One way to maximize the productivity of your apple trees is to understand the roles of spur and shoot leaves in flower bud formation and fruit development, and incorporate these roles into your management practices.
Why apples need both spur and shoot leaves to grow
Spurs and spur leaves
Spur leaves are the first leaves to form on an apple tree in spring, and comprise the majority of the tree canopy during the time from bud break and bloom time until fruit set.
Spur leaves may represent 30 to 50 per cent of the canopy during the entire season. On spur-type trees, spur leaves may comprise more than 60 per cent of the leaf area of a tree.
Spur leaves are therefore very important in providing carbohydrates produced by photosynthesis, for fruit growth.
Spur leaves are part of a spur. A spur is a compressed leafy shoot with either a vegetative or flower bud. Spurs can therefore be classified as either vegetative or reproductive.
A vegetative spur, also called resting spur, forms on one-year-old or older wood. It is called vegetative or resting, because it has a bud with no flowers for one season, and the spur will therefore have no fruit. A vegetative spur can have 6 to 20 leaves.
Resting spurs play an important crop regulatory role because the buds rest for a year before they develop into buds which contain flowers. It is a type of insurance for next year’s crop.
When spurs have gone through a vegetative or resting season, they may become reproductive spurs the following season.
While the buds rest, floral parts are developing inside the buds. Following their season of rest, they become reproductive spurs with buds that have flowers that will set fruit. Reproductive or fruiting spurs form on two-year-old wood.
Each reproductive spur has six to ten spur leaves which form a whorl below the flower cluster of five to six flowers.
These spur leaves are the only leaves on the spur until about flowering time, when one or two buds below the flowers sometimes form lateral shoots, called bourse shoots.
Bourse shoots can grow to about 25 mm or more. Sometimes the bourse bud may form another small whorl of leaves ending into another bourse bud, which is a resting bud and will not flower the following season.
Early in the season, the growing bourse shoots may compete with the developing fruits for nutrient reserves and assimilated carbohydrates produced by the spur leaves.
If bourse shoots grow too vigorously due to too much nitrogen fertiliser in spring, they become strong competitors and fruit set may be reduced.
(cont next month)
See this article in Tree Fruit August 2018