Relationships between defoliation, leaf area index, canopy reflectance, and forage yield in the alfalfa-leaf spot pathosystem

Field experiments were conducted to quantify the relationships between defoliation, leaf area index (LAI), canopy reflectance, and alfalfa yield. A range of disease levels was achieved by varying the frequency and type of fungicides used at two locations in Iowa (Ames and Nashua). Canopy reflectance (810 nm) was measured weekly using a hand-held, multispectral radiometer. Percentage defoliation was also obtained weekly by destructively sampling alfalfa stems from each plot and then visually counting the presence or absence of the primary leaf at each node of the main stem. LAI was measured with a leaf area meter by destructively sampling three 0.48 m diameter circles from each of 24 (1998) or 32 (1999) plots. Significant relationships between canopy reflectance (810 nm), percentage defoliation, yield, and LAI were detected using linear regression. Percentage reflectance (810 nm) models explained 15 and 12% more of the variation in yield and LAI, respectively, than models using percentage defoliation as the independent variable average over all the models. This study demonstrated that percentage reflectance measurements had a better relationship with alfalfa yield and LAI than visual percentage defoliation assessments, and that percentage reflectance (810 nm) can be used to quantitatively (and non-destructively) estimate yield and LAI in alfalfa.