Disease‐induced alterations in the reflectance spectrum of grape leaves

Abstract

Context and purpose of the study ‐ Phytopathogenic diseases impact the development and yield of grapevines, resulting in economical, social and environmental losses. Sick plants have their metabolism changed, leading to alterations in their reflectance spectra. Little is known on these alterations, and a better knowledge could be used in the development of sensors able to detect diseases through fast, non‐ destructive techniques. The present study was aimed at detecting spectral changes on the reflectance spectra of vines of cv. Cabernet Sauvignon, with early symptoms of downy mildew, powdery mildew, black‐ foot and Petri disease, describing the spectral domains where alterations are measurable with respect to healthy control vines. This information can be used to the development of low‐cost devices which can perform real‐time field measurements to early assessment of vineyard health status.

Material and methods ‐ Plants of Cabernet Sauvignon grown in pots and kept in a greenhouse were inoculated with the pathogens causing mildew, powdery mildew, black‐foot and Petri disease. In early stages of disease development, reflectance measurements were performed using a FieldSpec 3 spectroradiometer, which were compared with data from healthy plants. The investigation began with discriminant analysis, which revealed that symptomatic plants are indeed separated from the control ones. Reflectance spectra were therefore further investigated and alterations on the shape of the spectra, characteristic of each disease, were looked for. The disease descriptors were based on ratios between spectral features internal to a spectrum, a procedure which allowed the derivation of parameters intrinsic to each disease.

Results ‐ A set of thresholds, which are the intensity ratios of reflectance at selected wavelengths, was derived for the studied diseases. The selected wavelength ratios were 443/496, 443/573, 443/695, 443/1900, 496/573, 496/695, 516/1900, and 1900/2435 (values in nanometers), for which the spectra from symptomatic plants present shape changes of as much as 20% in reflectance with respect to healthy plants. The observed spectral deformations are larger for black‐foot and powdery mildew, but some wavelength ratios are also indicators of downy mildew and Petri disease. Data from near‐infrared are in general more useful, compared with measurements at 1900 and 2435nm.