Components of Resistance to Magnaporthe grisea in 'Coyote' and 'Coronado' Tall Fescue

The components of resistance in tall fescue to Magnaporthe grisea, the causal agent of gray leaf spot, were measured in growth chamber experiments. Cultivars ranging in susceptibility to M. grisea were selected: 'Kentucky 31' (susceptible), 'Rebel III' (moderately susceptible), 'Coronado' (resistant), and 'Coyote' (resistant). Plants were inoculated with nine M. grisea isolates representing five clonal lineages associated with tall fescue in Georgia. Compared to Kentucky 31, Coronado and Coyote exhibited longer incubation and latent periods, reduced rates of disease progress and lesion expansion, and lower final disease incidence, final foliar blight incidence, final mean lesion length, area under the lesion expansion curve, and area under the disease progress curve. No evidence of hypersensitive response was observed, all M. grisea isolates completed the disease cycle by producing secondary inoculum, and no differential response to isolates from different clonal lineages was detected in Coronado and Coyote. These results indicate that Coronado and Coyote have partial resistance to M. grisea. Measurement of resistance components using primary parameters and derived parameters yielded similar results. Foliar blight incidence data exhibited increased variation relative to other parameters and was less powerful for detection of M. grisea resistance. Measurements of incubation period, latent period, final disease incidence, and final mean lesion length were the most effective and efficient methods for detecting M. grisea resistance in tall fescue.