Relationships between defoliation, canopy reflectance and pod yield in the peanut-late leafspot pathosystem

Defoliation of peanut (caused by Cercosporidium personatum), and reflected radiation (800 nm wavelength) of sunlight from peanut canopies, were assessed weekly beginning approximately 60 days after planting. Visual and reflectance assessments obtained with a hand-held, multispectral radiometer continued until one week prior to digging. Critical-point yield-loss models were developed using either percentage defoliation or percentage reflectance values as the independent variable and pod yield (kg ha(-1)) as the dependent variable. Yield reductions in the four experiments ranged from 52.9 to 79.0%. Based upon coefficients of determination and the standard errors for predicted y-values (yield), the best critical-point models for either assessment method were obtained when peanuts were assessed 2-3 weeks prior to digging. Critical point models using percent reflectance data as the independent variable explained more of the variation in peanut pod yield and had lower standard errors of the y-estimate than critical point models using percent defoliation values as the independent variable. Percent reflectance values explained 96.3-99.3% of the variation in biomass. This study supports the hypothesis that measurements of healthy green leaf area (as estimated by percent reflectance) have a better relationship with pod yield than defoliation based assessments.