NUMBER AND SIZE OF FRUITS

It is well established that susceptibility to pit increases with fruit size.
Martin (53) showed that fruit diameter was highly correlated with susceptibility to pit in Cleopatra apples and many other workers have since obtained similar data.
Our chairman, Mr.
Perring, and others have shown that the concentration of calcium in the fruit drops as fruit enlargement takes place due to the failure of the developing fruit to maintain a sufficiently high rate of calcium uptake during the later part of the season.
Climatic factors and tree vigour also influence the rate at which calcium moves into the developing fruit.
In lightly cropping trees, where there is a relatively high leaf: fruit ratio, the growth rate of the fruit is high because of the readily available supplies of assimilates but the imbalance between leaves and fruits favours the movement of calcium into the leaves of the extension shoots rather than into the fruits.
Consequently any orchard treatment which promotes fruit size and vegetative vigour increases the risk of bitter pit.
Many workers favour the theory first advanced by Wiersum (99) that the high K:Ca ratio of fruits on light-crop trees is brought about because the rate of transpiration of apple fruits is much weaker than that of leaves.

Rootstock type

The type of rootstock may affect bitter pit development in a number of ways.
In the first case the rootstock may have an effect via its influence on tree vigour.
Thus we find that fruit produced by trees on more vigorous stocks are more likely to develop bitter pit (46). However, these effects may be masked by the soil type.
If the trees are growing in a fertile soil and not subject to water stress bitter pit is unlikely to be a problem even on very vigorous stocks.
On a poorer soil the same set of rootstocks may show marked differences especially where calcium levels are generally lower.

II Discussion Meeting on Bitter Pit in Apples

M.A. Perring, R.O. Sharples

https://doi.org/10.17660/ActaHortic.1974.45.3