Prediction of Xanthomonas harboricola pv. pruni infection on peaches

X. arboricola pv. pruni (Xap) is present on Prunus spp. in some European countries, and it is listed as an A2 quarantine pest by EPPO; its importance in Northern Italy has increased in the last decade. An empiric model predicting Xap infection has been developed in late ‘90s. Occurrence of the first seasonal infection was monitored in peach orchards of Romagna, in 1992 to 2008, and compared to model predictions: an infection was predicted when there were at least 3 successive rainy days, with temperature between 14 and 19°C; symptom’s onset was expected after one to four weeks of incubation. Xap symptoms appeared in 10 out of 17 years; first seasonal symptoms become visible between 19 May and 12 July. These infections were always correctly predicted by the model, with an average incubation period of three weeks. Five infection periods were predicted by the model that did not result in actual infection. In five years the disease did not appear at all. In four of these years the model did not predict infection all season long, while in one year it wrongly predicted two possible infection periods. Sensitivity, specificity and accuracy of the model showed that one would have somewhat more confidence in predictions of non-infections than in predictions of infections. In a practical use of the model, this would lead to some unjustified alarms.