Interactions of European Red Mite and Crop Load on Maturity and Quality, Mineral Concentrations, and Economic Value of `Starkrimson Delicious' Apples

Abstract

Fruit maturity, quality, calcium concentration and economic value of `Starkrimson Delicious' (Malus domestica Borkh.) apples, under a range of crop levels and European red mite [Panonychus ulmi (Koch)] cumulative mite-days (CMD), were best explained by local surface regression models involving CMD and crop load. Fruit from trees with low CMD and a light crop (125 fruit/tree, about 20 t/ha) were the most mature at harvest. Those fruit had higher ethylene concentrations, starch pattern indices, soluble solids concentrations, and watercore incidence at harvest than fruit from trees with low CMD and a normal crop (300 fruit/tree, about 40 t/ha), or with high CMD at any crop level. Those fruit also had higher incidences of watercore and internal breakdown after 4 months of cold storage. Calcium concentrations in fruit increased as crop load and CMD increased. Whole-canopy net CO2 exchange rate per fruit related better to fruit quality and calcium concentrations than either crop load or CMD alone, but was always a much worse predictor than local surface regressions. Low CMD and normally cropped trees had the highest crop value; lightly cropped trees had an intermediate crop value; while high CMD and normally cropped trees had the lowest crop economic value. Crop load should be considered when defining action thresholds for mites, and harvest schedules for apples should reflect crop load and mite populations on apple trees.