Evaluation of "ground sensing" optical sensors for diagnosis of Plasmopara viticola on vines

The present work considers the possible use of two commercial optical devices, the GreenSeeker RT100 and the Crop Circle, in detecting different levels of grapevine downy mildew symptoms. The analysis was conducted on vine leaves that had been picked from plants of cv. Cabernet Franc infected by Plasmopara viticola. Leaves were divided into eight homogeneous infection classes and then analyzed (on the leaves' adaxial surfaces) through the optical devices and a portable visible/near infrared (Vis/NIR) spectrophotometer used as tester. Data showed a linear relation between the percentage of symptomatic leaf area and the Normalized Difference Vegetation Index (NDVI) calculated through the optical sensors (R-2 = 0.708 for GreenSeeker; R-2 = 0.599 for Crop Circle; R-2 = 0.950 for the spectrophotometer). The regression obtained for GreenSeeker is more significant than the one obtained for Crop Circle. This fact suggests a greater capability of GreenSeeker than Crop Circle in detecting different disease levels and its possible use in diagnosis application in the vineyard. Finally, the NDVI measurements carried out through the two commercial sensors, showed lower values on abaxial surfaces than on adaxial surfaces, and a reduced range of values. Moreover, the identification of different infection classes was more difficult on the abaxial surface. This is due to both the different structure of the leaf tissue and the different symptoms of P. viticola on the abaxial and adaxial surfaces. The present work will allow, in the future, applying these optical devices to diagnosis directly in vineyards.