Canopy reflectance as a measure of disease in tall fescue

Measurement of changes in canopy reflectance of tall fescue (Festuca arundinacea Schreb.), which result from deterioration of tissues caused by disease organisms, is a potential unbiased method for quantifying disease. Canopy reflectance in eight spectral bands between 430 and 840 nm and 18 vegetation indices derived from the spectral bands were regressed against visual severity estimates of Rhizoctonia blight, caused by Rhizoctonia solani Kuhn, and gray leaf spot, caused by Pyricularia grisea (Cooke) Sacc. Reflectance within the 810-nm band exhibited the strongest relationship (19% less than or equal to r2 less than or equal to 63%) with visual severity estimates of Rhizoctonia blight and gray leaf spot. Reflectance in the visible or near infra-red wavelengths was similar to tall fescue blighted by either fungal pathogen. A significant (P less than or equal to 0.05) negative linear relationship was observed between canopy reflectance in the 810-nm band and the severity of either Rhizoctonia blight or gray leaf spot. As Rhizoctonia blight decreased with increases in the application rate of the fungicide flutolanil (N-[3-(1-methylethoxy)-phenyl]-2-trifluoromethyl benzamide) and the proportion of a resistant cultivar in tall fescue blends, canopy reflectance in the 810-nm band increased. However, models based on canopy reflectance had twice as much unexplained variability than models based on visual severity estimates of Rhizoctonia blight. Factors other than disease influenced variability in reflectance of light from tall fescue canopies