Incidence-Severity Relationship for downy mildew of grapevine in North Khorasan Province

Grapevine are important and valuable fruits that is contaminated in different parts of Iran, especially in North Khorasan Province, by the downy mildew of grapevine (Plasmopara viticola) and its yield decreases. Investigating the relationship between the incidence and the severity of the disease is one of the important concepts in the epidemiology of plant diseases. It is easier and faster to measure the incidence of the disease, but the severity of the disease is an important indicator.

In this study, the relationship between the incidence disease and severity disease of downy mildew of grapevine in North Khorasan Province (Bojnourd, Mane and Samalghan, Shirvan and Farooj) during 2018 and 2019. Incidence and severity of disease were recorded weekly in 80 selected vineyards from early April to stop the disease progression.

The incidence of the disease was 10.27% in the first year and 51.64% in the second year. The severity of the disease in the first year was between 1.2 to 6.6% (mean 3.4%) and in the second year it was between 6.4 to 44.6% (mean 25.82%). Investigation of the relationship between incidence and severity of disease using four statistical models (power, linear, quadratic, complementary logarithmic logarithm) power model was able to express this relationship in 31 vineyards more accurately. In 24 vineyards the incidence and severity of the disease were well fitted to the data by linear model.

Final models were evaluated based on R2, SEE and Ra2 of regression analysis of raw data. Based on these criteria, final model in Bojnourd as S = (0.534) × I (1.263), in Shirvan as S = (0.684) × I (1.472), in Mane and Samalghan as S = (0.4691)× I-0.01711 and in Farooj as S = (0.3484)×I-0.0078 showed relationship between incidence and severity of disease. This is the first report to evaluate the relationship between the incidence and severity of downy mildew of grapevine in Iran.