Pneumatic defoliation - Change in fruit growth (part 1)

In order to intensify the body colouring of medium- and late-ripening apple varieties, pneumatic defoliation is increasingly being carried out before harvesting, especially in valley floor areas. The Laimburg Research Centre (South Tyrol/Italy) conducted trials in this regard in autumn 2021. The fruit size was measured regularly in two practical facilities following pneumatic defoliation in order to record the effects, including any negative ones.

After defoliation, an increased incidence of light and temperature becomes possible. An even colouring and ripening of the fruits is the result.

The basic idea behind the defoliation of apple trees is to improve the incidence of light by partially removing the leaves. This is intended, depending on the variety, to significantly increase the colouring of the fruit. At the same time, leaf mass that the tree would normally need for photosynthesis is removed. This raises fears that defoliation before harvesting might have a major negative impact on fruit size development.

Correct timing is a very important factor in pneumatic defoliation: it should not be done too early on a commercial plantation, otherwise the period of reduced fruit growth will be even longer. Various trials have shown that intervening around two weeks before the planned harvest is sufficient to achieve a significant improvement in fruit colouring. Delaying the defoliation for as long as possible has the advantage that fruit growth remains for longer at a high level. Following defoliation, on the other hand, the fruits will need a few days so as to develop their body colouring, which can lead to a delay in harvesting. Nor is excessive defoliation desirable, i.e. the leaves should not be completely removed, as it must be assumed that this will increase the negative effect on fruit growth.

A test carried out by the Laimburg Research Centre showed that defoliated trees of the Braeburn and Fuji varieties showed about 0.30% less fruit growth after some 4 weeks. This relatively small loss in fruit size should normally be compensated by much better fruit colouring and is thus no longer be of economic importance.

Source: Laimburg Research Centre (South Tyrol, Italy): Christian Andergassen
Image: fruit-tec.com – REDpulse Duo