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 \u201890s.
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\ub0C; symptom\u2019s 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