Identification of regional climatic conditions favorable for development of European canker of apple.

Months of the year with high risk of European canker (Neonectria galligena) development in areas of the United States, Chile, England, and Northern Ireland were determined from published data. Moving-window analysis of long-term climatic data was used to classify disease risk in these areas in relation to rainfall and temperature conditions using the degree of agreement statistic. Greatest agreement occurred when it both rained on >30% of days/month and there was an average of >8 h/day with temperature of 11 to 16°C. When these thresholds were applied in eight validation areas in New Zealand, Australia, the United States, The Netherlands, and Denmark, areas with reported higher risk of disease tended to be areas where the thresholds were exceeded more often and by greater amounts. Areas at higher latitudes (>52°) with frequent summer rainfall appeared to be most prone to European canker, including the fruit rot phase of the disease, probably because summer temperatures were more favorable than at lower latitudes. The climatic thresholds derived for European canker could be useful for studies of disease establishment risk, surveillance, eradication, climate change impact assessment, and, possibly, for disease risk forecasting. The methods used in this study allowed conditions favorable for disease development to be identified even though quantitative regional disease data were lacking, and they could be useful for similar geoclimatic studies of other diseases.