Sep 5, 2013
Vine size control discussed during viticulture event

Vineyard leaf removal and managing excess vine size are two pivotal operations in high-density grape production.

Leaf removal in the fruiting zone is a classical vineyard management practice applied during the summer, from fruit-set to veraison, according to Paolo Sabbatini, assistant professor of horticulture at Michigan State University (MSU).

Presentations by Sabbatini and Tom Zabadal, an MSU horticulturist who addressed vine size management, were part of Viticulture Field Day held July 31 at MSU’s Southwest Michigan Research & Extension Center (SWMREC) in Benton Harbor. The event was co-sponsored by MSU Extension and the Michigan Grape Society.

Sabbatini said leaf removal improves clusters’ exposure and air circulation, which reduces conditions favorable to bunch rot complex diseases.

He cautioned, however, that leaf removal also can affect fruit negatively. Excessive leaf removal can lead to overexposed clusters – creating high light intensity and temperature – and reduced berry color in red cultivars.

Leaf pulling is essential to manage moisture on the leaves from rain, enabling exposure to the cluster when sunshine emerges.

“You want to reduce the rot and improve the quality of the fruit,” Sabbatini stressed. “If you (pull leaves) that cluster will dry a little faster for sure – leading to 50 percent less bunch rot.”

The effects of leaf removal on yield vary depending on timing and severity, he said. Carbohydrate supply at anthesis – the period during which a flower is fully open and functional – is a primary detriment of fruit set. Leaf removal within four weeks of anthesis generally reduces yield, cluster compactness and total amount of sugar per berry.

According to Sabbatini, if leaves are removed later in the season, or at reduced level, yield is not significantly affected and occasionally increases compared to the non-defoliated control.

Some of the grape-growing challenges found in the Great Lakes region include spring frost and winter injury, short and variable growing seasons and low heat accumulation.
“Under these conditions, it is difficult to achieve maximum fruit maturity – and crop control is a priority to assure high-quality wines,” he said.

Traditional crop-control strategies like winter pruning, node count adjustment and further fine-tuning by shoot and cluster thinning are time consuming, expensive and require skilled labor.

Manual leaf pulling at anthesis, however, works well.
“It reduces fruit-set and berry size, leading to looser clusters and improved fruit composition,” Sabbatini said.
Light and temperature are key factors in determining fruit quality and volume.

“Ultimately, you don’t need a lot of light exposure – you don’t need 100 percent under the sun,” Sabbatini said. “In Michigan, when 25 to 50 percent is exposed, that’s enough. When you expose it more, it doesn’t increase anything.”

Leaf pulling can take place at different times of the season, with varying results.

“From a practical point of view, it’s done around veraison, when the cluster starts to turn color. The problem is that sometimes it’s too late. If you do it two or three weeks before veraison, you will get more tonnage but not more color. It really depends on the style of wine you’re shooting for.

“Timing should be something you choose yourself in relation to the varieties and which styles of wines you’re shooting for,” Sabbatini said. “In whites, they are completely different from reds. Timing is crucial. The industry is choosing for veraison time. If you go a little ahead of veraison, you can get more benefit of fruit quality.”

Managing excess vine size

Using a 25-year-old SWMREC Cabernet Sauvignon vineyard that hasn’t received much care as his setting, Zabadal reviewed findings of vine shoot trials in determining optimal vine sizes.

“When we plant a vineyard, we take an educated guess as to the spacing between vines that we think will work,” Zabadal said. “We want a balance between the vine and its vegetative growth. We take that into account when we decide how far apart to plant. We only want so much growth that can be managed well on a trellis.”

He said a “perfect” vine has four attributes:

– It completely utilizes the trellis space provided for it.

– It fills that space with one and a half to two layers of leaves so all leaves function fully in sunlight.

– There is a balance between crop level on the vine and the leaf area needed to acceptably ripen that crop.

– The vine maximizes the sustainable yield of grapes of acceptable quality.

Symptoms of vines that are out of balance with excessive crop are that the fruit does not acceptably ripen, there is poor maturation of shoots into canes, declining or chronically small vine size and greater than average winter injury.

Excessive vegetative growth symptoms include numerous layers of grape leaves, unhealthy interior leaves, lack of fruitful buds in the renewal zone, inability to set a crop, difficulty achieving good spray coverage and poor fruit exposure to sunlight.

He pointed to the SWMREC vineyard, which has 123 shoots per every 6 feet – equaling about 20 shoots per foot.
“We know we cannot manage well 20 shoots in a foot of trellis,” he said. “The optimum number is around five. We don’t want to get stuck with too much vegetative growth or too much crop on a vine that has more capacity than our space allows.”

Zabadal outlined several factors that influence vine balance, ranging from soil drainage and fertility, weather, climate, variety and pruning methods, to the choice of a vine training system, vine and row spacing and crop adjustment practices.

He pointed to several options for getting vines to the appropriate size, but some, such as reduced fertilization and pruning, don’t always work, so more “radical measures” may be needed.

“You can change the training system, maybe taking the shoots and train them up and down so instead of being able to handle five shoots well, it can handle 10. That does help get the vines in balance.”

In a vineyard that has ample vine size capacity, when a vine is missing, the cordon can be extended and “it works wonderfully.”

“That may be the answer; take out and extend vines if they are the kind that can do that. If you’ve planted stuff on 3 and 4-feet spacing, and by June you have a trellis full and don’t know where to go, that’s trying to tell you something.”

A “powerful tool to bring vine size down” by a third when needed involves the placement of sod under the trellis to create competition for excessively large vines. Zabadal has conducted research showing sod under a trellis “competes right where it needs to be – where the preponderance of grapevine roots are. But if you don’t have a way to manage it, you create a mess.”

He said sod management options may include mowing or grass control chemicals.

Gary Pullano

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