Development of Xanthomonas arboricola pv. pruni epidemics on peaches

Infections of Xanthomonas arboricola pv. pruni (Xap) on peaches have become more important in northern Italy since the late '80s. Therefore, a three year (1993 to 1995) epidemiological study was conducted in two peach orchards affected by Xap in the district of Verona. Both disease onset and progress over time were considered. The incidence of affected leaves or fruits and disease severity were assessed in relation to meteorological data. Primary infections appeared on leaves between the end of May and the middle of July. The disease progressed with variable incidence and severity, the former ranging from very slight to more than 70% of leaves affected. Fruits were less severely affected. Rainfall played a predominant role in Xap infections. Primary infections were always established following at least 3 successive rainy days, with temperatures between 14 and 19°C. Disease progress was closely correlated with the number of rainy days after disease onset. The use of the latter variable in a logistic regression model accounted for 85% of variability in disease incidence and for 96% of variability in disease severity. A more complex model, also considering the time of disease onset, increased accountable variability to 93 % and 97%, respectively