Tree Growth Stage and Environment After Pathogen Inoculation Alters Susceptibility of Pear Trees to Phytophthora Canker

We investigated whether pear (Pyrus communis L. OHF97) tree growth stage, nitrogen (N) status, wound age, and infection environment influences susceptibility to Phytophthora syringae Klebahn. Actively growing trees were sus- ceptible to the pathogen when inoculated before terminal budset and cooler temperatures increased disease severity. In the autumn, during early stages of dormancy development after budset there was no relationship between temperature after inoculation and susceptibility and as trees became more dormant a negative relationship developed between temperature and susceptibility. Altering tree N status in the autumn after budset by spraying leaves with urea had no influence on sus- ceptibility, regardless of environmental conditions after inoculation. Lesions developed when fresh wounds (0 d to 1 d old) were inoculated after budset, but as wounds aged they were less susceptible to infection. These results suggest a combination of tree growth stage when inoculated with the pathogen and environmental conditions afterward play impor- tant roles in disease development. Differences in susceptibility may be a direct result of temperature on pathogen activity or an indirect effect of temperature on tree metabolic activity. Differences in tree susceptibility related to dormancy devel- opment may be related to stem N status or the ability of trees to produce a barrier to infection after stems are wounded. Surface wounds inflicted during handling after budset may serve as infection locations for P syringae; however the length of time between wound formation and exposure to the pathogen dictates whether infection will occur when trees are in the early stages of dormancy development.