2018 ASHS Annual Conference

Total non-structural carbohydrate (TNC) availability in apple trees (Malus domestica Borkh.) is affected by light availability, temperature and other environmental factors as well as physiological processes such as respiration, tree growth, and crop load. These factors affect photosynthesis and starch accumulation (energy supply) and alter energy requirements for tree growth (demand). The balance between carbohydrate supply and demand is particularly important for apple development at thinning time when fruitlets are 10-12 mm in diameter and competition for carbohydrates is high. If current season carbohydrate supply to competing fruitlets is low, the application of chemical thinners may be more effective resulting in a lower crop density than if carbohydrate availability is high. However, if starch concentrations, accumulated in the previous year are high, more carbohydrates may be available for fruitlets which may limit the effect of a chemical thinner application and result in a higher crop density.

‘Golden Delicious’ trees on M.9, M.26, and G.16 rootstocks were used to investigate the effect of fruit removal treatments at 29, 125, 154 and 182 days after full bloom (DAFB) in 2013 on glucose and starch concentrations in current-season shoots and 1- and 2-yr-old branch sections the following season. The effect of fruit removal on thinning efficacy the following year was also investigated and trees were sprayed with water (control) or a mixture of 0.95L Carbaryl 4L and 5 ppm of NAA (Fruitone N) in 378.5L of water. We hypothesized that trees with fruit removal treatments at 29 and 125 DAFB would have higher glucose and starch concentrations and higher crop density after thinner application the following year while trees with fruit removal at 154 and 182 DAFB would have lower glucose and starch concentrations and lower crop density after thinner application. Trees with fruit removal at 29 DAFB had greater return bloom than trees with fruit removal later in the season. Fruit removal and application of chemical thinner enhanced available glucose and starch concentrations (mg∙g^-1 dry weight) in actively growing shoots and 1- and 2-year-old wood; however, this depended on time of year and rootstock.