Detection of powdery mildew in grapevine using remotely sensed UV-induced fluorescence

Powdery mildew is caused by Erysiphe necator (syn. Uncinula necator), an ascomycete fungus. It can induce severe damage to a vineyard including yield loss and depreciation of wine sensory characteristics. This disease as other fungal diseases is thoroughly controlled by pesticides. In order to reduce the use of pesticides and to improve early detection for this disease, we studied the potential of UV-induced imaging fluorescence to detect and quantify the colonization of a leaf by E. necator. Cuttings of grapevine (vitis vinifera cv. Carbernet-Sauvignon) were grown in a greenhouse. Twenty-four leaves were collected and fourteen were inoculated with the fungus on their adaxial side, the remaining leaves were used as control. Images of fluorescence were taken in a lab at six combinations of emission/excitation wavelength (F440(UV), F520(UV), F690(UV), F740(UV) and F690(blue) F740(blue)). Data treatments include proper image correction, interest area selection, fluorescence ratios computation and edge detection. Using the spatial average of F(440)/F(520) for the whole leaf area it was possible to detect the fungus starting at three days after inoculation (DAI). Using the edge detection algorithm it was possible to detect and quantify the colonization of the leaf starting at three DAI, with a lower coefficient of variation compared to leaf spatial average.