Effect of nitrogen fertilization on the physiological damage and subsequent recovery of mite-infested apple leaves

Two experiments were conducted with field- and container-grown 'Delicious' apple trees to evaluate the effect of nitrogen fertilization on the physiology of apple leaves subjected to European red mite feeding, and the subsequent recovery of leaf function following mite removal. Nitrogen (N) fertilization affected the visual damage, and the photosynthetic response of 'Imperial Delicious' apple leaves to mite feeding. Visual damage occurred first, and remained greatest on the low N treatment throughout the experiment. High N trees exhibited a linear decline in Pn with increasing MD, in contrast to the quadratic decline found under low N treatment. With equal mite populations, high N trees retained higher Pn rates. Transpiration, dark respiration, relative water content, leaf nitrogen, and total chlorophyll were negatively related to MD, but specific leaf weight was positively related to MD. No interaction was found between N treatment and mite feeding for these parameters.

Following removal of ERM populations, 'Redchief Delicious' leaves subjected to greater than 4000 MD exhibited recovery in Pn by 28 days after mite removal. Leaves receiving less MD (1500-3000) demonstrated an inconsistent response over time. Transpiration and diffusive resistance showed no recovery. Relative water content, specific leaf weight, leaf nitrogen, and total chlorophyll linearly declined with mite feeding, and were unaffected by foliar urea treatment.