Grapevine Fruit Zone Leaf Removal

- In wine grape vineyards, fruit zone leaf removal is a critical canopy management practice that increases sunlight and temperature of grape clusters, which are measured by these sensors.

Cluster exposure to ambient light, typically leads to increased wine quality potential and better cluster fungal disease management, especially in humid grape-growing regions with high disease pressure, like Pennsylvania.

Relative to densely foliated shaded fruit zones, exposed fruit zones can have a greater airflow and reduce humidity and drying time.

Further, fungicide penetration and coverage on clusters are likely increased in open fruit zones.

Consequently, bunch rots are generally lower in exposed relatively to shaded grape clusters.

Grape cluster exposed to sunlight and increased temperature, tend to have a balanced primary fruit composition, namely an increase sugar to acid ratio, which is desirable for some cultivars and wine-making goals.

Secondary metabolites that impart varietals character are typically increased in grape exposed to higher sunlight and temperatures.

For example, Methoxyprazines, a compound class that produces green, vegetal wine aromas, tend to be lower in exposed grapes while Norisprenoides, a compound class that often produces floral, fruity wine aromas, is typically higher in sun-exposed grapes.

Note that the responses will depend on cultivar, vintage and the magnitude of fruit exposure.

If an insufficient number of leaves are removed, there may be a negligible difference in fruit chemistry.

And this is present when compared to shaded grape clusters.

This harvested Cabernet Sauvignon cluster has poor color development and fungal disease infection.

Both negative effects of excessive canopy shading.

Fruit zone leaf removal should be implemented after fruit set, between the baby size very stage and pea size very stage.

If leaf removal is applied to early or before bloom, the result can be reduced fruit set and cluster compactness, which can help moderate bunch rots in some cultivars.

However, the decrease fruit set can reduce cluster weights, so it's not recommended in all cultivars or when a yield reduction cannot be tolerated.

If leaf removal is not implemented in a timely manner, then all is not lost.

Some fruits only thinning between bunch closure and post-veraison is fine.

However, it may not be wise to completely defoliate the entire fruit zone in mid to late summer due to the threat of sun scald on berries that have developed in the shade since they were set in early summer.

The magnitude of fruit zone leaf removal depends on several factors, including when leaf removal takes place, cultivar, training system, rot susceptibility, and food composition goals.

In general, a fruit zone with zero to two leaf layers surrounding clusters, have lower fruit rot and better fruit quality potential, relatively to more share that fruit zones.

However, more modest leaf removal may be justified to reduce the risk of sunburn on the afternoon sun-exposed canopy side.

Fruit zone leaf removal is a canopy management practice that generally improves wine grape quality.

For more information on fruit zone leaf removal and canopy management, please visit the Penn State Extension Grape and Wine team's website at extension.psu.edu/grape-and-wine.

Thanks for watching and happy leaf pulling.