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Michigan State University researchers suspect there is something physiologically different about Honeycrisp apples that leads to such variability in return bloom.

"We’ve seen that between four to six fruits per trunk cross-sectional area is the breaking point for return bloom on Honeycrisp," Dr. Jim Flore said during the Great Lakes Expo in Grand Rapids, Michigan. "But even between the four to six fruits, return bloom the following year is still all over the place. Why is it not more clear-cut?"
He believes that something else may be going on within the tree and that it may have something to do with seed number.

As part of a thinning trial at two locations in Michigan, Honeycrisp trees were thinned to the four to six fruits per square centimeter of trunk cross-sectional area at bloom to the king or lateral flower. At harvest, seeds of the fruit were counted.

"Honeycrisp is odd in that the number of seeds vary more than the normal apple—from 2 to 3, to 14 to 16 seeds per fruit," Flore said. Most apples have five locules (where a maximum of two seeds reside), but with Honeycrisp, there are two to seven locules per fruit.

Also, one would expect more seeds to reside in the fruit that came from a king flower compared to the lateral flower fruit, he noted. But at both research locations, scientists found more seeds in the lateral fruit than in king fruit. Moreover, the seed numbers related to the fruit size.

"At both sites, the lateral fruit were bigger than the king fruit," he said, adding that researchers plan to go back to the trees this season to look at return bloom. They are interested in learning if seed number and fruit size influence return bloom.