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Helminthosporium Spot Blotch of Switchgrass in Pennsylvania. K. E. Zeiders, Research Plant Pathologist, USDA, ARS, U.S. Regional Pasture Research Laboratory, University Park, PA 16802. Plant Dis. 68:120-122. Accepted for publication 8 August 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/PD-68-120.

Spot blotch caused by Helminthosporium sativum was the most prevalent and important disease on leaves of switchgrass (Panicum virgatum) in Pennsylvania from 1976 through 1981. H. sativum was isolated from switchgrass in six counties. Isolates of H. sativum from switchgrass differed in virulence on switchgrass and other hosts. Isolates from switchgrass and deertongue grass (Dichanthelium clandestinum) were about equally pathogenic on the two species and on other susceptible hosts. Caucasian bluestem (Bothriochloa caucasica) was not susceptible to H. sativum from either host. Corn (Zea mays) and rice (Oryza sativa) were moderately to highly susceptible to H. sativum from switchgrass. Field disease ratings showed that Ky-729 switchgrass was the most resistant of 11 cultivars to spot blotch during each of 5 yr. NY-4006, PMT-788-77, and Ky-1625 were more resistant to spot blotch than seven other cultivars. The differential reactions of cultivars in the field indicate that genetic variability for resistance to spot blotch exists. At one location, severity of spot blotch on switchgrass cultivars was greater in plots low in available soil phosphorus (P) compared with the same soil fertilized with P. Severity of spot blotch increased with time if plants were not clipped. This buildup of disease can be alleviated considerably if the grass is cut twice or possibly three times per year. Disease severity was related to duration of high relative humidity and leaf wetness at two locations. Results of this study indicate that if switchgrass is grown to any great extent in the humid areas of the eastern and northeastern United States, spot blotch will probably be the most important disease.

Keyword(s): disease resistance, warm-season grass.