Integrated control of potato late blight: predicting the combined efficacy of host resistance and fungicides

Integrating cultivars that are partially resistant with reduced fungicide doses offers growers an opportunity to decrease fungicide input but still maintain disease control. To use integrated control strategies in practice requires a method to determine the combined effectiveness of particular cultivar and fungicide dose combinations. Simple models, such as additive dose models (ADM) and multiplicative survival models (MSM), have been used previously to determine the joint action of two or more pesticides. This study tests whether a model based on multiplicative survival principles can predict the joint action of fungicide doses combined with varieties of differing partial host resistance. Data from eight field experiments on potato late blight (Phytophthora infestans), where the severity of foliar blight was assessed and converted to AUDPC, were used to test the model. A subset of data, derived from the most susceptible cultivar, King Edward, was used to produce dose response curves from which parameter values were estimated, quantifying fungicide efficacy. These values, along with the untreated values for the more resistant cultivars, Cara and Sarpo Mira, were used to predict the combined efficacy of the remaining cultivar by fungicide dose combinations. Predicted efficacy was compared against observations from an independent sub-set of treatments from the field experiments. The analysis demonstrated that multiplicative survival principles can be applied to describe the joint efficacy of host resistance and fungicide dose combinations.