The problem child for return bloom
Honeycrisp sets fruit easily in young trees and can overset even in frosty years.
The large photo at left shows an ideal crop load for Honeycrisp apples.
Honeycrisp seem like the perfect apple. They have a wonderful name, provide an explosively crisp eating experience, produce attractive fruit with strong grower returns, are precocious and productive, and are loved by consumers. But they have a problem with alternate bearing, says Michigan State University's Phil Schwallier.
"Honeycrisp trees will come back with no return bloom," said Schwallier, who is coordinator of MSU's Clarksville Horticultural Experiment Station. "The variety has the ability to overset even in frosty years and sets fruit easily in young trees. And then, you have too many fruit on the trees and problems with return bloom."
Studies conducted by MSU show that the variety has the ability to set 150 to 200 percent of a crop on the tree, he said. By observation, trees with six to eight fruit per square centimeter of trunk cross-sectional area (TCSA) look like they have a desirable crop load. "Unfortunately, when you measure return bloom from that crop load the next year, you get a three percent return bloom. You need to get down to one-half to one-third of a crop load to get a normal return bloom."
Schwallier, who was part of a one-day symposium on fruit thinning and return bloom held during the Great Lakes Expo in Grand Rapids, Michigan, said that based on MSU data measuring crop load and return bloom research, they have found that five fruits per square centimeter of TCSA was the breaking point for Honeycrisp return bloom—a higher number of fruits and the return bloom is poor, he reported. "The key is to stay in the four to six fruit per square centimeter of TCSA range, but the best is five fruits."
He suggested that growers approach crop load management of young Honeycrisp trees with caution.
"Plan on using no more than mild thinning applications, and perhaps, better yet, hand thin these trees," Schwallier said. "It's so easy to knock the fruit off young trees, especially Honeycrisp. And besides, you then run into bitter pit problems."
Using mild to moderate rates of the chemical thinner NAA (napthaleneacetic acid) or MaxCel (benzyladenine) is necessary to avoid overthinning. Young Honeycrisp trees are sensitive to MaxCel, he explains. In some years, MaxCel gave exceptional return bloom, but in others, NAA was just as good as the hand-thinned or application of Sevin (carbaryl) at the late stage when fruit are 12 millimeter in diameter.
He believes that with young trees, hand thinning is the safest approach.
Schwallier defined young Honeycrisp trees as four to five years old in a high density planting on Malling 9 or 26 rootstocks. For Honeycrisp on M.7, trees are mature around seven years; on M.106, mature trees are around eight years.
Thinning trials on bearing Honeycrisp trees were conducted in 2007, comparing chemical thinning applications of Sevin and NAA with Sevin and MaxCel at petal fall and when fruitlets were at the 10-mm size stage. The untreated check had about 300 apples per tree; hand-thinned apples in the trial were thinned to 100 apples per tree.
The petal fall application didn't thin as well as the 10-mm stage thinning, Schwallier reported. Thinning results were similar for the two chemical thinning agent combinations tested.
In 2008, return bloom was assessed on the trees. "The petal fall application doubled our return bloom," he said, concluding that for bearing trees, moderate to normal rates of combination thinners should help return bloom. For a Sevin and MaxCel combination, he recommends around 50 to 75 parts per million; for Sevin and NAA, around 8 to 10 ppm.
MSU research shows that a carefully timed application of NAA or Ethrel (ethephon) can help increase return bloom. An application of NAA made five to nine weeks after full bloom, while bud initiation is occurring, can help return bloom, Schwallier said. The research also showed that an application of Ethrel, made five to six weeks after bloom at a rate of 200 ppm improved return bloom.
Summer applications of NAA showed an improvement in return bloom by 5 to 15 percent from the untreated control, while Ethrel improved return bloom by 10 to 15 percent from the control.
But Ethrel applications must be done with care, he warned. Although the growth regulator increases return bloom, he found that at high rates of 600 ppm, Ethrel hastened maturity, ripening fruit seven to ten days earlier, and increased preharvest apple drop.
"It's much safer to use NAA to enhance return bloom if it's used at the recommended rates," Schwallier said. Low NAA rates of five ppm or four to six ounces per acre showed no effect on fruit size; however, high rates will reduce fruit size and can impact stop-drop controls, even though it was put on in June or July, he added.
Schwallier predicts that future thinning programs for Honeycrisp will involve multiple things—blossom thinning applications at pink to petal fall, thinning at fruit set (10 mm stage), and follow up with a summer treatment of NAA.
"You don't want to let the petal fall window go by. If you have nice weather, put something on like Sevin, Sevin and NAA, or Sevin and MaxCel," he said, adding that Sevin combination thinners can be tank-mixed with insecticide applications.
"Once you're done with petal fall, you can do something else at fruit set if you need to," he noted. In some years, petal fall will give the best thinning results, but in years with cold weather during petal fall, thinners are not effective.
Though it will take multiple steps, Schwallier believes that growers have multiple opportunities to help themselves with the problem of return bloom on Honeycrisp.