COMBINED EFFECTS OF SPIREA APHID (HOMOPTERA, APHIDIDAE) AND NITROGEN-FERTILIZATION ON SHOOT GROWTH, DRY-MATTER ACCUMULATION, AND CARBOHYDRATE CONCENTRATION IN YOUNG APPLE-TREES

One-year-old apple trees were grown in pots in an unheated greenhouse with screened ends and were artificially infested with spirea aphid, A phis spiraecola Patch. Spirea aphid density increased at a faster rate on trees receiving higher rates of nitrogen application. Accumulation of dry weights in all tree parts (leaves, lateral shoots, trunk, root stock, and roots) during the growing season were affected by both spir ea aphid and nitrogen fertilization. Spirea aphid reduced accumulation of dry weights of all tree parts harvested at the end of the first gr owing season. These reductions were still present when trees were harv ested at the 10-leaf stage the next spring. Spirea aphid reduced later al shoot growth at the end of the growing season and at the 10-leaf st age. Dry weights of all tree parts increased with increasing rates of nitrogen. The percentage and amount of nonstructural carbohydrates in all tree parts were reduced by spirea aphid feeding and were positivel y related to nitrogen rate. At the 10-leaf stage in the second season, similar results were obtained. Foliar N concentration increased linea rly with increasing amount of N application in both infested and contr ol leaves. Also, differences in leaf N concentration were found betwee n infested and control leaves at each N application rate.