An Overview of Cluster-Zone Leaf Removal Strategies for Cool Climate Vineyards
By: Maria Smith and Michela Centinari
The grapevine canopy encompasses all of the above ground portions of the vine formed by the shoot system, including leaves, fruit, primary and secondary (lateral) shoot stems, and tendrils (Figure 1). Managing the canopy is a labor intensive process requiring a range of techniques such as winter pruning, shoot and cluster thinning, shoot positioning, hedging, and cluster-zone leaf removal (abbreviated in text as leaf removal) that span both the dormant period and growing seasons. For large and dense vine canopies with shaded clusters, canopy management practices including hedging, shoot thinning, and leaf removal are recommended, as these traits may result in reduced fruit and wine quality , increased disease pressure , and susceptibility to winter damage .
In this blog post, we provide a comprehensive overview on the practice of leaf removal, with an emphasis on how timing and intensity can affect and enhance the result of this management strategy in cool and humid climate grown grapevines.
Leaf removal (defoliation, leaf pulling, etc.) can be categorized into two distinct time periods:
- “Traditional” leaf removal – performed between fruit set and veraison
- “Early” leaf removal – performed between pre-bloom [i.e. immediately before bloom (E-L scale, stage 19) and fruit-set (E-L scale, stage 27)].
Traditional leaf removal is commonly recommended to improve the canopy microclimate of the fruiting zone, while early leaf removal is used for crop load regulation and decreasing cluster compactness. Both methods of leaf removal serve to enhance berry quality and decrease disease incidence.
Keep in mind the importance that timing and severity of implementation have with careful considerations for:
- Impact on yield parameters;
- Impact on fruit composition, aromas development, wine chemistry and sensory attributes;
- Risk of disease incidence;
- Risk of sunburn on fruit clusters.
Traditional Leaf Removal:
In cool climates traditional leaf removal is mostly considered beneficial. While it may be performed any time between fruit set and veraison, the “ideal” time is generally considered 2-4 weeks before veraison . During this pre-veraison period, light exposure and berry temperature are important factors that influence post-veraison fruit composition (sugars, acids, and aromas) and berry color development [4, 5]. While there are many complex interactions between light and temperature, it is thought that temperature is the primary factor regulating metabolic processes responsible for developing mature berry color [discussed in: 6].
The intensity of ‘traditional’ leaf removal usually varies between 40% and 100% in the cluster-zone, which may require removing two to five leaves around the clusters . The intensity is usually higher in red than white wine grape varieties, though varies based on canopy density and the climate of the growing region. Growers wishing to use traditional leaf removal should keep these in mind when deciding on when and how many leaves to remove. Partial defoliation (one side of the vine) can help mitigate the risk of sunburn in certain climates. For instance, in cool and sunny environments, only partially defoliating on the cooler side (e.g., the East-facing side of a North-South oriented row) may be needed . The risk of sunburn is highest in high-sun environments, and is less of a concern for many regions in the Eastern US where overcast conditions are common. Because of the overcast conditions, air and berry temperatures do not usually reach high enough levels for a sufficient time to cause heat damage.
“Early” leaf removal:
Early is a relative term. In the case of traditional leaf removal, “early” refers to soon after fruit-set. Studies conducted as recently as 10 years ago have come to redefine early as leaf removal between pre-bloom (immediately before bloom) to fruit-set (Figures 2 and 3).
Most of the studies pertaining to pre-bloom, trace-bloom, and fruit-set leaf removal have been performed in Mediterranean countries (Italy, Spain) on over-cropping varieties such as Sangiovese, Trebbiano, and Tempranillo. Removing basal and fully functional leaves at pre- and trace-bloom reduces the carbohydrate supply to the flowers compared with non-defoliated vines, leading to a reduction in fruit-set and likely to fewer berries per cluster . On the other hand, when basal leaves are removed at fruit set, the decrease in berry size is the principle factor contributing to reduced compactness and yield effects. As a result, the most consistent effects produced from early (e.g. within 4 weeks of bloom) leaf removal have been:
- Reduced yield and decreased cluster compactness [8, 9 and references within]
- Increased berry skin thickness [7, 10]
- Improved berry composition (increased Brix, decreased pH) and increased phenolic concentration [10, 11]
The impacts of early leaf removal may be beneficial for over-cropping varieties, and may also benefit tight-clustered varieties that are highly susceptible to rot (Figure 4). From the results of the above-listed studies, early leaf removal has been suggested and adopted as a means for controlling cluster rot diseases through decreasing cluster compactness (ELR 2014 FL Riesling , ) and crop load regulation on over-cropping varieties as a means to increase quality . However, keep in mind the potential for negative consequences that early leaf removal can have for excessive reduction in fruit-set in years where unfavorable weather has already reduced fruit-set. Early leaf removal is not a suggested practice for varieties that produce low yields.
Early leaf removal also has the potential to impact vine vegetative growth. Pallioti et al. (2011) found vine size (as measured by pruning weight), cane diameter and main leaf area significantly reduced after one year of treatment implementation as compared to non-defoliated vines. However, no negative consequence to vine carbohydrate storage was found, presumably due to higher rates of lateral leaf area development and higher photosynthetic capacity measured in the remaining leaves. While one might expect a decrease in bud fertility due to a loss in carbohydrate source production during a critical time for bud formation, only one study  thus far has found a reduction in bud fertility as a result of ELR. The lack of decrease in bud fertility may in part be related to higher local light exposure to the retained basal buds from winter pruning.
The extensive literature published on this topic suggests that the effects of early leaf removal on yield reduction, berry size, fruit composition and total phenolic accumulation vary based on timing (pre- vs post-bloom), intensity of leaves removed, variety, and climatic conditions of the growing site [7, 10, 11].
Current research from the Centinari lab at Penn State University aims to improve our understanding of how early leaf removal regulates yield parameters, fruit composition and affects winter survival in Pennsylvania-grown hybrid Chancellor and V. vinifera Grüner Veltliner.
Effects of leaf removal on berry aromas development and berry composition:
Modifications of sunlight and temperature conditions in the fruiting zone as a result of leaf removal impact (increase or decrease) simultaneously the concentrations of a variety of secondary metabolites and other fruit composition parameters. Individual grape phenolic and aroma compounds have different sensitivity to changes in light and temperature , therefore their responses to canopy microclimate from leaf removal modifications vary. In cool climate regions, increased cluster sun exposure is often associated with an increase in berry temperature into favorable ranges for the synthesis of anthocyanins and the formation (i.e., monoterpenes) or degradation (i.e., methoxypyrazines) of volatile aroma compounds . Increased sun exposure can also result in a decrease in juice titratable acidity (TA) at harvest, mainly due to a decrease in malic acid concentration. This may be desirable in cold and cool climates where in some vintages juice TA can be undesirably high.
The timing of leaf removal influences aromas development
Recent studies from Cornell University highlight the importance that timing of leaf removal has on aromas development in Cabernet Franc, Merlot and Riesling, three important varieties grown in the Eastern US.
- Methoxypyrazines: 3-isobutyl-2-methoxypyrazine (IMBP) is the primary contributing factor to the unpleasant green “vegetative” and “earthy” aromas associated with under-ripeness in cool climate grown Cabernet franc and other Bordeaux winegrape varieties. Therefore, high concentrations of IBMP negatively impact the perceived quality of the wine. In general, light exposed clusters accumulate less IMBP than shaded clusters. Thus, increasing sun exposure through leaf removal can be beneficial for decreasing levels of IBMP at harvest. However, the timing of leaf removal implementation should be taken into careful consideration. A study conducted in NY found that early season (10 to 40 day after bloom) basal leaf removal was more effective than post-veraison leaf removal in reducing IBMP accumulation in berries in Cabernet Franc and Merlot field grown vines in the Finger Lakes and Long Island, respectively .
- C13Nor-Isoprenoid: C13 Nor-Isoprenoid is a class of volatile compounds believed to be derived from carotenoid degradation. Included in this class of volatile compounds are beta-damascenone, responsible for floral and honey-like aromas, and TDN (1,1,6-trimethyl dihydronaphthalene), responsible for “petrol” like aromas. While “petrol” aromas may be beneficial in aged-Riesling wines, they are generally perceived negatively in young Rieslings and in high concentrations. Timing of post-fruit set leaf removal (2, 33, and 68 days past fruit-set) was shown to affect levels of pre-cursor compounds associated with “petrol” aromas in Riesling. Mid-season cluster sunlight exposure (33 days following fruit-set) was reported to have the highest amount of pre-cursor compound concentrations in juice and wine, suggesting this time should be avoided if a reduction in “petrol” aromas is desired (Kwasniewski study summary ).
- Monoterpenes: This class of volatile compounds is responsible for many of the pleasant aromatic qualities found in V. vinifera varieties such as Riesling, Muscats, and Gewürztraminer and hybrids Traminette and Vidal Blanc. Many studies have shown that pre-veraison cluster exposure to sunlight effectively increases concentrations of monoterpenes in Gewürztraminer, Riesling, Golden Muscat, and Traminette. However, few studies have evaluated monoterpene concentrations in response to the timing of leaf removal. It seems, though, that leaf removal before mid-season is best for enhancing accumulation post-veraison (Reviewed in: ).
Effects of leaf removal on the incidence of fungal diseases:
Another important benefit for grape clusters in a humid growing climate is cluster exposure for air circulation and spray penetration, which serve to decrease the incidence of fungal diseases and increase the effectiveness of fungicidal sprays. Numerous studies over the years have consistently found that leaf removal regardless of timing decreases the incidence of fruit rot, in particular the bunch rot fungus Botrytis cinerea and powdery mildew [2, 17]. For more information on fruit rots, see: 2015 Post Bloom Disease Management Review.
Implications for leaf removal on bud fruitfulness and hardiness:
Warm temperatures and light exposure are essential in developing healthy bud tissue for increasing bud fruitfulness and bud cold hardiness. Sanchez and Dokoozlian (2005) showed a direct and positive relationship between shoot light exposure and bud fruitfulness for Chardonnay and Cabernet Sauvignon vines under warm climate conditions of the San Joaquin Valley region of California. Wolpert and Howell (1985) found primary bud cold hardiness in Michigan-grown Concord increased from 4.5°F (2.5°C) to 10.8°F (6.5°C) in well-exposed buds to sunlight compared to those under shaded conditions during winter acclimation.
Leaf removal mechanization:
Leaf removal is both labor and time intensive to implement in the vineyard. The costs associated with leaf removal implementation may be not fully recovered from the sale of grapes. This is particularly true for the hybrid wine grape varieties since the price of grape currently averages $600-1200 per ton for inter-specific hybrid varieties (PSU Ag Alternatives; for NY and VA wine grape prices, see: NY Finger Lakes grape prices 2014; Virginia grape prices 2014). Therefore, it is important for growers of hybrid varieties to maintain low production costs. Mechanization in other regions has shown promise as an effective cost-saving measure for implementing leaf removal for vine yield control in vertical shoot positioned vines (VSP) using a bladeless pulsed air system that shred leaves without damaging fragile shoots and inflorescence .
Mechanizing early leaf removal is a major topic of interest for the Pennsylvania wine industry. Current research supported by the Pennsylvania Wine Marketing Research Program focusing on pre-bloom mechanical leaf removal is being conducted by Penn State (PI B. Hed; co-PI M. Centinari) at the Lake Erie Grape Research and Extension Center and in collaboration with grower cooperators for feasibility of use on VSP trained V. vinifera and high wire trained hybrid varieties in Pennsylvania (Figure 5).
Conclusions from leaf removal studies:
Leaf removal strategies can be effective for improving both grape and wine quality. However, careful considerations for your site’s growing conditions and vine variety should be made before deciding to use leaf removal. Timing and intensity are the most important factors to consider in regards to the vine variety. If you are interested in trying either early leaf removal or traditional leaf removal techniques in your vineyard, begin with a small selection of vines to implement treatments. Be sure to record crop yields and fruit composition (Brix, pH, TA) and quality (fruit aromas and flavors, etc.) data to compare among the treated and untreated vines.
For more information:
- Grapevine canopy management: Richard Smart and Mike Robinson – Sunlight into Wine: A handbook for winegrape canopy management.
We would like to acknowledge the Pennsylvania Wine Marketing Research Board for their support in funding early leaf removal research projects in Pennsylvania.
Maria Smith is a viticulture PhD candidate with Dr. Michela Centinari in the Department of Plant Science. She specializes in cold stress physiology of wine grapes. She is currently funded by the John H. and Timothy R. Crouch Program Support Endowment, an endowment founded and funded by the Crouch brothers, original owners of Allegro Winery in Brogue, PA.
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