Growers have long believed full-season foliar calcium sprays will reduce bitter pit, but two Cornell University horticulture researchers have discovered it’s only half the solution: Reduced potassium fertilization fills out the rest of the equation.
Lailiang Cheng, Cornell University horticulture associate professor, and Mario Miranda Sazo, Cornell Cooperative Extension fruit specialist, made this discovery based on ratios of calcium to potassium, phosphorus and magnesium found in Honeycrisp peels and flesh.
This year, for the first time, Miranda Sazo recommended western New York growers reduce their potassium fertilization rather than replenish supplies taken up by the previous year’s crop.
Based on their findings, lower potassium uptake results in higher levels of calcium in Honeycrisp fruit.
“Until now, we have been telling Gala, Empire and McIntosh growers whose orchards produce 1,000 or 1,500 bushels per acre, they needed to apply 70 to 100 pounds of potash (K2O) per acre to replenish what the trees took from the soil,” he said.
That application is far more likely to set the stage for bitter pit in Honeycrisps. “For Honeycrisp, we may need to lower the (potassium application) rate by 25 to 30 percent,” Miranda Sazo said.
At this point, Miranda Sazo and Cheng are not saying potassium should be brought down lower than that because it is still important to fruit development and sugar accumulation. They are continuing application rate testing this year.
Testing is being done in Michigan because its sandier soils hold less potassium. “This allows us to alter the soil potassium level faster,” Cheng said.
More than just calcium
Increasing fruit calcium level by means of foliar sprays still plays a major role in bitter pit mitigation, but it is by no means a panacea.
In the early 2000s, Cheng did some research with Dave Rosenberger, Chris Watkins and Steven Hoying using foliar calcium sprays to control bitter pit. “It was effective in some years, but in others, not as effective,” Cheng said.
One of the things they found was the incidence of bitter pit in Honeycrisp is negatively associated with the fruit cortex (flesh) calcium level.
“Overall, in a best-case scenario, fruit calcium level explains about 45 to 50 percent of the variation in bitter pit incidence,” he said. “In other years, this relationship only explains less than 20 percent.”
That led them to conclude that there was a lot more to bitter pit than fruit calcium concentration alone.
In 2006, Cheng’s research group did a study of Gala macronutrient and micronutrient requirements. They discovered when calcium was partitioned between fruit and tree leaves, 86 percent of calcium went to leaves and only 14 percent went to fruit.
Three years later, Cheng found Honeycrisp leaves had an even higher level of calcium than Gala leaves, and consequently, even lower fruit calcium levels.
Based on these two facts, Cheng decided to compare Gala and Honeycrisp in terms of fruit calcium level in balance with other fruit nutrients, such as phosphorus, potassium and magnesium.
He also wanted to evaluate Honeycrisp fruit with and without bitter pit, comparing nutrient levels in both the peel and the cortex.
So he and Miranda Sazo found a block of Honeycrisp with moderate amounts of bitter pit alongside a Gala block with no bitter pit at all in western New York. They took leaf and fruit samples from both and analyzed them.
They duplicated the results of Cheng’s 2009 study that Honeycrisp leaves had even higher calcium levels than Gala leaves. In addition, they found Honeycrisp leaves had lower levels of phosphorus, potassium and magnesium.
Next, they looked at the fruit samples. They separated the peels from the cortex tissue and analyzed them for the presence of nutrients.
“The first thing that caught my eye was in both the cortex and peel tissues, Honeycrisp fruit had only half the calcium that Gala had,” Cheng said.
In Gala and Honeycrisp cortex tissue, they found no significant differences in the potassium, magnesium or phosphorus levels. Once they examined Honeycrisp peel tissues, however, they found almost 50 percent higher levels of potassium and phosphorus.
Then they calculated the ratios of potassium to calcium and phosphorus to calcium. Both were much higher in Honeycrisp than in Gala, which Cheng said depicted a nutrient imbalance associated with bitter pit susceptibility.
Visually healthy peels on Honeycrisp apples with bitter pit had lower calcium content than those Honeycrisp free of the disorder, but had higher concentrations of potassium and phosphorus. Pitted peels had the lowest calcium level and the highest concentrations of potassium and phosphorus.
The pattern was clear. Wherever potassium levels were higher, calcium levels were lower.
And because they observed the level differences in peels and not the cortex, Cheng and Miranda Sazo think peel nutrient analysis is a far better indicator of bitter pit development than calcium levels in the cortex. “Peel nutrient levels are very consistent with bitter pit development,” Cheng said. •
– by Dave Weinstock
Bitter pit reduction begins at ground level
Preventing or mitigating bitter pit in Honeycrisp begins before trees are planted, especially in regions like western New York, where soils tend toward acidity.
Applying lime to bring soil pH up to optimal levels — 6 to 6.5 — followed by the right rootstock are the very first steps growers can take to reduce bitter pit incidence.
Liming sites before planting is just one more way to increase calcium availability. “I’ve known growers whose orchards get very acidic by the second or third leaf. Under those conditions, calcium is not fully available to the roots,” said Mario Miranda Sazo, Cornell Cooperative Extension fruit specialist.
Sample soil from both the topsoil layer — zero to 8 inches — and the subsoil layer, from 8 to 12 inches. “Do it before planting, and that will tell you how much lime you have to apply,” he said.
Cornell Cooperative Extension personnel also recommend three to four foliar sprays of 1.5 to 2 pounds of 78 percent calcium chloride per 100 gallons, at two-week intervals beginning seven to 10 days after petal fall.
Follow this with two additional sprays at 3 to 4 pounds of calcium chloride per 100 gallons at four weeks and two weeks prior to harvest.
Rootstock plays an important role in bitter pit susceptibility as well. “Based on data I’ve seen in the NC-140 trials, Honeycrisp trees on Bud 9 seem to have less bitter pit than those on M.9 or M.26,” said Lailiang Cheng, Cornell University horticulture associate professor.