It has long been recognised that large apples are generally more susceptible to rotting and most physiological disorders than smaller fruit. However, in today’s market, it is not an option to produce small fruit in order to maximise storage potential.
The challenge to the grower is to ensure that trees do not under- or over-crop and that fruit are not over-sized. Advice on achieving adequate initial fruit set and on subsequent thinning of the crop is provided in Part 1 of the Guide.
The level of cropping is not always within the control of the grower and low fruit numbers may occur in ‘off years’ in biennial bearing trees or due to adverse weather conditions.
The higher susceptibility of larger fruit to disorders such as bitter pit and senescent breakdown is related to the generally lower concentrations of calcium and higher concentration of potassium and magnesium.
Growers should be aware of the need to supplement the uptake of calcium into the developing fruit by the use of foliar calcium sprays and to ensure that samples are taken prior to harvest for an analysis of the mineral composition.
These actions are particularly required for lightly cropping trees and where the fruit size is large. Although increasing the size of apples may generally reduce storage potential there may be positive effects on the eating quality of the fruit.
Thinning Cox trees to remove all but the first axillary flowers or fruitlets increases the dry matter content of fruit at harvest and the firmness of fruit at harvest and after CA storage.
The greatest increase in firmness was achieved by thinning in the period 5-15 days after full bloom (FB) and to a lesser extent in the period 30-40 days after FB. In view of the risks associated with the thinning of flowers it may be appropriate to delay thinning until 35-40 days after FB.
In a MAFF Link (Agro-Food) project significant improvement in the texture of Cox apples was achieved in 6 commercial orchards by hand thinning to one fruit per cluster in late June (approximately 5 weeks after FB).
Late thinning of the crop by hand may be impractical and too expensive and may not provide a sufficient increase in fruit size (see Part 1 of the Guide). A strategy of chemical thinning followed by hand thinning appears to be most appropriate and is likely to provide the size required for profitability whilst improving the eating quality of the fruit.
A recent review on flower and fruitlet thinning was conducted by ADAS (AHDB Project TF 215) evaluating the benefits of chemical, mechanical and hand thinning and the options of combining different techniques to manage fruit number. Deciding on the optimum number of fruitlets can be aided by a number of tools and models. The MAFCOT Equilifruit tool, developed in France uses cross sectional area of branches to estimate the optimum numbers of fruitlets to leave after hand thinning. Other crop load management tools and fruitlet growth models have been developed to help growers estimate final fruit size and yield. Research in AHDB Project TF 225 has investigated thinnig strategies to increase fruit dry matter.
Clearly it is imperative that Cox and other dessert apple trees are not allowed to over-crop. This will produce too many under-sized and under-coloured fruit and will lead to a delay in harvest date in an attempt to improve these characteristics. Equally in cultivars such as Braeburn, that have a tendency to produce oversized fruit in UK conditions, thinning can lead to large fruit that tend to be more prone to internal browning disorders.
Late harvest will compromise the duration of storage.
The importance of adequate leaf area in relation to sugar content and colour development has long been recognised.
The adverse effect of over-cropping on red colour development is particularly acute in Jonagold orchards. Consequently adjustment of crop load by thinning is particularly important for this cultivar.
Early experiments on Delicious carried out in the US showed that it was possible to increase the area of solid red on apples from 23 to 58% by increasing the ratio of leaves to fruit from 10 to 75.
Concerns about light crops and oversized fruits relate primarily to cultivars such as Cox’s Orange Pippin and Bramley’s Seedling that are susceptible to calcium-deficiency disorders.
Other cultivars such as Gala normally achieve high concentrations of calcium in the fruit and for these there may be less of a concern that light crops may compromise storage potential.