Quantitative assessment and determination the relationship between incidence and severity of rapeseed Sclerotinia stem rot disease in Mazandaran province

Plant diseases assessment is necessary for developing effective integrated diseases management strategies. Amount of disease, is usually evaluated with two quantities, incidence and severity. Relationship between incidence and severity is important concept in plant diseases epidemiology. Sclerotinia stem rot is a rapeseed important disease in the world and Iran. For measuring disease incidence and severity and determination relationship between them, 240 rapeseed fields were investigated in Mazandaran province during years 2010, 2011, 2015 and 2016, and disease amount recorded in weekly surveys. Based on final incidence and severity, there were significant differences (P<0.01) between regions and years. The highest incidence and severity (34.02 and 23.70 percent) was in year 2016 and the lowest of them (12.85 and 6.32 percent) in year 2015. Between regions, Galogah and Sari had the highest and lowest disease infection respectively. In order to determine relationship between two mentioned quantities, the related data, were fitted with different mathematical models by linear regression analysis method that based on statistics such as coefficient of determination (R2) and standard error of estimates, linear, allometric and square root models had good fitness with data. The linear model, was selected as the final model because of the simplicity and having low computational complexity. Based on slope of linear model equation, there was significant difference (P<0.01) between four years of investigation. The equation of the model for years 2010 and 2016 was S=0.72(I)-0.99 and for years 2011 and 2015 was S=0.55(I)–0.61. Weather conditions had significant effects on disease infection in different years.