Between wet summers and a short growing season, it’s not easy growing grapes in the Great Lakes region.
Moisture can linger in tight-clustered varieties, notably whites, and feed bacteria and fungi that cause bunch rot, while a short season can mean that red grapes, in particular, sometimes don’t have enough heat accumulation and time on the vines to develop full flavor and character.
Mechanized leaf removal, however, may give growers the edge they need to overcome both issues, according to a study by researchers in Michigan State University’s Horticulture Department.
Mechanized or not, leaf removal is beneficial. When employed before bloom, fruit-set is reduced, and the grape cluster is more open and loose. This allows water to drip through or evaporate faster, so rot is ultimately reduced.
In addition, the lower number of berries per cluster allows them to develop more fully, said Joshua VanderWeide, a Ph.D. graduate research assistant in Associate Professor Paolo Sabbatini’s MSU research group. When leaves are removed post-bloom, foliage is reduced, which also gives the ripening grapes more access to the sunlight and warmth that can aid their development.
“Traditionally, workers remove leaves by hand, and while they come off pretty easily, you have to count and keep track of how many you remove to maintain consistency, and you also have to make sure you don’t pull off the fruit with the leaves,” VanderWeide said. “And if you’re removing leaves right before bloom (rather than after fruit-set), you might only have five or six days to get the whole job done, and that can be a problem if you have hundreds of acres but not enough workers to do it.”
That’s where automation can help, VanderWeide said. With this study, the researchers wanted to examine the potential for it in the Great Lakes climate. Primary funding came from a $20,000 grant from the Michigan Grape and Wine Industry Council, $29,000 from Project GREEEN at MSU, and the support of Lemon Creek Winery in Berrien Springs, Michigan.
Man vs. machine
For their tests, the MSU researchers used a Collard pneumatic leaf remover, which mounts to the front of a typical vineyard tractor and is equipped with spinning air compressors that can be adjusted to match the canopy height and width. Air from the compressors shreds the leaves to remove them.
The test plot included 90 vines: 45 Pinot Grigio as a representative tight-clustered white, and 45 Merlot as a representative loose-clustered red.
For the experiment, conducted using the top cordon of a Scott Henry (SH) trellis system at Lemon Creek Winery, the researchers removed six leaves manually or leaves from a zone of six leaf nodes (about 70 centimeters) mechanically just prior to flower bloom (June 7 for Pinot Grigio and a few days later for Merlot).
They also did the same manual and mechanical leaf removal just after flower bloom (June 27 for Pinot Grigio and a few days later for Merlot).
In addition, they added a control treatment, which was the industry standard of manual, six-leaf removal at veraison. The researchers conducted the trials in 2016 and repeated them in 2017.
In both trials, they found mechanization made short work of the task. The manual method generally required eight laborers who together could complete about 1 acre per hour, VanderWeide said, whereas the mechanized method involved only one worker, who ran the tractor, and could cover about 2 acres per hour.
The main interest with Pinot Grigio was whether the mechanized leaf removal system could alleviate rot. In 2016, which saw above average rainfall from veraison to harvest, they found that prebloom, machine leaf removal resulted in a 45 percent reduction of rot, with a 70 percent reduction in the manual treatment compared to the control. Yields improved in both instances, despite the lower number of grapes per cluster, VanderWeide said.
In 2017, a much drier year, they had no rot in the prebloom, manual-treated grapes, and 5 percent rot in the mechanical-treated fruit compared to 10 percent rot in the control clusters. Sugar content of grapes in machine-treated vines also increased by about 10 percent in both years, he said.
For Merlot, the researchers’ primary interest was the quality of harvested fruit.
“We were surprised to find much improved sugar content (2 to 3 Brix higher in both years), as well as improved fruit quality measurements, including an average of about a 32 percent increase in total anthocyanins with prebloom mechanized leaf removal vs. prebloom manual leaf removal,” he said. “We just didn’t anticipate we would find that much improvement in quality. When you mechanize a vineyard practice, you just hope that it turns out as good as something you do by hand, but in terms of fruit quality, the mechanized treatment performed better. We found it very interesting and are currently investigating why this was the case.”
VanderWeide plans to continue his analysis of leaf removal by studying its impact on different varieties.
“We generally see a consistent reduction in fruit set and number of berries, especially with the prebloom machine treatment, but in order to fine-tune this practice, we have to look at and try to understand cultivar-dependent responses,” he said, noting that researchers in other parts of the world have found a good deal of variation in the extent to which leaf removal affects fruit quality among reds in particular.
The results of the 2016-2017 study alone, however, show that mechanized leaf removal has a place in Great Lakes vineyards.
“It saves time, which is perhaps the most important reason to do this, and it requires fewer people,” he said. On top of that, “this practice can be catered to different viticultural goals, whether it’s to reduce fruit set and rot in tight-clustered grapes, or to reduce fruit set and improve the quality of fruit in reds.” •
—by Leslie Mertz