Estimation of disease severity of potato virus y based on disease incidence in Golestan province tobacco fields

Potato virus Y, the type member of the genus Potyvirus, family Potyviridae, is one of the most destructive viruses of tobacco worldwide. To determine the relationships between incidence (I) and severity (S) of PVY disease in five cultivars in tobacco fields in Golestan province, several surveys were conducted during July to October 2017 at 15 days intervals in tobacco fields of Gorgan, Aliabad and Minoodasht. Two disease indicators (I and S) were evaluated in 50 plant samples for each farm. The presence of PVY was detected by double antibody sandwich-ELISA (DAS-ELISA) using PVY-specific polyclonal antibodies. The rate of PVY infection was different among tobacco cultivars and the maximum and minimum infection rates were belonged to PVH03 (33.6%) and Burley (6.2%) cultivars respectively. Based on the statistical analysis of data, there was a significant difference for disease indicators on different tobacco cultivars, as the maximum and minimum symptom severity mean value were evaluated in PVH03 (4.44) and Burley (1.82) cultivars, respectively. Also, Square Root model (Sqrt transformation of I and S) in 60 % of the fields (R2 = 82.32 %), and Allometric model (natural logarithm transformation of I and S) in 30 % of the fields (R2 = 89.90 %) showed the best fitness for the collected data, andwere the best model to describe I-S relationships in PVY disease in tobacco fields. Based on the regression slope of Sqrt transformed values of I and S, the cultivars were classified in three groups, including PVH03 (with the highest slope of 1.005), NC100 (with the moderate slope of 0.729) and K326, Burley and Basma (with thelowest slope of 0.6 – 0.66).