Development and validation of a standard area diagram set to estimate severity of grapevine downy mildew on Vitis labrusca

The aim of the present study was to develop and validate a standard area diagram set (SADs) to evaluate severity of grapevine mildew (Plasmopara viticola) for Vitis labrusca cultivars. SADs contain 10 disease severity levels (0.5; 2; 5; 10; 20; 30; 40; 50; 60 and 72%). Validation was performed using a sequence of 50 images assessed by 16 raters with no previous experience in quantifying plant disease severity. By Lin's concordance correlation coefficient analysis, there was an improvement in u (no SADs = 0.361, with SADs = -0.030), upsilon (no SADs = 1.456, with SADs = 0.997), C-b (no SADs = 0.864, with SADs = 0.979), r (no SADs = 0.897, with SADs = 0.937) and rho(c) (no SADs = 0.774, with SADs = 0.918) parameters, showing an increase in accuracy and precision of the estimated disease severity using SADs when compared to the non-use of SADs. Reliability improvement was also observed, with increase in R-2 values when SADs were used (no SADs = 0.790, with SADs = 0.846). Therefore, SADs improved accuracy, precision and reliability of grape downy mildew severity estimates in Vitis labrusca.