Sabbatini is studying the timing of and effect that removing leaves around grape clusters (leaf pulling) has on grape ripeness, and the flavors and aromas in the resulting wine.

“Leaf pulling is all about changing the microclimate of the grape cluster,” he said. “It creates more air circulation, and allows for better ripening, spray coverage and reduction of bunch rot.”

Sabbatini was interested in finding the optimal time to do leaf pulling, because he noticed a great variation in the way growers dealt with the practice in the Michigan vineyards.

“Some growers do it early in the season, some do it late or some are not doing it at all,” he said. “Our Project GREEEN study is looking at the best time to expose the clusters by leaf pulling in red grape varieties, such as Pinot noir, which is the most highly planted red vinifera variety in Michigan.

“If growers remove the leaves too early, then they are removing the ‘working’ leaves for the cluster that use the sunlight for photosynthesis (sugar production),” he said. “This is not good. But if it’s done too late, then the skin of the grape berries is not used to the sunlight and gets burned. This can lead to the skins developing strange flavors in wine. Berry ripening depends on high temperatures and high UV, which aids in the production of anthocyanins and a better accumulation of phenolics, which wine makers like.”

Pinot noir is a difficult grape variety to grow and vinify in Michigan. It is sensitive to bunch rot because the grapes are very tightly clustered and thin skinned. And in Michigan, there is a lot of rain in the fall near ripening, which means more chances for bunch rot. Sabbatini also looked at optimal timing of leaf pulling to reduce cluster compactness in Pinot noir.

“We found that when leaves are removed at bloom, the carbohydrates, fundamentally important for fruit set, are also removed. Consequently there is less berry set, making the grape cluster looser and less susceptible to bunch rot,” he said. “We also tested this on Chardonnay, and found that leaf pulling before bloom had no effect on cluster compactness, but at bloom the berries per cluster were reduced, and there was a better grape skin to pulp ratio with the Chardonnay, which is good. Tests on grape skin to pulp ratio in Pinot noir are still being analyzed.”

He also looked at optimal times for leaf pulling for ripening Pinot noir, and found there was no effect on ripening if the leaves were pulled very early, but there was a huge negative effect if leaves were pulled too late.

“Leaf pulling around veraison aided in ripening the clusters,” he said. “Growers can also do cluster thinning at that same time.”

As the demand for Michigan-sourced vinifera wine grapes increases – and as wine production increases by 14 percent per year in the state – there is tremendous potential, and a vast amount of work yet to be done.

“We have a lot to learn about vineyard management and making wine from the grapes grown in the state, in comparison to the thousands of years of winemaking history in Europe,” Sabbatini said. “Because the industry is so young, there is still much to learn about what wine grape varieties can be grown here and the vineyard manipulation that needs to be done to grow grapes that make great wine.”

Michigan’s developing wine grape growing industry may face different challenges than other grape growing regions in the United States and around the world, but Sabbatini sees a lot of opportunity here.

“In this Great Lakes region, the lake effect on the climate is huge,” he noted. “It is very well positioned to grow vinifera grapes well, because that lake effect helps lengthen the season. Strategically, Michigan wineries are also in a good position when it comes to the whole local food and wine movement.

“When I came here in 2007, there were 45 wineries,” he added. “Now there are 70, and that is despite the dismal economy. People are still investing in this industry. I am very happy to see graduate and undergraduate students working with me for few years, leaving the viticulture program at MSU, and becoming employed by Michigan and U. S. wineries.”

“Between the jobs, sales and agricultural tourism it brings in to the state, it has a huge economic impact that continues to grow,” Sabbatini said.