Effect of Rhizoctonia resistance in tall fescue on fungicide efficacy: A turfgrass IPM model

Effects of resistance in tall fescue cultivars to R. solani on efficacy of the fungicide flutolanil were studied in a controlled environment and the field. Six levels of Rhizoctonia resistance were formulated in seed blends of the cultivars Kentucky 31 (moderately resistant to R. solani AG-1-Ia) and Mojave (susceptible) in ratios ranging from 100:0 to 0:100% Kentucky 31:Mojave. Models developed from severity data of Rhizoctonia blight to describe the effect of increasing Rhizoctonia resistance in a blend described a linear decrease in disease progress as the proportion of Kentucky 31 was increased in a blend.; Resistance components of size, area, and expansion rate of lesions caused by R. solani were examined on plants of Kentucky 31 and Mojave. Kentucky 31 had significantly wider leaves and lesions than Mojave. However, a significantly greater expansion rate of the lesions was detected in Mojave, compared to Kentucky 31, and lesions covered a greater proportion of leaf width on Mojave than on Kentucky 31.; Blends of tall fescue cultivars were treated with six different rates of the fungicide flutolanil ranging from the label rate of 3.43 kg a.i. ha{dollar}sp{lcub}-1{rcub}{dollar} to 1.14 kg a.i. ha{dollar}sp{lcub}-1{rcub}.{dollar} Incorporation of Kentucky 31 in a blend significantly {dollar}(Ple 0.05){dollar} improved the efficacy of flutolanil in suppressing the incidence and severity of Rhizoctonia blight in two out of three studies. Lack of significant interaction between cultivar blend and flutolanil rate indicated that the integrated effects on the suppression of Rhizoctonia blight were additive. Applications of flutolanil at less than label rates failed to limit disease to acceptable levels ({dollar}<{dollar}10% severity) on any of the cultivar blends tested. Combined effects of lower shoot density, wider leaves, and a slower lesion expansion rate in Kentucky 31 delayed the onset of exponential increase in the progress of Rhizoctonia blight and improved the efficacy of flutolanil, as compared to Mojave.; In field experiments, reflectance of near infra-red (midpoint 810 nm) light from a tall fescue canopy had the best relationship with visual disease severity estimates. However, other environmental and agronomic factors influenced canopy reflectance reducing accuracy in measuring disease.