U.S. Department of Agriculture-Agricultural Research Service, Waste Management & Forage Research Unit, Mississippi State, MS 39762
Excised leaves of bermudagrass were inoculated with mycelium of isolates of Bipolaris, Exserohilum, Curvularia, and Drechslera spp. in water agar plates to evaluate differences in susceptibility of leaf tissue, virulence of pathogens, and quantitative resistance of bermudagrass genotypes. Isolates of nine 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 or genera. Severity of symptoms induced by most isolates increased progressively from younger to older leaves. Within and across leaf positions, numerous significant differences in virulence of isolates within fungal species and between species were observed. Among 40 randomly selected bermudagrass genotypes, a continuous quantitative gradient was observed for mean scores of disease severity in excised leaves inoculated with E. rostratum. Numerous significant differences were observed within this gradient, and severity of symptoms in the most susceptible genotypes was approximately double that in the most resistant. When intact 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 and significant (P = 0.05) correlations between results with excised leaves and intact foliage were observed. However, the range of differences in disease severity between genotypes was more narrow in intact foliage than in excised leaves. Results indicate that the excised leaf inoculation technique can be used to evaluate the relative resistance of bermudagrass genotypes to E. rostratum for use in programs to breed for quantitative host resistance.