Influence of rootstock, temperature and incubation duration on bacterial canker severity caused by Pseudomonas syringae pv. syringae in peach

Introduction . Bacterial canker, caused by Pseudomonas syringae pv. syringae , is a damaging disease of stone fruit worldwide. The effects of rootstock, temperature and incubation duration on bacterial canker in peach were assessed using both field and laboratory inoculation assays. Materials and methods . Both field and laboratory experiments were conducted to study the effects of rootstock, temperature and incubation duration on disease severity in peach. All inoculations were achieved with P. syringae pv. syringae strain B3A. Bacterial inoculations were applied to 1-year-old shoots of peach trees [Prunus persica (L.) Batsch]. After inoculation, the inoculated shoots were allowed to incubate either under field conditions or in a cold room at different temperatures [constantly at 0 °C, constantly at 14.4 °C, and in a fluctuating temperature regime of 12 h at 0 °C (night) and 12 h at 14.4 °C (day)] for excised shoots. The lesions were determined 1 to 6 weeks after inoculation to determine the effect of incubation duration. Results and discussion . The field experiment using peach grafted on three rootstocks (Nemaguard, K119-50 and P30-135) showed that shoots on Nemaguard developed the longest lesions and shoots on K119-50 the shortest among all three rootstocks. Shoots on Nemaguard had significantly lower bark calcium and higher nitrogen concentrations than those on K119-50 and P30-135. A negative correlation was found between lesion length and bark calcium concentration and the [calcium / nitrogen] ratio. Laboratory experiments with excised shoots on Nemaguard, K119-50, P30-135, Lovell and Guardian rootstocks growing in a second orchard showed inconsistent results. Shoots from Nemaguard developed significantly smaller lesions than those on K119-50 and P30-135. Shoots on Guardian and Lovell also developed significantly smaller lesions than those of shoots on K119-50 and P30-135. Temperature fluctuation during incubation (0 °C to 14.4 °C) had no effect on shoot lesion length compared with those incubated constantly at 14.4 °C, but produced significantly longer lesions than shoots incubated constantly at 0 °C. These inconsistent results suggest that, in the absence of major predisposing factors (i.e. , ring nematodes or low soil pH), rootstocks may play a minor role in peach susceptibility to bacterial canker even under favorable disease development conditions.