Effects of molybdenum sprays on the growth, yield and fruit quality of ‘Red Jonaprince’ apple trees

Abstract

The aim of the study was to examine the responses of mature ‘Red Jonaprince’ apple (Malus domestica Borkh.) trees/M.26 to molybdenum (Mo) sprays. The experiment was conducted in Central Poland in 2017-2018 at high-density orchard growing on strongly acidified coarse-textured soil with low Mo availability. Trees were sprayed with Mo in the form of sodium molybdate before bloom (at pink bud), at bloom (when 5-20% of flowers were open) or after bloom (14 days after petal fall). In each spray, the Mo rate was 286 g ha-1. Trees that were not treated with Mo served as the control. Except for manganese (Mn), the concentrations of the essential macro- and micronutrients in the summer leaves of the control trees were within optimum/sufficient ranges. The summer leaf Mn concentrations were high but not toxic. Prebloom and bloom Mo sprays increased the activity of nitrate reductase (NR) and SPAD chlorophylometer values for leaves at 21 days after full bloom (DAFB), Mo concentrations in flowers and seeds, the number of seeds in an apple, fruit Ca concentration (on average by 23% for both Mo spray treatments), and the Streif index values of the fruit at harvest. The postbloom Mo spray increased the summer leaf concentrations of nitrogen, magnesium, iron and Mo, and the NR activity and SPAD values of leaves at 21 DAFB and at harvest. No Mo spray affected tree vigour, fruit set and yield, mean fruit weight, skin russeting and blushing, and the soluble solids concentration and flesh firmness of apples at harvest. It is concluded that when the apple flower Mo concentration, at least in triploid varieties, is as low as 1.5 mg kg-1 DW, a prebloom or bloom Mo spray is recommended to improve Ca-related apple quality.