If your sprays are not penetrating into the fruit zone…then your crop is not being fully protected and your spray dollars are simply being wasted. While spray penetration is one of the top reasons for practicing leaf removal, leaf removal practices can also impact several characteristics of the resulting wine. Read on to learn more about why and how leaf removal impacts your crop and your wine.
What is leaf removal?
Leaf removal, also called leaf pulling, thinning, or tunneling, is the process of removing leaves in the fruit zone to impact the microclimate around the clusters by allowing more airflow and sun exposure.
Why is leaf removal used in the vineyard?
Reduce Disease Pressure
One of the primary reasons for growers to remove or thin leaves in the fruit zone, especially in warm, humid climates, is to improve air circulation and allow more sun exposure around clusters to lessen fungal disease and bunch rot pressure. Fungal diseases of grapes prefer a damp dark environment, so allowing better air flow around the fruit with some sunlight penetration can greatly reduce these disease pressures for growers.
Improve Spray Penetration
Another key reason that growers may benefit from leaf removal or thinning is to improve spray efficacy. If your sprays are not fully penetrating to the fruit zone because leaves are blocking the clusters, then your crop is not being fully protected and your spray dollars are being wasted.
Improve Fruit Quality
Growers also leaf thin to change the fruit quality in some way and the resulting wine character. Some herbaceous or vegetative notes that some varieties hold can be decreased by leaf removal. Leaf removal can also be used to influence the titratable acidity, change the pH, and sometimes even reduce the potassium levels of your resulting fruit. Potassium levels affect the mouthfeel of the wine and resulting wine pH, and pH affects the wine’s stability for aging.
More sun exposure to the fruit has been shown to increase some polyphenols, phenols, and other precursors to flavors in wine, but some studies have also shown that too much sun exposure can have detrimental effects on wine quality, so like everything else in grape growing (and in life), finding the right balance is essential.
Change Fruit Set or Berry Size
A final, but less common reason for leaf thinning is to alter fruit set. Extreme fruit zone leaf removal at the onset of flowering, or even before flowering or bloom, can remove some of the resources for the flower and can reduce the numbers of berries set, berry size, and even loosen up the cluster morphology. A grower might choose to use this for very tightly clustered varieties in an effort to reduce late season bunch rot, however it requires heavy leaf removal and this may not be ideal for all climates, especially where sunburn is an issue. It is also very labor intensive.
When should leaf removal be done?
Leaf removal timing depends on your climate and your objectives and generally falls into 3 main time periods.
Early season leaf removal is completed shortly before bloom or before pre-bloom or right at the onset of trace-bloom. Early leaf removal is most commonly used for growers trying to either reduce berry set (which is the case with very early and heavy leaf removal) or for varieties that are susceptible to brunch rot or Botrytis flower wilt where increased airflow and spray coverage is needed for the flowers as well as the newly developed fruit.
Most commonly or traditionally, leaf removal is done immediately after fruit set and before bunch closure when berries are about peppercorn size or 2-3 mm in diameter. Opening up the fruit zone during this time allows for improved spray penetration, increased airflow and sunlight exposure for the developing berries. Allowing the developing fruit to get early sun exposure provides essentially a base tan, if you will, so that when the full summer sun comes it does not hinder ripening by burning the berries.
Late season leaf thinning at berry softening or version can be done if adequate leaf pulling was not completed earlier in the season, if there are issues with bunch rot or spray penetration, or if additional laterals and new growth have covered over the fruit zone. Leaf thinning at this point in the season has to be done cautiously to avoid sunburn to the fruit. To mitigate sunburn risks, growers can choose to leaf pull only on the early morning side of the canopy or pull only every other leaf or just those leaves immediately in front of clusters.
To learn more about what research data shows about the benefits of leaf removal and how mechanical leaf removal works in a vineyard listen to vineyardundergroundpodcast.com/vu020 with guests Dr. Cain Hickey from Penn State University and seasoned grape grower Galen Troxel from Galen Glen Vineyards in Pennsylvania.