Submitted to: Journal of Production Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 12, 2000
Publication Date: N/A
Interpretive Summary: A major challenge for American agriculture at the end of the twentieth century is to continue to produce large quantities of plant and animal products for a rapidly growing population without causing serious pollution of the water, soil, and air. Concentrated animal production facilities, where large numbers of animals are produced in small, confined spaces, can be major sources of environmental pollution on account of large quantities of waste materials that are generated. The major polluting substances in animal wastes are inorganic nutrients, especially phosphorus and nitrogen, that enrich water to toxic levels. To remove these elements without environmental pollution, animal waste materials are applied to pastures, and when the grass is cut for hay and removed, excess nutrients that were incorporated into it during growth also are removed. Diseases of grasses that prevent their normal growth and the incorporation of waste-derived nutrients can inhibit or destroy this system of nutrient removal. In this study, six Helminthosporium-type fungal pathogens were isolated from diseased bermudagrass at three sites in Mississippi where swine effluent was being applied. Differences in the frequency and virulence of these pathogens were related to the severity of symptoms in the field. All symptoms, including death of crowns, stolons, and roots, were reproduced in bermudagrass following inoculations of foliage with the most virulent pathogens, and these were reisolated from all symptomatic tissues. Yield reductions of 48-63% were observed in diseased samples following growth in the greenhouse. These results indicate that Helminthosporium-type diseases may seriously inhibit forage production by bermudagrass grown for nutrient removal on swine waste application sites.
Technical Abstract: Plant diseases that reduce growth of forage crops on animal waste application sites may limit removal of waste-derived nutrients and disrupt control of environmental pollution. However, no studies have documented the importance of plant diseases on waste disposal sites in the southeastern United States. During 1998, leaf, stem, and root disease symptoms were observed in common bermudagrass (Cynodon dactylon [L.] Pers.) on three swine waste application sites in Mississippi. Severity ranged from small, individual patches of diseased plants to large, coalesced patches that caused extensive dieback and little forage production. Species of Exserohilum, Bipolaris, and Curvularia grew or sporulated from 82-100% of assayed tissues, and six species were identified among 266 isolates. E. rostratum and B. spicifera were the predominant species in the most severely diseased stand. In inoculation experiments with bermudagrass, E. rostratum was the most virulent pathogen; B. stenospila, B. hawaiiensis, and B. spicifera were intermediate in virulence; and C. lunata and C. geniculata were least virulent. All species were reisolated from symptomatic leaf tissue initially, and species of Exserohilum and Bipolaris later were reisolated from necrotic crowns and roots. In the greenhouse, mean dry-matter yields of diseased turf were 37-52% of yields of healthy turf. These results indicate that Helminthosporium diseases may reduce growth and survival of bermudagrass grown for nutrient removal on swine effluent application sites, and that E. rostratum and Bipolaris spp. are the most damaging pathogens.