Automatic detection of powdery mildew on grapevine leaves by image analysis : optimal view-angle range to increase the sensitivity

Powdery mildew is a major fungal disease for grapevine ( Vitis vinifera L.) as well as for other important specialty crops, causing severe damage, including yield loss and depreciation of wine or produce quality. This disease is thoroughly controlled by uniform spraying of vineyards with agrochemicals according to a calendar, which can easily result in ten to fifteen fungicide applications in several grapevine-growing areas. Since primary infections are localized in discrete foci rather than being uniformly diffused, there are potential benefits linked to the development of systems able to detect initial infection foci and operate targeted treatments instead of the current homogenous and unselective sprayings. Proximal optical sensing is a major candidate for becoming the preferred technique for identification of foci for powdery mildew in grapevine and other specialty crops, but detection sensitivity of symptoms in the early-middle stage can yield largely limited results due to the combination of small dimensions, low density, and spatial arrangement of thin fungal structures.This study investigated how the detection sensitivity (i.e., the portion of diseased tissue correctly recognized by the system) can be improved, especially for early-middle symptoms by means of sensing measurements carried out from an angle, rather than perpendicularly to the leaf's surface. To this aim, a multispectral imaging approach was applied to 35 grapevine leaves (10 used as calibration and 25 as validation samples) that were imaged at five different view angles from 0° (camera perpendicular to the leaf surface) up to 75°. Detection sensitivity was evaluated by applying to the validation images an algorithm based on the combination of two spectral indexes. The used algorithm was separately trained basing on the calibration set of images.Overall results indicate that detection sensitivity generally increases as the view angle is increased, with a peak value obtained for images acquired at 60°. In particular, for tissue with early-middle symptoms, the algorithm's sensitivity exhibits a dramatic improvement, from 9% at 0° up to 73% at 60°.Provided that the adopted training system results in rather homogenous leaves orientation, these findings suggest that field-sensing systems for detecting initial foci of grapevine powdery mildew can achieve improved results by providing the capability of measuring the canopy from a view angle in the range of 40-60°.