Paolo Sabbatini, MSU associate professor of horticulture, is tackling this problem by developing viticulture techniques that alter grape cluster microclimates, thwart disease and improve fruit quality.
November 22, 2014
As the fourth largest grape-producing state in the United States, Michigan is home to more than 15,000 acres of vineyards that fuel a $790 million industry. The Michigan Grape and Wine Industry Council explains that a tremendous growth in sales and production of Michigan wineries has created demand for high-quality, Michigan-grown grapes that is challenging to meet. Growers must overcome cold winters and cool, wet summers, which take a severe toll on vines and encourage diseases that detract from producing the perfect bottle of wine.
Paolo Sabbatini, Michigan State University (MSU) associate professor of horticulture, is tackling this problem by developing viticulture techniques that alter grape cluster microclimates, thwart disease and improve fruit quality.
“At this time in history, there is a surplus of wine production,” Sabbatini explained. “A new grape-growing region emerges in the world every year — every state in the United States now grows grapes and produces wine. No one saw that coming 20 years ago. As a result, people are looking for the best wines, and the market is responding by providing very competitive prices.”
In short, Michigan grape growers and winemakers are under pressure to produce wines of the highest quality. Doing so enables them to earn an opportunity to compete in the larger U.S. market and, in the future, the global market.
Wine grapes thrive in temperate climates that boast warm, dry summers and mild winters. By way of photosynthesis, grapevines use the summer heat and sun to produce sugar and acid, develop color and accumulate heat for physiological ripening, which includes the development of color, skin and pulp texture, seed color and texture, tannins and other important fl vor compounds.
Though the climate is considered temperate compared with those of other states in the Great Lakes region, Michigan’s summers are often shorter and cooler than those in other similar regions of the world. This results in fewer opportunities for grapes to take advantage of the sun and warm, dry days. Each year, Michigan grape growers work in earnest to overcome this disadvantage, and with the help of Sabbatini and others, have done so successfully.
Sabbatini is helping them overcome a third climate-related struggle: preharvest rain.
“September and October mark the nearing of harvest time; growers have worked diligently to get and keep grapes on the vines so that they can develop the varietal characteristics needed to make great Rieslings, Pinot Noirs, Chardonnays and other wines,” he said. “But it’s very humid and wet in Michigan during these months — perfect conditions for fungi to cause cluster rot. This disease can completely destroy grape clusters in a matter of days or make potentially high-quality grapes an unmarketable product.”
Sabbatini explained that growers currently have two options when seasonal rains come prior to harvest: leave the grapes on the vine and risk rot and rain damage, or pick them early and sacrifice quality and critical flavor compounds.
“Growers need a third option,” he said. “Grape clusters are susceptible to rot at that point in the season because the grape varieties grown in Michigan have compact clusters with the berries touching one another. When it rains, water is trapped between the fruit and they’re wet for many hours. Fungus then moves in and destroys the grape cuticles, resulting in rot.”
The research question then becomes “How do we keep grape clusters on the vines during the wet, rainy period at the end of the season without losing them to rot?” His answer is to alter the cluster’s microclimate by lowering cluster density.
Sabbatini reasoned that if he could reduce the number of fl ers in a cluster, he could reduce the number of berries that would develop. This should work because the number of fl ers a grapevine produces directly correlates to the number of berries a grape cluster will contain.With the support of Project GREEEN, Sabbatini developed a vineyard management technique that involves removing the leaves around fl er clusters at bloom time. Doing so temporarily eliminates the fl ers’ source of photosynthesis. The result is that the vines produce fewer fl ers per cluster and, subsequently, less dense grape clusters.
“This works — not only does it give you less cluster rot, but it allows every berry in the cluster to be exposed to the sun,” Sabbatini said. “Normally, the berries on the inside of the clusters don’t get a lot of sun or heat, but now they can, which means that the fruit quality also improves. Our research shows that there was also less humidity and more heat in the clusters’ microclimates.”
To address concerns about possible decreased yields, Sabbatini plans to test various pruning techniques this winter with the goal of leaving more buds in the spring.
“If we’re reducing the number of berries per cluster, then we need to increase the number of clusters per vine so that growers don’t lose profi s,” he concluded. “Growers are hopeful about this approach — and I am, too. We have to get around the issue of cluster rot, and we have to improve fruit quality because there’s nothing magical you can do. If the quality of the grapes is low, the quality of the wine will also be low.
“In comparison with much of the world, we are a very young region of wine producers, but we’re overcoming problems every year, and we’re giving consumers what they want — Michigan wine made from Michigan grapes.”