A Brief Review of Fruit Zone Management Principles

Cain Hickey and Michela Centinari reviewed the fundamentals of canopy fruit zone management as part of the Penn State Wine and Grape Team's weekly webinar series.

Fruit zone leaf removal is timely when implemented during late bloom into the BB-size or pea-size berry growth stages. Below is just a brief review of fruit zone management fundamentals. Use the words below in tandem with the attached slides for a review on how fruit zone management can be effectively implemented in your own vineyard management program.

Bunch rots. 
Leaf removal is unnecessary to manage bunch rots in arid/dry regions like eastern Washington and California, but fruit zone leaf removal aids in bunch rot management in PA and other humid growing regions in the eastern US. Leaf removal is more critical to manage rots in rot-prone cultivars relative to those that are less prone (see below about "prioritization of leaf removal"). The slides provided (above) show several cases in which bunch rot was reduced in exposed relative to shaded clusters. The photo of the Cabernet Sauvignon cluster, below, was taken after harvesting that cluster from a deeply shaded canopy; this is a good visual that shows excessive shade can result in rot development.

This Cabernet Sauvignon cluster has poor color development and Botrytis bunch rot. It was harvested from a shaded canopy. This photo summarizes the effects that shaded fruit zones can have on rot and wine quality potential in humid regions. Photo credit: Cain Hickey.

Fruit composition.
In general, exposed clusters have enhanced "varietal character," meaning that increased radiation and temperature enhance some of the "positive" sensory impact compounds associated with a particular cultivar. Exposed clusters can have reduced "negative" sensory compounds, such as those associated with undesirable, herbaceous aromas in some Bordeaux reds (e.g., Cabernet Franc). Well-exposed clusters also tend to have lower acidity than those ripened in shade. Thus, one must consider winemaking goals and use fruit zone management to target grape compositional attributes that can increase the chances of achieving the desired wine style. As an extreme example, acid preservation is key for sparkling wine production while color and aroma enhancement, in addition to lower acidity, may be desirable for the production of some white and red wines. In these two unique cases, fruit zone management (in tandem with harvest date) can be used to target the desired compositional attributes.

The shaded (left) and exposed (right) sides of the same two clusters. This photo exemplifies how radiation can change the color (and likely the primary and secondary metabolite profile) of grape berries. Musts composed predominantly of grapes similar to those in the left photo will likely be good for sparkling or "fresh" wine styles while those made primarily with grapes similar to those in the right photo are predicted to produce less acidic, more aromatic wines. Photo: Cain Hickey.

As with bunch rot management, leaf removal may be more critical to enhance color in red grapes grown in the eastern US compared to drier, warmer regions of California, eastern Washington, and southern Oregon. In fact, there are several cases in the attached slides (above) where exposed red grapes had greater anthocyanins and phenolics than shaded red grapes; those studies were conducted in humid growing regions. Studies conducted in hotter and drier regions in the western US have shown that well-exposed red grapes had reduced anthocyanin levels compared to shaded grapes, and this was particularly true on the "hot" or "afternoon sun-exposed" canopy side (e.g. on the west side of north/south-oriented rows). The variable cloudiness experienced in humid, eastern US growing regions attenuates the radiant heating of exposed grape clusters; clouds act as "large leaves" that preclude radiation penetration to the fruit zone. Fruit zone leaf removal thus may be considered a less "risky" practice in the eastern US, where radiation, heat, and air movement around the fruit zone can result in rot-free grapes with a desirable and balanced primary and secondary metabolite profile.

When, how, and to what extent? 
Common questions regarding optimal leaf removal practice are "when should I remove leaves?", "how should I remove leaves?", and "how many leaves should I remove?". 

When? 
In general, leaf removal is ideally implemented between late bloom and pea-size berries. Implementing leaf removal between these growth stages will aid in spray penetration to, and air movement through, the fruit zone at the beginning of the critical period for cluster disease protection (bloom through bunch closure). Pre-bloom (or "during bloom") leaf removal to excessive amounts (e.g. removal of greater than 4 leaves per shoot) can cause a precipitous decrease in crop yield; therefore, it is judicious to moderate the extent of leaf removal when it is implemented before or during bloom. Implementing leaf removal before pea-size berries will allow berries to acclimate to ambient radiation outside of the canopy starting at an early stage of berry development. That said, some remedial leaf thinning between bunch closure through veraison, and even after veraison, will create a microclimate that is less favorable to rot development and one that will improve wine quality potential. In many cases, a "follow-up" leaf removal pass, or two, will be needed after the initial effort in order to maintain a favorable fruit zone microclimate as lateral shoots and leaves continue to grow throughout the growing season in a humid climate. 

Pea-size berries, an arbitrary growth stage terminus for the implementation of "early season leaf removal". Photo: Rachael White.

How? 
Leaf removal has been traditionally performed manually by removing leaf blades and petioles and lateral shoots from the fruit zone by hand. Manual leaf removal is still more common than mechanized leaf removal. However, "air pulse" and "cutting" types of tractor-mounted, mechanized leaf removal machines are becoming popular in eastern US vineyards. In fact, a few vineyard and winery enterprises in PA employ the use of a mechanical leaf removal machine. Mechanical leaf removal offers the advantage of practice efficiency, which results in leaf removal implementation across sizable acreage within a narrow time-frame. While speculative, and variable on a case-specific basis, mechanical leaf removal may be worthwhile in vineyards larger than 10 or 15 acres, as upfront cost of the machine may be offset in only a few growing seasons (assuming that leaf removal is viewed as a meaningful practice and is consequently manually implemented across the whole vineyard). It is safe to say that upfront costs will be recouped faster with increasing vineyard size; it would take several growing seasons to offset the upfront costs of a mechanized leaf removal machine in a 5-acre vineyard.

To what extent? 
In general, leaf removal to an average of one or two leaf layers surrounding the fruit zone should offer some rot management and fruit composition benefits relative to unmanaged fruit zones. In general, removal of two or three leaves per shoot from the fruit zone region will produce an average of one or two fruit zone leaf layers. Leaf removal to greater extents (as in the Merlot photo, below) can reduce rots, increase anthocyanins and phenolics, and reduce acidity, collective traits that may be desirable in the production of some red and white wine styles. Fruit zone leaf removal to such magnitudes (zero leaf layers) may not be commercially feasible due to the amount time and labor required to do so. 

A Merlot fruit zone with zero leaf layers from a RESEARCH (not commercial) plot. While such fruit zones may improve spray penetration and rot management and produce less acidic grapes with desirable primary and secondary chemistry, commercial implementation to such extents would be difficult. Photo: Cain Hickey

Refinement for fruit zone leaf removal prioritization.
Fruit zone leaf removal is a vineyard practice used to manage cluster diseases, fruit composition, and crop yield. Like other vineyard practices, the effectiveness of fruit zone management is optimized when it is refined by region and cultivar. Since manual leaf removal (by hand) can be laborious, refining practice by prioritization may help define situations where leaf removal is most needed. Consequently, leaf removal implementation across several acres may not seem overwhelming. As an extreme example, and as mentioned above, fruit zone leaf removal should be a greater priority for fruit quality production in humid relative to arid/dry climates. The climate in southeast PA is humid, subtropical while the climate in northeast and northwestern PA is warm-summer humid, continental. Thus, while rot management is important across PA, one could make a case that rot is particularly pervasive in southeast PA and therefore leaf removal for rot management may be of greater importance in southeast PA relative to northwestern PA. However, leaf removal is still an important tool for improving fruit composition under the cooler conditions and shorter growing seasons typical of north and central PA regions.

Leaf removal may be "high priority" in white-berried, tight-clustered, thin-skinned, vinifera cultivars, that are highly susceptible to rot. Leaf removal may be "lesser priority" in many hybrid cultivars but "greater priority" in vinifera cultivars. Leaf removal could be prioritized based on phenology. For example, Chardonnay breaks bud and blooms early; therefore, it may be judicious to perform leaf removal earlier in Chardonnay (or Merlot) relative to Petit Verdot or Cabernet Sauvignon. Leaf removal may be unnecessary in cultivars like Norton, Chambourcin, and Charodnel. This is not to say that fruit zone leaf removal should be completely avoided in "low priority" situations. But developing a plan based on cultivar rot susceptibility and/or wine quality potential "gained" will help determine where to commence seasonal leaf removal and where it may be okay to wait for a couple weeks.

While some PA regions are warmer than others, all are humid and are characterized by variable rainfall and cloudiness throughout the growing season. Source: Wikipedia

Practical "pros and cons". A practical disadvantage to fruit zone leaf removal is that it requires labor, time, and/or money, regardless if manually or mechanically implemented. Bird and vertebrate pest pressure is variable across sites. However, when clusters are exposed, pests can easily target their bounty. Therefore, the need for pest management and bird netting may be heightened when leaf removal is implemented to the point where clusters are easily visualized. Harvesting visible clusters is much easier and efficient than harvesting clusters surrounded, and hidden, by foliage. Therefore, a benefit to leaf removal is the ability to efficiently harvest; some may make a final "leaf removal pass" nearing the harvest date for this reason. 

Due to leaf removal and the subsequent ability to visualize clusters, harvest will be more efficient from the forefront fruit zone relative to the background fruit zone. Photo: Cain Hickey

Bird netting and other pest management measures may be important to prevent depredation and crop yield loss.

For More information

Cain Hickey slide presentation on leaf removal

Michela Centinari slide presentation on Penn State research trials on leaf removal