It will be a few more years before new precocious, dwarfing rootstocks for sweet and tart cherries become available from the Michigan State University rootstock breeding program.
“We don’t have enough data yet for growers to decide whether they want to use them or not,” said Amy Iezzoni, who developed the rootstocks. “The data so far are promising, and our strategy now is to plant more plots and get more looks at them. In the meantime, we’re taking other steps to make sure that they will be available without delay when the decision is made to release them.”
Iezzoni traveled to eastern Europe in the 1990s and brought back cherry selections that she planted in a large collection at Michigan State University. She has been breeding with these plant materials to develop new scion varieties of tart cherries.
Early on, she decided to also evaluate the selections for traits that would be important in rootstocks. About 100 were initially selected and planted at the Michigan State University horticultural research station at Clarksville.
After several years of culling out rootstocks of no or limited value, she reduced the number to 11 in 2007, and she is now working with five rootstocks—all named after Michigan counties. These are Crawford, Lake, Clinton, Cass, and Clare. “Names are easier to work with than numbers,” she said.
From the beginning of her rootstock project, Iezzoni received support from the Pacific Northwest and has been working with Tom Auvil at the Washington Tree Fruit Research Commission and Dr. Matt Whiting at Washington State University. The Research Commission and the Oregon Sweet Cherry Commission have provided funding, as have the Northwest Michigan Horticultural Research Foundation and the Michigan Cherry Committee, which recently provided support for Armillaria resistance rootstock breeding as part of her work as well.
Willow Drive Nursery, Ephrata, Washington, grew 264 trees of the test rootstock selections with Bing as the scion, for planting at WSU’s Roza Farm in Prosser in 2009. They have been evaluated there since.
Trunk cross-sectional area measurements from 2012 showed that Clare is the smallest of the five, with Lake and Cass slightly larger and just a bit smaller than Gisela 5. Crawford and Clinton are just a bit larger than Gisela 5, but all are smaller than Gisela 6. On a percentage scale, Clare is predicted to be the smallest, rated at 35 to 50 percent of full-size trees on seedling rootstocks. Lake, Cass, and Crawford are rated at 40 to 50 percent of full size, and Clinton at 50 to 60 percent of full size, equal to Gisela 5.
Although other rootstocks are not in the WSU plot, it is possible to extrapolate their size relative to the MSU rootstocks. Krymsk 6 is 60 to 80 percent of full size; Gisela 6 and Krymsk 5 are 80 to 90 percent of full size, while Gisela 12, Mazzard, Colt, Maxma 14, and Mahaleb are rated at 90 percent or more of full size.
Flowering data showed that all five of the MSU rootstocks had more flowers per node than Gisela 6, and all but Clare have more flowers per node than Gisela 5.
Data from 2012 showed that Cass and Clinton produced the largest cherries of the five MSU rootstocks. Cherries on Cass were larger than those on Gisela 5, which otherwise produced the largest cherries among all the other rootstocks.
To put the rootstocks on a somewhat faster track, they were sent to the National Clean Plant Network in Prosser for virus certification. Four of them, sent in 2011, have emerged from there, and the fifth (Crawford, sent in 2012) will be done by this fall, Iezzoni said.
The four that were completed have been sent for liner production to Duarte Nursery and Protree Nurseries in California; Cameron Nursery and Teak Nursery in Washington; and Copenhaven Farms, North American Plants, and Willamette Nursery in Oregon. One thousand liners of each of the four rootstocks are being grown and will be sent to finished-tree nurseries this spring and grown for budding this fall. These budded trees will be planted in test plots in spring 2015, Iezzoni said.
“We have put together a liner pipeline ahead of time so liner availability and tree cost are not future roadblocks,” Iezzoni said. “The strategy is to get virus certified and genetically verified rootstock into the hands of liner nurseries as soon as possible.” Genetic verification is done in Iezzoni’s lab at MSU.
“Another goal is to give the nurseries an opportunity to gain experience propagating rootstocks and making finished trees,” she said.
In Michigan, two plots of Montmorency tart cherries are growing on the MSU rootstocks at the Northwest Michigan Horticultural Research Station at Traverse City, where they are being grown at standard spacing and also at close spacing for evaluation of their suitability for over-the-row harvesting. The smaller trees can be treated like berry bushes and harvested with blueberry harvesting equipment rather than conventional trunk shakers—at least, that’s what researchers and some growers are trying.
Trunk shaking requires the grower to grow a strong trunk, which puts off fruit production for up to six years. Growers want earlier production, and the smaller trees are more precocious.
In the Traverse City plantings, Montmorency trees on Clinton, Lake, Clare, and Cass rootstocks are being compared trees with trees on Mahaleb.