Development and Validation of a Standard Area Diagram Set for Estimating Downy Mildew (Peronospora sparsa) Severity on Rose Leaves

Downy mildew, caused by the oomycete Peronospora sparsa, is a recurrent problem in greenhouse rose production in the Ecuadorian highlands, yet no validated aid exists for estimating its severity on foliage. We built a standard area diagram set (SAD) from 98 symptomatic compound leaves of rose cv. ‘Niky’ collected in a commercial greenhouse in Cayambe, Pichincha. Actual severity was measured on 400-dpi scans with Assess 2.0, and seven photographic diagrams represent 3, 8, 12, 16, 22, 32 and 42% diseased leaf area. Twenty-seven inexperienced and four experienced raters then estimated severity on 40 leaf images, first unaided and then with the SAD. Agreement was quantified with Lin’s concordance correlation coefficient (ρc). Unaided, most raters overestimated severity. With the SAD, mean ρc rose from 0.45 to 0.66 for inexperienced raters (95% bootstrap CI of the gain, 0.12 to 0.28) and from 0.19 to 0.76 for experienced raters, through gains in precision (r) and accuracy (Cb). Mean location bias changed sign, from overestimation to slight underestimation. A drawn colour version with identical reference values is also provided, and a calibrated Monte Carlo simulation indicated that the current diagram levels are adequate. The SAD improves visual estimates and is ready for field testing.