plots had been planted to switchgrass for the past 21 years. Disease lesions covered 75 to 80% of leaf tissue per patch and were also evident on stems. To identify the pathogen, center portions of diseased leaves were cut into 20- to 30-cm segments, surface disinfested (95% ethanol for 30 s, 10% bleach for 1 min, and 95% ethanol for 30 s), and dried. Disinfested leaves (5-cm sections that included a leading edge of a lesion) were plated on potato dextrose agar (PDA). Plates were incubated at 22°C. Within 12 h, white, fluffy, aerial mycelium developed. Viewed from above, colonies were tan to cinnamon in color with a dark brown-to-black substratal stroma on and in the agar, which appeared brown as viewed from below the petri dish. No spores were observed. Morphological characteristics of colony and hyphal growth were identical to those of S. homoeocarpa F.T. Bennett (1). Pathogenicity studies were conducted with 6-week-old ‘Alamo’ switchgrass grown from scarified (2), surface-disinfested seed. Nine (9 × 9-cm2) pots with 18 plants each were inoculated with 20 mycelial plugs (6-mm diameter) per pot, taken from 3-to-5-day-old fungal cultures. Two control pots were inoculated with sterile PDA plugs and subjected to the same conditions. Plugs were placed on leaf surfaces and around the plant crowns. Plants were subjected to high humidity by enclosure in a plastic bag and incubated in a growth chamber at 25/20°C with a 12-h photoperiod. Plastic bags were removed after 48 h. Leaf spots appeared as early as 2 days postinoculation, with full symptoms after 2 weeks for eight of nine replicates. Control plants had no symptoms. The fungus was cultured from leaf spots and stem lesions of inoculated plants as described above. The same disease and fungus were observed, completing Koch's postulates. The internal transcribed spacer (ITS) regions of ribosomal DNA from the original isolate used for inoculation and from the isolate recovered from plants in the pathogenicity assay were amplified with PCR with primers ITS4 and ITS5 (4). PCR amplicons of ~565 bp were sequenced; sequences of amplicons from the original isolate and reisolate were identical and submitted to GenBank (Accession No. HQ850151). The sequence had 99% homology with several S. homoeocarpa isolates in GenBank, including three isolates from buffalograss in Oklahoma (Accession Nos. EU123800, EU123802, and EU123803). The mitochondrial small subunit region was amplified from the original isolate with primers NMS1 and NMS2 (3). The resultant 536-bp fragment was sequenced and submitted to GenBank (Accession No. HQ850152), but no S. homoeocarpa sequences were available for comparison. To our knowledge, this is the first confirmed report of switchgrass as a natural host for S. homoeocarpa, extending the known host range for the pathogen.
References: (1) F. T. Bennett. Ann. Appl. Biol. 24:236, 1937. (2) K. D. Gwinn et al. Crop Sci. 31:1369, 1991. (3) K. N. Li et al. Appl. Environ. Microbiol. 60:4324, 1994. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, NY, 1990.