Although Mother Nature has taken a severe toll on the Michigan tart cherry crop, a bright spot for the industry is growing in a greenhouse on the campus of Michigan State University (MSU): seedlings that have shown, in three generations so far, resistance to the most costly disease facing Michigan tart cherry growers – cherry leaf spot.
The disease is caused by a fungus that thrives in humid conditions. It attacks foliage, causing leaves to yellow and eventually fall from the tree. Premature defoliation can severely weaken the tree and eventually lead to tree death if winter temperatures are severe. Fungicides, the primary means of controlling the disease, must be repeatedly applied from petal fall to harvest.
MSU AgBioResearch scientist Amy Iezzoni is determined to develop a genetic solution -- a new tart cherry cultivar that is naturally resistant to cherry leaf spot and would not require the use of chemical sprays.
“Cherry leaf spot is caused by a fungus, so anytime there is moisture it becomes very active, spores are produced and it spreads quickly,” said Iezzoni, a professor in the MSU Department of Horticulture. “It’s a major challenge in Michigan. In fact, the disease requires more pounds of chemicals per acre to control than any other cherry disease or pest.”
Iezzoni and doctoral student Travis Stegmeir have identified a chromosome region in cherry that contains a gene that controls resistance to cherry leaf spot. It is found in a wild cherry known as Prunus canescens, which produces fruit too small to market.
“This cherry bears fruit the size of a pea that no one would even consider making a pie from,” she said. “But we’ve been breeding the Prunus canescens with Montmorency – the major tart cherry variety in Michigan with consistent fruit quality and yield – and we’ve been able to transfer the resistance through three generations.”
Iezzoni said that Prunus canescens is able to “recognize” that the cherry leaf spot fungus is there and destroys it, preventing defoliation of the tree. Ultimately, Iezzoni wants to develop a new cultivar that has this resistance ability and produces a highly marketable fruit.
“We’ve collaborated with a group in Germany that is using the same source of resistance, but they’ve crossed the resistant source with diploid sweet cherry, which is simpler genetically,” she said. “Our genetic results are in collaboration with the breeding program in Dresden, Germany. We are using the information gleaned by working with our German colleagues and translating it to our Montmorency cherry – a more complicated, absolutely necessary variety for Michigan’s tart cherry population.”
Much of Iezzoni’s work has been made possible through the RosBREED project – a U.S. Department of Agriculture Specialty Crop Research Initiative grant that leverages emerging DNA sequence information to improve the quality of cherries and other rosaceous fruits, including apples, peaches and strawberries.
Iezzoni said that finding the specific plant gene responsible for the resistance – next on her list – will lead to benefits somewhat similar to those of explorations of human genetics.
“In medicine, they can determine who’s a carrier for a cystic fibrosis gene without the patient being challenged by the disease,” she said. “They do a genetic diagnostic test. In plant science, it’s much like asking which seedlings are carrying the genetics that are necessary to reach my research goals.
“I don’t know the gene yet, but I do know the general location, and I know what markers are linked to it,” Iezzoni adds. “I can begin doing the DNA diagnostics without knowing the gene because I know approximately where it is.”
Iezzoni, who owns Bel Lago Vineyards and Winery in Cedar, Mich., with her husband, Charlie Edson, knows firsthand the hard work, extensive labor and high costs that Michigan fruit production requires. She is disheartened by this year’s tart cherry crop loss, but she said that it inspires her to continue to do all she can to help.
“In a year like this, when you have a cherry crop loss, growers will not have to control certain insects if there is no fruit on the trees,” she said. “But they do still have to keep the leaves on the trees. It’s a control cost they will have this year, even though most of them will not have any tart cherry income. To make matters more frustrating, they will also have to spray longer than usual because of the early warm-up this year, and that’s an added cost.”
Iezzoni is hopeful that the fourth generation of seedlings growing in the campus greenhouse will show both resistance and high fruit quality. Plans are underway to accelerate the screening of the seedlings, she said.
“I’ve been working in the dark for so many years, and now we’re finally getting there,” Iezzoni said. “We’re able to use our DNA knowledge and our knowledge of what chromosome regions are needed from what parent to increase breeding efficiency.”