Forecasting infections of the leaf curl disease on peaches caused by Taphrina deformans

An Israeli model forecasting leaf curl disease on peaches caused by Taphrin a deformans was validated in the Emilia-Romagna region of northern Italy, d uring a three-year period (1996-1998), in 13 cases (year x location x culti var). When the peach trees are susceptible to infection, the model uses mat hematical functions to calculate the risk of infection on the basis of weat her conditions (daily rainfall greater than 10 mm, and maximum air temperat ure greater than 5 degrees C), and it forecasts periods of possible symptom appearance based on the length of incubation. Peach trees became susceptib le to infection between the end of January and mid March, when the first le af buds attained phenological stage C, i.e. appearance of leaf apex. The tr ees remained susceptible for at least 9 weeks: the last infection occurred in mid-May. Since most of the leaf curl onsets observed in the orchards fell within the range of model forecasts, the model proved to be accurate in signalling bo th the first seasonal infection and repeated infections during the primary inoculum season. Few errors occurred, caused either by conditions of rainfa ll and temperature lower than the thresholds fixed in the model, or by disc repancies between forecast and actual length of incubation. Infection occur red also at 3.1-3.5 degrees C, and with 9.6 mm rainfall. Thus, thresholds s hould not be accepted too rigorously, and perhaps temperature should not be considered as a limiting factor for infection under the conditions of the present work. The length of incubation showed high variability: it was 23 d ays long on average, with a 95% confidence interval ranging from 20 to 27 d ays, and extreme values of 9 and 33 days.