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  • Author or Editor: S. Laywisadkul x
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Recent field observations by growers suggest that increased nitrogen (N) content in nursery trees resulting from foliar sprays with urea in the autumn increases tree susceptibility to infection by Phytophthora syringae. We investigated the effects of soil N availability and spraying pear (Pyrus communis ‘OHF 97’) trees with combinations of urea, chelated copper ethylenediaminetetraacetic acid (CuEDTA), and phosphonate-containing fungicides on stem N concentration and susceptibility to infection by P. syringae. Increasing soil N availability increased susceptibility to P. syringae and increased N and amino acid concentration in stems. Spraying trees with urea in the autumn increased concentrations of N and amino acids in stems and had no significant effect on tree susceptibility when stems were inoculated with P. syringae before or after urea sprays. Spraying trees with CuEDTA decreased stem N concentrations and had no significant influence on tree susceptibility to P. syringae when stems were inoculated before or after CuEDTA sprays. These results suggest the relationship between tree susceptibility to P. syringae and tree N concentration may be specific to the form of N, delivery method, or timing of N applications. Trees had higher N concentrations in stems in November than in October and were more susceptible to P. syringae when inoculated in November, suggesting that environmental factors and increasing tree dormancy may be responsible for changes in susceptibility to the pathogen. Spraying trees with fungicides containing fosetyl-aluminum in October or November decreased tree susceptibility to P. syringae. The effects of fungicides containing fosetyl-aluminum on susceptibility were similar regardless of whether trees were sprayed or not with urea or CuEDTA, suggesting that these fungicides can be used in combination with urea or CuEDTA sprays for reducing disease severity caused by P. syringae without impacting growers' objective of increasing tree N content with urea or enhancing early defoliation with CuEDTA.

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