Diseases like downy mildew (DM) and powdery mildew (PM) are characterized by whitish symptoms on leaves of many plant species. Hyperspectral imaging (HSI) has been successfully used for the detection and identification of various diseases associated with different symptoms. Proximal HSI (400–1000 nm) was tested under controlled conditions for its potential to differentiate among whitish disease symptoms on leaves of apple and grapevine due to DM, PM, and a non-melanized mutant of apple scab at the leaf and tissue (microscopic) level. Spectral traits were analyzed by using difference spectra and spectral ratios, spectral vegetation indices like NDVI, and average brightness and half NIR increase introduced here and were confirmed by supervised spectral angle mapper classification. Although similar, spectral signatures of whitish symptoms were significantly different and could be used for spectral separation of diseases; differences were greater on the tissue level than on the leaf level. However, disease detection and differentiation were affected by spectral differences between plant species, leaf sides, the variability of symptoms in space and time, and the integrity of superficial pathogen structures. In the case of similar disease symptoms, additional spectral information on the effects of pathogens on plant metabolism, e.g., leaf water patterns, supports spectral differentiation of leaf diseases.
Oerke et al. (Tue,) studied this question.