UMN apple breeding program: Desirable traits and the selection process

Madeline Wimmer- UMN Fruit Production Extension Educator

Image: University of Minnesota (UMN) researchers evaluate hundreds of apples each year to search for unique, well-performing apple varieties that match or surpass the Honeycrisp standard. Seen in this photo is David Bedford, Jack Tillman, and Sarah Kostick, UMN apple researchers and breeders; and Kate Scapanski, UMN apple researcher.

With its original release in 1991, it’s been a while since the Honeycrisp apple was released and began a revival in apple consumption throughout America. The Honeycrisp apple was and still is adored for its balance of sweetness and tartnes, juiciness, and its signature crisp texture. To this day, a number of apples that are just as excellent in quality have been released by the University of Minnesota (UMN) fruit breeding program: Zestar!®, SweeTango®, First Kiss® (aka Rave®), and, most recently, Kudos®, and Triumph®.

Today, the efforts to breed apples through traditional breeding methods continue for the UMN fruit breeding program to find apples with desirable fruit traits, disease resistance, and good tree production performance. Because apple trees are woody perennial crops and take a few years to begin bearing fruit, the process from the time a cross is produced from seed until a variety is released can take many years. Last week, I had the opportunity to head out to the UMN Horticulture Research Center and follow our researchers as they assessed unreleased potential apple varieties during their second stage of evaluations and learn about what it really takes to become the next apple variety that is released.

Fruit evaluations

Flavor
Apple fruits are evaluated in the UMN breeding program both qualitatively and quantitatively. Flavors and aromas are noted by researchers who have an ability to recognize nuance in flavor. Similar to the diverse flavor profiles possible in grape production, there are a multitude of flavors to observe in apples: pineapple, peach, cherry, vanilla, anise, berry-like, or—one of my favorites we sampled during the evaluation—a mixture of raspberry with potentially a hint of cranberry.

Images: UMN researchers select a few representative apples from a variety being evaluated, cut them in half, and spray each one with iodine. This helps researchers determine how much starch is in each apple - the iodine binds with the starch and turns the flesh bluish/black - which indicates how ripe the fruit is.

Ripening
Growers in Minnesota and other places with shorter growing seasons do well to grow apples that ripen before the risk of fall frost becomes a possibility. Thus this aspect plays a part in selecting apple varieties. Our apple researchers use the starch-iodine test to track apple ripeness over time, and this also determines when apples are evaluated each year.

Image: A penetrometer is an instrument used to measure apple firmness, which relates to apple texture. For testing purposes, researchers first remove the apple skin, and then press the penetrometer on the apple flesh to get a reading.

Texture
Crispness and juiciness are two traits that are highly valued by the UMN apple breeding program. This is partly how Honeycrisp revolutionized the apple world, and it may be somewhat surprising to hear that some apples can actually offer a juicy crisp that goes beyond Honeycrisp. Apple firmness, or its ability to handle compression can be measured using a penetrometer, which gives some indication of the apple flesh texture. Penetrometer readings vary both by variety and harvest timing with values usually decreasing as apples ripen. Oddly, penetrometer readings can occasionally be misleading. During the evaluations, for example, we tried one apple with an abnormally low penetrometer reading, but it had a texture beyond anything I had previously tasted in apples. Subjectively, I would say the texture was similar to thousands of micro water balloons, filled with juice, exploding in every bite. UMN researchers refer to this texture as a tender-crisp: a tender apple that is both juicy and crispy.

Tree evaluations

The sensory experience of an apple is an important component for consumers, but from a production standpoint, knowing how well an apple tree performs is just as important. In regards to diseases, apple scab resistance has been incorporated into UMN and other apple breeding programs since the mid 1900s. For example Honeycrisp has one apple scab resistance gene making it somewhat resistant, and Triumph® has even more protection because it has two resistance genes. The researchers also look at other tree performance traits such as winter hardiness, vigor, productivity, and overall growth habit.

Overall evaluation

Apples in the UMN breeding program are overall evaluated on a scale from 1-9. In order to “pass” or move up in priority within the breeding program, an apple must surpass a 5, which is based on the variety Gala. If it’s not better than a Gala, it won’t pass the initial rounds of evaluation. The next standard is based on Honeycrisp, which is placed at 7.5. If an apple is meeting or surpassing Honeycrisp qualities, it’s going to be prioritized. Right now, the most recent apple cultivated varieties that have been released include both Kudos and Triumph, in which Triumph currently has limited tree availability, which will change as more trees are propagated in nurseries.


Learn more about UMN apple cultivated varieties:
MN Hardy: All University of Minnesota apple varieties




This article was reviewed by David Bedford and Jack Tillman.