Pruning and canopy management are two of the most important aspects that affect grape crop yield and quality. While dormant pruning is accepted as standard and necessary, several other practices can affect not only crop yield and quality but wine flavor and consumer acceptance.
PhD candidate Maria Smith discusses the impact of canopy management on vinifera and French American hybrids. She says understanding the basics of crop load management can go a long way in knowing why and how grapes can be manipulated for optimizing both fruit quality and vine quality.
The concept begins with understanding the vital role of photosynthesis — the conversion of carbon dioxide to glucose, which is the useable form of energy for the plant. “Carbon is accumulated and stored over winter,” said Smith. “It provides the initial energy source for new vine growth in spring, vegetative growth below and above ground during the growing season, and energy to produce defense compounds as well as immune response to both insects and disease pathogens.”
Smith noted that the grape fruit itself can account for up to 75 percent of the total carbon allocated by tissue type in the vine, so manipulating fruit or leaves can have a major impact on the plant.
“We use this manipulation for the concept we call vine balance,” said Smith. “This is maximizing yield without causing detriment to vegetative growth.” Balanced vines will produce sufficient vegetation to ripen fruit, produce enough carbohydrates to mature wood for both cold hardiness and optimal bud performance, and also provide enough carbohydrates to provide initial growth in the following season.
However, vine balance should not be confused with vine size. Vine size is a function of site conditions such as the availability of water and soil nutrients, rootstock selection, and disease and insect pressure. Vine balance is the combination of vine size and crop management decisions such as spacing, training and canopy management choices.
Smith says an undercropped vine produces excessive vegetative vigor, which leads to cluster and leaf shading. This occurs because there is a lot of vegetation, small clusters, or few clusters per shoot. Overcropped vines can produce a lot of clusters per shoot, and those clusters are often large. Shoots are stunted and photosynthesis shuts down earlier.
A balanced vine is measured by crop load, which is estimated by the Ravaz index. Smith says the Ravaz index is simple to use and practical for field use. The Ravaz index represents the crop weight at yield to dormant pruning weight. Smith suggests sampling blocks that represent a fairly homogeneous segment of the vineyard.
Balanced crop loads are determined by both the vineyard site and the vines. “You don’t want to cluster thin an imbalanced vine or an undercropped vine,” said Smith. “You’ll usually dormant prune or shoot thin.”
The benefits of canopy management are numerous and include increased sunlight exposure that increases total soluble solids, Brix, and flavor and aroma development in berries. “It also helps with vine training and maintaining structural architecture and integrity,” said Smith. “Shoot rejuvenation provides healthy shoots for maximum bud fruit fullness in the next season, as well as overall long-term vine health.”
Dormant pruning done prior to budbreak at the end of winter is standard practice, and it’s the first step in achieving a balanced crop load. Proper dormant pruning establishes vine shape and establishes a good base for shoot thinning, which is done in mid spring. Shoot thinning opens the canopy and reduces shoot density, and Smith says growers should shoot thin when shoots reach about 18 to 24 inches. “The longer you wait, the harder it is to remove the shoots,” she said. “Shoot spacing should be about three to five shoots per linear foot of canopy, and select shoots for both desired positioning and fruitfulness.”
Cluster thinning helps fine-tune the crop load, and should be used only in over-cropping vines. “It increases leaf area to fruit ratio so fruit reaches maximum maturity,” said Smith. “Timing is important. If you cluster thin too soon before fruits are pea-sized, the fruit can overcompensate in size, which negates the effect of the thinning.” While cluster thinning helps bring vines into balance, it doesn’t necessarily increase fruit quality. It’s also a time-consuming and expensive practice when done manually.
Smith describes another practice, early leaf removal, as a relatively new management tool. Although leaf removal isn’t new, early leaf removal is performed either prior to fruit set or just prior to bloom. “At bloom, it’s called trace bloom leaf removal,” said Smith. “About five percent of the flower clusters should be open when you perform this. It reduces yield by decreasing fruit set, and also decreases cluster compactness.”
Smith says there are some advantages of early leaf removal over cluster thinning. “Early leaf removal is easier to mechanize,” she said. “It’s more cost-effective to implement in the vineyard. It reduces cluster compactness and opens the canopy, which can reduce bunch rot; especially in tight clustered varieties. It can also increase sunlight and air circulation, which improves spray penetration, and improves fruit and wine quality.”
However, there are some drawbacks to early leaf removal. Unlike cluster thinning, early leaf removal doesn’t allow the estimation of final yield. There’s also a risk of excessive decrease in fruit size, especially in a cool, rainy spring. “We don’t know if early leaf removal affects the accumulation and distribution of carbohydrates in the vine itself,” said Smith, “and that’s something we’re actively investigating.”
Smith describes a 2014 and 2015 Chancellor crop load study. The variety was selected because it’s the poster child for overcropping. “We used 25-year old vines at the Lake Erie Research and Extension Center,” she explained. “All treatments were effective in decreasing yield and crop load by about 50 percent relative to the control.”
Also significant is that the results also showed the degree to which crop load management can affect wine quality. Cluster thinning and the two early leaf removal treatments in 2014 affected quality with significantly higher Brix. However, these differences went away in 2015 because crop load was reduced as yield was reduced.
“In 2014, despite the differences (higher) in Brix, we didn’t see any differences in consumer preference,” said Smith. “But we found in the 2015 wine that cluster thinning treatment is preferred, suggesting that crop load management can affect wine quality.”
The costs associated with canopy management can be high, and will vary according to labor costs and market prices of grapes; and how fast workers can perform treatments in the vineyard. Smith says in research trials, additional costs for canopy management can likely be recovered with a marginal increase in the price of a bottle of wine.