Container grown pear trees were used to determine the effects of the following factors on the susceptibility to Phytophthora syringae: 1) tree nitrogen (N) status and foliar sprays of urea and copper-EDTA (CuEDTA); 2) dormancy and environmental conditions; 3) wound and wound age; and 4) chemical and biological control measures. Lesion size and disease incidence from a stem assay test were used to evaluate tree susceptibility to the pathogen.
Increasing rate of N fertigation during the growing season and spraying trees with urea increased the concentration of N and amino acids in stems. CuEDTA had no influence on stem N concentration. Urea had no influence on tree susceptibility while the effect of CuEDTA varied between experiments (years) however the combined application of urea and CuEDTA had no influence on disease severity. Increasing plant N concentration increased susceptibility of plants to P. syringae.
Lesions caused by P. syringae were greater in November than in October, due to the colder and wetter conditions that favor disease development. Decreasing temperatures after inoculating pear trees with P. syringae increased disease severity. Trees incubated in the greenhouse showed smaller lesions than those in the lath house. The lesion size increased in the lath house from October through December. The progression of the disease increased due to the cooler and wet conditions that favored the development of P. syringae. Both actively growing and dormant trees were susceptible to P. syringae when trees were exposed to low temperatures after inoculation. Wound was important for P. syringae infection and disease incidence and lesion size decreased with increasing wound age.
The application of either Aliette (6 g/l) or PhytoFOS (1%) was effective in preventing disease when applied as a foliar treatment 1 week or 2 days respectively before inoculation under all treatment conditions.
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