Smaller rootstocks and properly timed thinning could help growers reduce the occurrence of bitter pit in Honeycrisp apples, researchers say.

Essie Fallahi, University of Idaho pomology and viticulture program director at UI’s Parma Research and Extension Center, said the popular Honeycrisp variety is susceptible to bitter pit. He and his team, along with Fruitland, Idaho-based Henggeler Packing Co., are part of a Honeycrisp quality study, now in its third year.

Dark depressions on the apple’s skin indicate bitter pit and its underlying cell breakdown. The condition compromises taste, texture and crop value. It can appear near harvest or in storage.

Contributors include calcium deficiency or a high level of potassium compared to calcium, Fallahi said. Bitter pit could develop on the tree as the apple matures, or the tree could be predisposed to it.

“We are trying to work with thinning and the use of different rootstocks to reduce the chance of bitter pit,” he said.

Evaluations are done at harvest and after 90 days in storage, said Sara Mahdavi, a post-doctoral fellow at UI-Parma.

So far, it looks like Honeycrisp thinning is best done later than for Red Delicious and Fuji, Fallahi said. “But we cannot do that extremely late, or quality and yield are adversely affected.”

The team also found that if the process starts with some preliminary chemical thinning and is followed by manual work, thinning a five- or six-fruit bunch down to one produces the best yield and lowest incidence of bitter pit, he said.

“Rootstock with more growth, and with more potassium and nitrogen, has more chance of producing large, jumbo fruits susceptible to bitter pit,” Fallahi said.

Fallahi said election of dwarf or semi-dwarf rootstock shows promise. Researchers continue to study this, and in about a year aim to publish a joint paper on rootstocks.

Smaller rootstocks’ apparent benefits include that the leaf-to-fruit ratio can be better and more easily adjusted to acceptable conditions, he said. That way, there is not an oversupply of foliage, so apples get more sunlight and develop better color.

As for minerals, “the ratio between fruit potassium and fruit calcium is extremely important in predicting the occurrence of bitter pit,” Fallahi said. “Therefore we have to be extremely careful to apply several (courses) of pre-harvest calcium, but not excessive nitrogen and potassium.”

Cornell, Michigan State, Washington State and Utah State universities, and UI have been working on the project. Other work includes evaluating rootstock performance in different locations and growing environments. Researchers aim to learn more about fruits predisposed to bitter pit through biochemical and molecular analysis.

Idaho’s participation is helped by a USDA Specialty Crop Block Grant through the state Department of Agriculture, and by several state fruit organizations.

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