Early sensing of peanut leaf spot using spectroscopy and thermal imaging

Early and late leaf spot are the most devastating, important fungal foliar diseases affecting peanut. The economic loss caused by peanut diseases justifies the necessity to develop more reliable detection methods. Point and image spectroscopy and thermal imaging were used in this study for the early detection of peanut leaf spot. The spectral reflectance factors differ significantly according to the health condition. The leaves of the healthy peanut showed a decreasing reflection in 1015 nm, whereas the heavily diseased leaves showed an increasing reflection. At the thermal infrared range, affected plants show a higher temperature than healthy ones. A presymptomatic decrease in leaf temperature using thermal imagery was found about 1.3°C lower than the healthy leaves. However, the diseased plant’s temperature was 2.2°C higher than that of the healthy one. The temperature difference allowed the discrimination between the infected and healthy leaves before the appearance of visible necrosis on leaves. Two simple indices, early leaf spot index (ELSI) and late leaf spot index (LLSI), were developed to allow early prediction of the peanut disease severity. The disease severity estimation using ELSI and LLSI has an overall accuracy of 78% and 89%, respectively. This demonstrates the efficiency of the proposed indices to estimate the peanut disease.