MSU Extension Grapes

In Michigan’s expanding wine industry, the need for higher fruit quality is no longer optional, it is essential for establishing a reputation in a competitive national market. As producers work to elevate the profile of Michigan-grown wines, achieving consistent, high-quality fruit is a foundational priority for growers and winemakers alike. However, vineyards in northwest and southwest Michigan face persistent challenges.

The growing season is relatively short, which limits the time for full physiological fruit ripening. Excessively vigorous vegetative growth, driven by excessive rainfall and often by the excessive use of chemical fertilizer by the growers, leads to shaded fruit zones and dense canopies. Combined with persistent seasonal humidity, these conditions create an ideal environment for fungal infections and fruit rot, particularly in compact-clustered Vitis vinifera cultivars such as Pinot Noir, Chardonnay and Riesling, which are the most widely planted cultivars and represent flagship varieties of Michigan’s wine industry.

To compound the challenge, growers are under increasing pressure to reduce chemical inputs, especially fungicides, which are already heavily relied upon to manage annual outbreaks of downy mildew, powdery mildew and Botrytis. These environmental constraints and market demands have created an urgent need for practical, scientifically validated viticultural strategies that enhance fruit quality while supporting sustainable production.

Over the past two decades, the viticulture program at Michigan State University (MSU) has taken a leading role in addressing this need through close partnerships with growers, Extension educators and research collaborators. One of the program’s most impactful contributions has been the development and validation of early leaf removal, a pre-bloom canopy management technique that involves the selective removal of basal leaves near the fruit zone approximately 10 to 15 days before flowering.

Unlike traditional cluster zone leaf removal, which typically occurs post-fruit set or at veraison, early leaf removal is applied during a critical developmental window. This timing influences hormonal and carbohydrate allocation within the vine, leading to reduced fruit set, looser clusters, enhanced sunlight penetration, and improved berry exposure from the earliest stages of fruit development.

Under the leadership of Paolo Sabbatini, PhD, professor of viticulture in the Department of Horticulture, and Joshua VanderWeide, PhD, assistant professor and viticulture specialist, MSU’s early leaf removal research has evolved from localized trials in commercial Michigan vineyards into a comprehensive scientific portfolio. This body of work spans from on-farm demonstration plots for grower-led innovation to high-resolution metabolomic studies and global meta-analyses synthesizing dozens of independent trials. Taken together, the research presents a compelling case for early leaf removal as a climate-resilient, quality-enhancing and biologically intelligent canopy management strategy, one that is exceptionally well-suited to the conditions of cool-climate wine regions like Michigan.

The following will present the physiological, data-driven results that underpin early leaf removal, as demonstrated through a robust body of research published in peer-reviewed scientific journals. These findings have directly shaped the practical application of early leaf removal in vineyards across Michigan and have been embraced by many of the state’s leading wineries. Notably, the technique has also been widely adopted by vineyard management companies who have now integrated early leaf removal into their core portfolio of canopy practices offered to clients. Their endorsement reflects a strong alignment between cutting-edge academic research and the operational priorities of modern viticulture, namely, fruit quality, sustainability and long-term vineyard resilience.

The following presents key findings from that research in a summarized, practical format. If grape growers or winemakers are interested in the original scientific manuscripts briefly summarized here, please contact Sabbatini at sabbatin@msu.edu.

Rethinking canopy timing: The biological rationale for early leaf removal

Traditional leaf removal, typically performed after fruit set or at veraison, is designed to expose grape clusters to sunlight and improve airflow to reduce fungal disease. However, by this stage, the clusters have already formed and are often tightly packed, making them more vulnerable to Botrytis and sour rot, necessitating fungicides chemical applications to protect them from rotting (Figure 1).

Early leaf removal works differently. By defoliating the basal leaves around the inflorescences just before bloom, growers can alter the source-sink relationship and hormonal signaling in the vine during a critical period of floral development. This leads to reduced berry set, resulting in looser clusters that are less prone to rot. Looser grape clusters significantly reduce the risk of rot by enhancing air circulation within the fruit zone. When berries are spaced apart, air can flow more freely between them, allowing moisture from rain, dew, or high humidity to evaporate more quickly. This reduces the duration of wetness on the berry surface, which is a key factor in limiting fungal growth.

In addition, loosely formed clusters experience less internal pressure between berries. In compact clusters, berries are tightly pressed together as they grow and swell, which increases the likelihood of skin damage, microcracks or bruising, especially during heat events or following rainfall. These micro-injuries serve as entry points for pathogens such as Botrytis cinerea or the organisms responsible for sour rot. By reducing berry-to-berry contact, looser clusters maintain skin integrity and act as a natural barrier to infection.

In addition, the early increase in sunlight exposure initiates flavonoid biosynthesis and can improve color development, skin thickness and antioxidant levels. In this way, early leaf removal is not merely a physical adjustment of canopy architecture but a developmental intervention that fundamentally impact grapevine physiology.

Early trials in Michigan vineyards: Crop load and fruit composition

The first set of experiments on early leaf removal research at MSU began in 2008 with field trials conducted in collaboration with commercial vineyards in Michigan’s Leelanau Peninsula, Old Mission Peninsula and the Lake Michigan Shore region. The MSU team applied manual early leaf removal to Pinot Noir, Merlot, Chardonnay, Riesling and Cabernet Franc, comparing one-sided and two-sided pre-bloom defoliation with post-fruit set or no defoliation controls and several levels of intensity of early leaf removal (numbers of leaves removed or percentage of leaf area retained on the shots).

Results from these multi-season trials showed that early leaf removal significantly reduced fruit set, producing looser clusters with more uniform berries. Fruit ripened more evenly, with higher sugar accumulation, lower titratable acidity, and greater skin pigmentation in red varieties. These compositional shifts contributed to more stable wine color and balanced sensory profiles. The early improvement in fruit-zone microclimate reduced disease incidence, particularly in tight-clustered cultivars. The field-based nature of these experiments demonstrated early leaf removal’s practical value and its adaptability across diverse vineyard blocks in Michigan (Figure 2).

Grapes under the microscope: Metabolomics reveal deeper impacts

To better understand how early leaf removal influences grape chemistry at the molecular level, MSU researchers turned to metabolomics: the study of the full set of small molecules (metabolites) produced by a plant during its physiological processes. In plant physiology, research is used to understand how vineyard practices affect berry chemistry, flavor compounds and fruit quality at the molecular level. This approach provides a detailed snapshot of grapevine responses to environment and management.

In one of the most detailed biochemical studies by the MSU viticulture team, researchers investigated how early mechanical defoliation affected Merlot grapevines, analyzing berry composition using ultra-high-performance liquid chromatography coupled with mass spectrometry. The data revealed that early leaf removal-treated berries contained significantly higher levels of flavonoid compounds, including quercetin, myricetin, catechins and malvidin glycosides. These compounds are critical for wine color stability, mouthfeel and antioxidant potential. Notably, the biosynthesis of these compounds was initiated early and sustained throughout ripening, suggesting that light exposure at pre-bloom had lasting effects on fruit metabolism.

Furthermore, the results demonstrated for the first time in scientific literature that mechanical early leaf removal could replicate the biochemical benefits of manual treatments. This provides a scalable and cost-effective approach for larger vineyard operations.

Beyond the vineyard: Health-enhancing compounds in early leaf removal grapes

As interest in functional foods and nutraceuticals has grown, researchers at MSU in collaboration with Ilce Medina, associate professor in the Department of Biosystems and Agricultural Engineering and director of the Food and Health Engineering Laboratory, explored whether early leaf removal could influence not just wine quality, but also the health properties of grape products. Focusing on proanthocyanidins, polyphenolic compounds with known antioxidant and anti-inflammatory activity, the team measured total phenolics and antioxidant capacity in early leaf removal-treated Merlot, Cabernet Franc and Pinot noir grapes using chemical assays.

The results were compelling. Grapes from early leaf removal vines had higher levels of total phenolics and significantly stronger antioxidant activity than untreated controls. The most notable increases occurred in the berry skins, where light-induced metabolic changes appeared to activate protective pathways early in development. This study suggests that early leaf removal can enhance the nutraceutical value of grapes, offering benefits for juice, dried skin supplements and other health-oriented grape products beyond wine.

Disease defense: Early leaf removal as a sustainable rot management strategy

One of the most consistent and practical benefits of early leaf removal has been its role in reducing fruit rot. In a multi-year study on Pinot grigio and Pinot noir, MSU researchers compared early leaf removal with cluster thinning and post-veraison defoliation under varying weather conditions. Vines that received pre-bloom leaf removal, especially two-sided, exhibited significantly lower rates of Botrytis and sour rot at harvest. In addition to improving air circulation, early leaf removal resulted in a thicker berry skin and more open clusters, which dried more quickly after rainfall. These changes limited the environmental conditions that promote pathogen spread, offering a non-chemical pathway to disease suppression.

The study found that integrating early leaf removal with other balanced canopy practices, such as shoot positioning and light cluster thinning, yielded the highest-quality fruit with the lowest disease risk, even under adverse weather conditions.

A global perspective: Meta-analysis confirms early leaf removal Benefits

Following the positive results in Michigan, MSU researchers conducted a comprehensive meta-analysis of the scientific literature to evaluate the broader relevance of early leaf removal. This work synthesized findings from more than 50 studies conducted in various grape-growing regions across North America, Europe and the Southern Hemisphere. The meta-analysis confirmed that early leaf removal consistently resulted in moderate yield reductions due to fewer berries per cluster, without significantly compromising berry size.

Importantly, early leaf removal was shown to increase anthocyanin and flavonol concentrations, enhance grape coloration and lower rot incidence in climates with high rainfall or dense canopies. This synthesis validated early leaf removal as a reproducible strategy that is especially beneficial in vigorous, cool-climate vineyards like those in Michigan. It also provided a framework for predicting early leaf removal outcomes based on cultivar, climate and timing, offering growers a more evidence-based pathway to implementation.

Take-home message: Applying early leaf removal effectively

The evolution of early leaf removal from experimental trial to mainstream vineyard practice has been driven by rigorous science, regional collaboration and a deep understanding of grapevine physiology. Michigan State University’s work in this area has demonstrated that early leaf removal is far more than a canopy adjustment, it is a developmental strategy that empowers growers to control quality, composition and sustainability outcomes from the start of the season.

As climate variability increases and the demand for higher quality fruit continues to grow, early leaf removal offers a scalable, scientifically validated tool for improving vineyard performance across Michigan. From reduced rot to better wine color and improved harvest timing, its impact is clear: early leaf removal changes the physiology of the grapevine in ways that benefit growers and winemakers.

To maximize the benefits of early leaf removal for fruit set control and fruit quality enhancement, growers should follow these key guidelines:

Timing is critical

• Apply early leaf removal before bloom or at the latest during full bloom. This developmental window is essential for influencing carbohydrate allocation that affect fruit set.
• Avoid applying early leaf removal after fruit set, as its effectiveness on cluster structure and berry development drops significantly.

Target the right leaves

• Remove approximately six to eight basal leaves per shoot, focusing on the fruit zone (the lowest third of the canopy where clusters emerge).
• This typically represents about 40% of the shoot’s total leaf area at that stage of growth.

Defoliate precisely

• Target one or both sides of the canopy depending on variety sensitivity (e.g., Pinot Noir may require softer defoliation) and vine vigor (high vigorous vines require a more intense defoliation).

Expect key outcomes

When applied correctly and at the right time, early leaf removal can result in:

• Reduced fruit set and looser cluster architecture
• Improved air flow and sunlight penetration
• Lower risk of rot and fungal infection
• Enhanced phenolic maturity, color and aroma compound development
• Earlier and more uniform ripening

Integration with other practices

Early leaf removal works best when combined with:

• Low cordon training systems (e.g. Guyot) with spur or cane pruning
• Proper shoot positioning and shoot tucking before the manual or mechanical early leaf removal

By following these guidelines, grape growers can use early leaf removal not just as a disease-prevention tool, but as a strategic, quality-focused intervention that shapes the entire vintage from the outset.