Shoots and shoot leaves: Spur leaves are not the only leaves apples need for growth.
Why apples need both spur & shoot leaves to grow (part 4)
(continued from last issue)
Spur leaves age after 60 to 90 days and their capacity to photosynthesise declines.
Each day an apple grows, it requires additional carbohydrates for cell development and sugar. Spur leaves cannot keep up with the demand of carbohydrates for fruit growth.
In early summer, shoots and bourse shoots stop growing because they can no longer compete with the nearby apples for carbohydrates.
Shoots and bourse shoots form terminal buds, and the leaves begin to send carbohydrates to the apples, which depend increasingly on this supply right up to harvest.
A shoot is defined as the current-season’s growth from a bud and includes the leaves. A shoot is also called a lateral, pencil or dart, and must not be confused with water-shoots.
In early summer, when bourse shoots and laterals stop growing, they also send a flower-forming signal to buds. Summer pruning should therefore only be restricted to removing strong upright parasitic water-shoots.
Thus, a strong reproductive spur will have good growth of spur leaves during the period of flowering up to at least 90 days after flowering, and strong, but not excessive growth of bourse and shoot leaves.
The carbohydrates produced by shoot leaves only move about 300 to 1500 mm within the tree frame. Therefore it is important to have a balance of spurs and shoots along the entire length of the fruit-bearing wood.
Severe summer pruning late in the season to enhance skin colour of apples, can reduce fruit size and soluble solids (sugar).
See this article in Tree Fruit Nov 2018