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- Author or Editor: Douglas G. Pfeiffer x
`Campbell Redchief Delicious' trees were treated with a factorial combination of five European red mite (ERM) populations (range = 0 to 3000 cumulative mite-days per leaf (CMD)), and four crop densities (CD) (range = 2 to 12 fruit · cm-2 trunk cross sectional-area) during two growing seasons. Yield was related positively to CD and negatively to CMD. In general, fruit weight, fruit surface red color, and fruit soluble solids concentration declined with increasing levels of CMD and CD. For most response variables, there was a significant CMD by CD interaction. Fruit were graded into six size categories and crop value was calculated. Crop value generally increased with increasing CD and declined with increasing CMD, mite feeding reduced crop value most at high CD. Crop value estimates were used to calculate economic injury levels. We propose economic injury levels of 1000, 500, and 300 CMD, respectively for spur `Delicious' trees with light, moderate, and heavy crop loads.
White apple leafhopper (WALH; Typhlocyba pomaria McAtee) feeding damage on apple (Malus domestica Borkh.) leaves was examined with scanning and transmission electron microscopy. WALH created feeding holes in the (lower) abaxial epidermis, with no visible exterior evidence of cell injury to the adaxial (upper) epidermis. Feeding holes were located in areas of the leaf with high stomatal density and were near stomata. Groups of cells in the palisade layers were empty or contained coagulated cell contents. Adjacent, apparently noninjured, palisade cells contained an abundance of starch granules, possibly indicating that photoassimilate export was impaired. Spongy mesophyll cells abaxial to the feeding area were left intact as were the epidermal cells adaxial to the feeding area. External views of either epidermis and internal leaf views of injured cells indicated no cell wall collapse.
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.