—by Matt Milkovich

Michigan State University doctoral student Chayce Griffith discusses the movement of calcium through apple xylem during the university’s CA Clinic in Muskegon, Michigan, in July. When xylem vessels break down, they stop delivering calcium where it’s needed, leading to bitter pit. (Matt Milkovich/Good Fruit Grower)

Honeycrisp growers dream about the day they won’t have to worry about the postharvest disorder bitter pit showing up in storage and eating into their profits.

That day might be just a little bit closer.

Michigan State University professor Todd Einhorn’s lab recently revealed more about the mechanisms of bitter pit that could be harnessed to help growers manage it: Incidence of the disorder isn’t simply related to final fruit size but rather to the fruit’s rate of growth during a crucial period.

“In Honeycrisp, people say that a light crop load and big fruit equals bitter pit, but we believe that it’s a transport disorder that begins once fruit growth exceeds a threshold,” Einhorn said.

The finding stems from several years of research into the relationship between bitter pit and crop load management, supported by Michigan Apple Committee funding.

It’s well understood that calcium deficiency plays a role in bitter pit, but Einhorn’s work shows that the disorder starts in the xylem, the vascular tissue that transports water and dissolved minerals throughout the fruit. Unlike other minerals such as potassium and magnesium, calcium can only move through the xylem. If the xylem breaks down, calcium can’t spread throughout the fruit, and bitter pit is more likely to develop. This is a recurring problem in varieties such as Jonagold and Honeycrisp.

There are cultivars with greater resistance to bitter pit, but market forces don’t always allow growers to choose them.

“There is some latitude in choosing resistant rootstocks, but at the moment Honeycrisp is king, and Honeycrisp pits,” said Chayce Griffith, a graduate student and research assistant in Einhorn’s lab who has been conducting much of the bitter pit research.

In susceptible apple varieties, xylem function gradually declines as the season progresses, and it eventually breaks down completely. The fruit is still growing but isn’t getting the calcium it needs. The earlier the xylem breakdown, the more susceptible a cultivar is to bitter pit.

And when xylem breaks down, it can’t be repaired.

“Kiwi fruit has the ability to repair xylem pathways, but apple doesn’t,” Griffith said. The xylem is “sort of like a garden hose. When it breaks, you can’t get water down the end anymore.”

In Honeycrisp, xylem breaks down between five and eight weeks after full bloom, a period of rapid growth when an apple really needs calcium. To prevent bitter pit, growers need to keep the xylem as functional as possible for as long as possible. Applying foliar calcium sprays is a common practice, but it doesn’t guarantee bitter pit-free fruit if the transport pathways are broken, Einhorn said.

The next step for the Einhorn lab is to find ways to temporarily slow fruit growth in the crucial period five to eight weeks after bloom, ideally delaying xylem breakdown while ensuring the fruit still reaches an optimal size by harvest.

Einhorn’s lab has investigated the effects of plant growth regulators containing auxins and abscisic acid on xylem functionality. Now they’re investigating horticultural strategies to regulate fruit growth. They’ll continue to study chemicals, rates and timings, he said. •