The relationship between SPAD-501 meter readings (SPAD) and total chlorophyll content (TCHL) was evaluated for `Delicious' apple (Malus domestica Borkh.) leaves grown in various environments. Regression models were developed between SPAD and TCHL for each of six separate experiments and were evaluated for statistical coincidence. SPAD was linearly related in a positive manner to TCHL in five of the six experiments; however, models differed between experiments, particularly between field- and greenhouse-grown trees. Thus, the relationship between SPAD and TCHL must be determined for each experiment.

Greenhouse-grown `Imperial Delicious' apple (Malus domestics Borkh.) and `Redhaven' peach (Prunus persica Batsch.) trees were inoculated during the summer with three densities of European red mite (ERM) (Panonychus ulmi Koch) and twospotted spider mite (TSM) (Tetranychus urticae Koch). As ERM- and TSM-days increased, net photosynthesis (Pn), transpiration (Tr), and total chlorophyll content (TCHL) of apple leaves decreased linearly. At similar densities, TSM was more damaging than ERM to apple leaf gas exchange. Water-use efficiency (WUE) of apple declined similarly with increasing mite-days for both mite species. Specific leaf weight (SLW) of apple increased with TSM-days. Pn, Tr, TCHL, and WUE of peach declined linearly with increasing ERM- and TSM-days, and the rates of decline were similar for both mite species. Mites did not affect peach SLW. These results indicate that greenhouse-grown peach is more tolerant than apple to mite feeding.

The effect of growing conditions on the physiological damage caused by mite feeding was evaluated. Control and twospotted spider mite (Tetranychus urticae Koch)-infested `Imperial Delicious' apple (Malus domestica Borkh.) and `Redhaven' peach [Prunus persica L. (Batsch.)] trees were grown under winter and summer greenhouse conditions. Mite feeding reduced net photosynthesis (Pn) of apple in both winter and summer, with equivalent regression models (Pn on mite days). Total chlorophyll content (TCHL) was decreased, and specific leaf weight (SLW) was increased by mite feeding in both winter and summer. In peach, Pn was also reduced by mite feeding, with equivalent regression models in winter and summer. SLW was unaffected, and TCHL was reduced by mite feeding in both seasons. Transpiration (Tr) of apple and peach was inconsistently affected by mites in winter and summer. In another experiment, control and European red mite (ERM) [Panonychus ulmi (Koch)]-infested `Red-chief Delicious' apple trees were grown in the summer under both greenhouse and field conditions. ERM feeding negatively affected Pn in both the greenhouse and the field, but reductions were less under field conditions. Mites reduced Tr 28% in the greenhouse and 12% in the field. TCHL, SLW, and leaf N content (LFN) were reduced by 16%, 24%, and 26%, respectively, in the greenhouse. In the field, TCHL was not significantly reduced, and SLW and LFN were reduced by 6% and 13%, respectively.