Accumulated rainfall thresholds for applying protectant fungicides on potato cultivars resistant to Phytophthora infestans in Ecuador

In Ecuador, susceptible potato cultivars must be protected from P. infestans with frequent fungicide applications, but resistant cultivars require much less fungicide. Ecuadorian farmers do not have access to equipment that would allow for sophisticated decision support systems, however, farmers can easily measure accumulated rainfall. In this experiment 5 resistant and one susceptible cultivar were grown without fungicide protection, sprayed with protectant fungicide weekly and after each 30 and 50 mm of rainfall. A minimum spray interval of 10 days was used with rainfall thresholds. Disease severity was high on all cultivars grown without fungicides, and no fungicide treatment gave ample control for the susceptible cultivar. All fungicide treatments controlled disease on the 5 resistant cultivars, but the 50 mm threshold resulted in the lowest number of applications. This threshold appears to be a useful indicator for fungicide timing on resistant cultivars under these conditions.