Fruitlet growth

After fruit set, the growth of the persisting fruitlets is brought about by a combination of cell division and cell expansion.  It is generally accepted that the majority of cell division takes place in the first 6 weeks ceases within 4 weeks after pollination and cell expansion continues throughout to harvest.

A number of factors, other than crop loading are known to influence cell division and expansion. These are:

• Photosynthesis and carbon supply to the fruitlets as influenced by temperature and light.
• Production and movement within the tree of natural plant hormones.
• Tree health and nutrition.

Photosynthesis and carbon supply to the fruitlets

The demand for assimilates (carbohydrates produced by photosynthesis) increases rapidly following bloom, peaking at about 4-6 weeks after full bloom and then  remaining fairly stable through until harvest.

• There are two periods of potential limitation of carbon supply; one about 2-4 weeks after blooming and the other in the last few weeks prior to harvest. 

Production and movement within the tree of natural plant hormones

Fertilised ovules (i.e. developing seeds) produce hormones.

• Gibberellins are found in apple seeds have are implicated in the growth of fruits.
• Gibberellins are also essential for the development of apples with good skin finish and freedom from russet.

Tree health

The general health of trees is important and trees suffering significantly damaging pests or diseases will inevitably produce fruits of smaller size and quality at harvest.

• Most of the apple trees grown in the UK are free from the viruses and phytoplasmas that might influence fruit size and quality at harvest.
• However, care should be taken to ensure that any variety or clone used is as free as possible from these diseases.
• The disorder known as chat fruit reduces fruit size on some varieties, such as Lord Lambourne, and it is important that growers ensure that their young trees are free of this problem.