Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2005
Publication Date: 10/1/2005
Citation: Pratt, R.G. 2005. Variation in occurrence of dermatiaceous hyphomycetes on forage bermudagrass over years, sampling times, and locations. Phytopathology. 95:1183-1190. Interpretive Summary: Bermudagrass is widely grown on swine, poultry, and dairy farms where large quantities of animal wastes must be disposed without causing pollution of the environment. Animal wastes contain high levels of phosphorus (P), which causes eutrophic water pollution when it moves into surface water. When animal wastes are applied to bermudagrass, the P is absorbed by healthy, growing grass, and eutrophic water pollution is reduced or prevented. Plant diseases caused by "dematiaceous hyphomycetes", a group of related fungal species, infect bermudagrass and reduce its growth, survival, and ability to absorb P and prevent environmental pollution. Multiple species of these fungi are known to infect bermudagrass, but their variability in occurrence between farms, over years, and at different times within the growing season is not known. In this study, species of fungi present in samples of diseased bermudagrass collected from three swine farms during three years, at eight sampling times per year, were determined and compared. Eight species were observed in 3,600 samples. Most samples were infected by more than one fungus, and some by up to five. The eight fungi differed greatly in overall frequency and could be grouped into major and minor species on that basis. Different species predominated at different farms, but most did not vary greatly in frequency between years or during growing seasons. Results indicate that these diseases of bermudagrass on swine waste application sites are caused primarily by complexes of fungi rather than by single species. Components of the complexes differ between farms but usually do not vary greatly in frequency of occurrence over years or during growing seasons. More studies are needed on kinds of interactions that may occur between these disease-causing fungi that occur together on bermudagrass.
Technical Abstract: Leaf samples of forage bermudagrass with symptoms suggestive of infection by species of Bipolaris, Curvularia, and Exserohilum (dematiaceous hyphomycetes) were collected on three swine waste application sites in Mississippi at eight sampling times during each of 3 yr. Samples were assayed for presence of pathogens by observing sporulation on plated leaf tissue. Among 3,600 samples assayed, 8 species of the three genera were observed. Sporulation by one or more species of dematiaceous hyphomycetes was observed on 97% of plated symptomatic leaf samples. On all sites within all years, numbers of leaves infected with 1-5 pathogen species differed significantly (P=0.01) from a Poisson distribution; on most sites, more leaves were infected with two or three pathogens and fewer with four or five pathogens than expected. Significant (P=0.05) variation was attributed to pathogen species, sampling times, and species x time interactions. E. rostratum, C. lunata, and B. cynodontis were the most frequent species (47-69 %) across years and sites; B. spicifera and C. geniculata were intermediate in frequency (31 and 13%); and B. hawaiiensis, B. sorokiniana, and B. stenospila were least frequent (<1-2 %). Frequencies of individual species usually did not differ significantly between sampling times. For the five most common species, significant differences in frequency between locations, as determined by Chi-square tests, were commonplace. Results indicate that dematiaceous hyphomycetes typically infect forage bermudagrass on swine waste application sites in complexes rather than as individual species; that E. rostratum, C. lunata, and B. cynodontis are the predominant species in complexes; and that frequencies of individual pathogens often differ significantly between locations. Therefore, diseases caused by dematiaceous hyphomycetes on forage bermudagrass primarily involve dynamic complexes of major and minor pathogens that vary significantly in frequency over time and between locations.