A comparative study on disease progression curves of grapevine downy mildew in North Khorasan province

Grapevine downy mildew caused by Plasmopara viticola is the most important factor in decreasing grape production in temperate and rainy regions. To investigate the disease progression curve in vineyards of North Khorasan province during two years of 2018 and 2019, Scoring of the target vineyards was carried out each week on a regular schedule from mid-April at the bud swelling time from the vineyard. The incidence rate and severity of the disease in 80 vineyards from four counties of Bojnourd, Maneh and Samolghan, Shirvan and Farouj were calculated. Linear regression was fitted to disease time progression curves with five population growth models as exponential, monomolecular, logistic, log-logistic and gompertz models. The model fitness was determined based on statistics as coefficient of determination (R2), standard error of estimates (SEE) and adjusted coefficient of determination (Ra2). The results showed that exponential, monomolecular, logistic, log-logistic and Gompertz models were fitted with 5, 27.5, 12.5, 17.5 and 37.5% of the disease in the studied gardens, respectively. Accordingly, Gompertz model with 96.76% coefficient of determination was selected as the most suitable model to describe disease progression in North Khorasan province. Rates of increase (rG) per unit of disease in each vineyard was 0.005 to 0.048 (with an average of 0.026). This is the first report of a study of the progression curve of grapevine downy mildew in Iran.