Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2003
Publication Date: 10/1/2003
Citation: Pratt, R.G. 2003. An excised-leaf inoculation technique for evaluating host-pathogen interactions and quantitative resistance of bermudagrass genotypes to dematiaceous hyphomycetes. Phytopathology. 93:1565-1571. Interpretive Summary: Bermudagrass is a major forage grass that is grown on animal waste application sites to absorb excess nutrients from wastes and prevent their release into surface waters to cause pollution. Effective use of bermudagrass for nutrient absorption and pollution control requires maximal forage production with minimal losses to fungal diseases. The best way to minimize disease losses is to breed populations of bermudagrass with resistance to the most virulent (damaging) fungal pathogens that cause disease, but few such populations have yet been developed. Effective breeding for resistance to a virulent fungal pathogen requires precise identification of the most resistant individual plants in a population. In this study, an excised-leaf inoculation technique was developed and used to evaluate both the virulence of fungal pathogens and the resistance of individual bermudagrass plants to one of the major pathogens. Leaf sections from bermudagrass stems were excised, placed on agar, inoculated with numerous species and strains of fungal pathogens, and evaluated for symptom development. Severity of symptoms differed between fungal pathogens and between leaves of different ages. The most virulent pathogens and the leaves in which resistance could best be expressed were determined. Among forty individual bermudagrass plants, significant differences in resistance to one major pathogen were identified, and the highest levels of resistance also were expressed when the fungus was applied to whole plants. These results demonstrate that the excised-leaf inoculation technique can be used to evaluate the virulence of different fungal pathogens of bermudagrass, and also to identify the most resistant plants for use in a breeding program.
Technical Abstract: Excised leaves of bermudagrass were inoculated with mycelium of isolates of Bipolaris, Exserohilum, Curvularia, and Drechslera spp. in plates of agar to evaluate differences in susceptibility of leaf tissue, virulence of pathogens, and quantitative resistance of bermudagrass genotypes. Isolates of 9 species of pathogens induced similar symptoms of light-to dark-brown necrosis and bordering chlorosis in excised leaves that were not distinct for individual species of genera. Severity of symptoms induced by most isolates increased progressively from younger to older leaves. Across leaf ages, numerous significant differences in virulence of pathogens were observed, and differences between isolates within species were often as great as differences among species. Among 40 randomly selected bermudagrass genotypes, a continuous quantitative gradient, with numerous significant differences within it, was observed for mean scores of disease severity in excised leaves inoculated with E. rostratum. Severity of symptoms in the most susceptible genotypes was approximately double that of the most resistant. When foliage of genotypes from the resistant and susceptible extremes of the gradient was inoculated with spores of E. rostratum, corresponding differences in severity of symptoms also were observed, but the range of differences was more narrow than in excised leaves. Results indicate that the excised leaf inoculation technique can be used to identify bermudagrass genotypes with relative phenotypic quantitative resistance to E. rostratum for use in programs to breed for quantitative host resistance.