Spectral characteristics of peanut crop infected by late leafspot disease under rainfed conditions

Abstract

A leafspot susceptible peanut cultivar (cv JL 24) was sown during kharif season of 2005 in rainfed region of Andhra Pradesh, India. Five disease levels (scale 0-4) were created in the field by differential fungicidal spray schedule, and each treatment was replicated four times. Spectral data was recorded at 2 nm intervals using a portable spectroradiometer within a spectral range of 300-1100 nm. The loss of leaf pigments (Chlorophyll a & b) due to diseases was quantified using spectrophotometer. The red and infrared reflectance values between 620- 680 and 770-860 nm, respectively were used for calculating Normalized Difference Vegetation Index (NDVI). Significant reduction in chlorophyll content was observed only when the disease reached a stage of scale 2 and above. The ratio of chlorophyll a to b showed a declining trend as the number of spots per leaf increased. From the spectral reflectance studies typical chlorophyll absorption bands (350 – 500 and 620-690 nm) of healthy and diseased plants could not be differentiated. However, the difference was evident in the chlorophyll reflection bands (522- 600 nm). The infrared spectral region between 700-850 nm was found to be sensitive to canopy disease stress. The reflectance (%) in this part of spectrum was higher for healthy plants compared to diseased plants. But, the low level of disease intensity (scale 1) was not differentiated by the spectral reflectance. The NDVI value for 82 days old healthy crop was in the range of 0.45 to 0.50, while the same for diseased plants was between 0.34 and 0.45. This study finds potential application of remote sensing techniques in detection of plant diseases.