Mechanised Pre-flowering Grapevine Leaf Removal: Benefits and Risks

Pre-flowering leaf removal is a recent practice in viticulture. As well as reducing fungal disease pressure, it limits yield and improves grape composition. An Agroscope study shows that mechanising pre-flowering leaf removal saves effort but also involves risks.

Grapevine leaf removal in the cluster area is a common practice in temperate and cold climates.  It is generally done between flowering and cluster closure to create a less favourable microclimate for fungal diseases. When done before flowering, leaf removal limits berry formation and reduces yield with a view to respecting production quotas. Moreover, it may also have a positive impact on berry composition.

A promising method for Swiss conditions

A previous Agroscope study showed that pre-flowering leaf removal had a positive effect on five grape varieties in Swiss environmental conditions. Conducted over a five-year period on Doral and Gamay grape varieties, the present trial confirms these findings. In addition, the possibility of mechanising this method was examined. A low-pressure dual-airflow leaf stripper (Fig. 1) was tested in comparison with manual leaf removal.

Effects on yield and berry quality

Compared to post-flowering leaf removal, pre-flowering leaf removal had an impact on yield. In the Agroscope trials, this method caused a 20% to 30% annual decrease in the number of berries per cluster, and a proportional decrease in estimated yield (Figure 2). It also gave rise to an average 2% increase in sugar concentration, as well as a 3% decrease in total acidity, although this effect was not significant for all years. The absence of Botrytis cinerea during the period of the trial prevented confirmation of the effectiveness of leaf removal in preventing the development of fungal diseases.

The mechanical approach further reduces yield …

The mechanical approach allowed for effective pre-flowering leaf removal without damaging the fragile shoots, although the loss of a few flower buds was observed. It had a stronger impact on the number of berries per cluster and on their maturation (-1% sugars, +5% total acidity) than manual leaf removal. With mechanical leaf removal, the lateral shoots grew and partially covered the cluster area, whilst manual leaf removal eliminated these shoots completely (Figure 3). Furthermore, the estimated yield was up to 20% lower compared to manual leaf removal at the same date (Figure 2).

… and involves less effort in cluster-thinning work

Compared to post-flowering leaf removal, mechanical leaf removal reduced cluster-thinning work for Doral and Gamay varieties by 69% and 27% respectively (Figure 2). No difference was observed between the Doral wines, whilst the Gamay wines from mechanical leaf removal tended to be less bitter (-7%), with smoother tannins (+6%) than those from manual leaf removal (p<0.10).

Mechanical leaf removal can reduce fruitfulness

Despite the positive impact on Gamay wines, pre-flowering mechanical leaf removal was too intense and caused a loss of fruitfulness compared to post-flowering leaf removal (-10 % and -8 % on average for Doral and Gamay, respectively). If leaf removal is too intensive, in young vines or in the presence of a significant water deficit or nutrient deficiency, the method can cause excessive yield losses and affect the long-term vigour and fruitfulness of the vine.