The effect of nitrogen source and quantity on disease expression of Neonectria ditissima in apple

The effects of nitrogen on the interaction between apple trees and European canker caused by Neonectria ditissima are not well understood. Previous field and laboratory studies have shown that nitrogen affects N. ditissima disease development, germination and germ-tube growth in vitro but the type of nitrogen source has not been examined in vivo. Therefore, the aim of this study was to determine the effects of root-applied nitrogen from different sources on the development of European canker on inoculated potted trees. One-year-old ‘Royal Gala’ trees were planted in a low-nitrogen growth substrate and treated with a range of concentrations of calcium ammonium nitrate (CAN) or other nitrogen sources (Ca(NO3)2, KNO3, (NH4)2SO4, NH4NO3, urea, YaraMila™) at equivalent molar rates of nitrogen as the highest CAN treatment. Treatments were applied during the growing season (Nov to May). The control treatment received no applied nitrogen. Bud and leaf scar wounds were inoculated at leaf fall with N. ditissima conidia. Tree growth and health, disease progression and leaf nitrogen content were monitored. The rate of nitrogen application affected tree diameter and leaf nitrogen content while the nitrogen source mainly affected tree survival, powdery mildew incidence, leaf weights, leaf nitrogen and European canker symptom expression. Trees treated with (NH4)2SO4 had the lowest survival rates and highest leaf nitrogen content. Disease expression was highest with NH4NO3 and lowest with KNO3 applications. The control plants (which received no additional nitrogen), showed the least amount of both growth and disease expression. Applications of CAN, even at the lowest rate (20 g), increased disease susceptibility. Increasing rates of CAN applications did not significantly increase disease incidence. Nitrogen concentration is an important factor in the disease development of European canker of apple. Field evaluation is recommended to further validate these results.