Mature and well-managed apple trees develop approximately similar numbers of spur and extension leaves in each season. These leaves are vital for producing, by photosynthesis, the food stuffs (sugars, carbohydrates) that power the growth of the fruits.
It is reasonable to suggest, therefore, that optimum fruit numbers that can be developed to full size by the tree, are related closely to:
- Tree size, especially canopy volume, and
- Light interception.
If light interception and temperatures were similar in most seasons, then the numbers of fruits that the mature tree could grow to full size would also be similar in most seasons.
- Climatic conditions differ slightly in most seasons, however, and some adjustments to optimum fruit numbers need to be made in the light of these seasonal variations.
- Thinning to ideal numbers of fruit per tree, is more logical and gives less variable effects than the often used strategy of thinning to specific numbers of fruit (1 or 2) per floral bud.
Trials conducted in France on six-year-old Royal Gala/M.9 trees at a spacing of 4 x 2 m showed that cropping loads of 120, 180 and 240 fruits per tree were equivalent to 5, 7 or 9 fruits per square centimetre of trunk cross sectional area.
- In these trials, in which the crop loads were established by thinning 20 days and 50 days after flowering, the two lighter crop loads produced 90% of their fruits in the top grades.
- The highest crop loading produced only 70% in the top grades.
- Large differences in ripening were noted with 58% or fruits ready for harvest at the first picking date on the lightest cropping trees, but only 27% on the heaviest cropping trees.