2.3.8.c When should leaf thinning be carried out ?

When should early leaf thinning be carried out?

Early leaf removal is the most effective to reduce the risk of Botrytis cinerea. Its effect is twofold: direct, as a preventive method to fight against the fungus's development by reducing the humidity of the microclimate; and then indirect, as early leaf removal can lead to a decrease in the number of berries per bunch by the plant's natural compensation phenomenon, the bunches are then less compact and less prone to Botrytis contamination.
The best time to intervene is around fruit set.
Early leaf thinning makes the treatments more effective and reduces the risks of scorching by enabling gradual acclimatization to sunlight. However, this practice also has its drawbacks. Leaf thinning at the beginning of the growth cycle means removing young, photosynthetically active leaves. This can lead to a decrease in photosynthetic activity and a decrease in the migration of photoassimilates towards the bunches. When leaf removal is performed at this stage, the number of leaves removed should be limited. In this case, it can be done after trellising operations. Leaf removal around flowering also encourages the synthesis of tannins and anthocyanins.

Should leaf thinning be carried out between bunch closure and véraison?

Leaf removal during this period can have several advantages, provided it is carried out in moderation. Ventilation of the area surrounding the bunches by removing some leaves is beneficial for the ripening of the berries and ensures they stay healthy. This practice is only recommended if it is done gradually, as there is a risk of sun scalding, particularly in hot regions.

Is leaf thinning during ripening worthwhile?

Leaf thinning during ripening can be problematic. Thinning out the leaves two to three weeks before the harvest can help the grapes to ripen and remain healthy. However, it can also be detrimental to yield and quality because, in very hot weather, bunches that are not used to the sun may be scalded. If high temperatures are forecast, it is unnecessary to protect the grapes against Botrytis. Moreover, removing any leaves reduces the supply of nutrients and the period of exposure to the sun may not be sufficient to obtain a significant improvement in ripeness.

Is a late leaf removal a few days before the harvest necessary?

Very late leaf thinning, i.e. a few days before the harvest date, has no advantages for the yield or the quality of the grapes (polyphenol concentrations). Removing leaves just two or three days before the harvest does not sufficiently affect the exposure of the leaves to the sun. However, this operation is often carried out for two reasons:

• For some late grape varieties, the harvest date can be as late as October. There can be abundant rain at this time of year, which can then compromise the vines' health. This is when there is the highest incidence of Botrytis due to humidity. Leaf removal a few days before the harvest allows more air to circulate around the bunches and reduces the risk of fungal diseases.

• To facilitate the grape-pickers' work and reduce the number of leaves harvested with the grapes, that possibly end up in the vats.

Late leaf thinning exposes the grapes to the sun, so there can be a risk of scalding. Too much thinning of the leaves a few days before harvest can also slow the lignification of the young branches. It is then necessary to make sure that the vines do not suffer too much water stress.