Balancing the crop load is particularly important with Honeycrisp, which is more susceptible than other apple varieties to problems involving mineral nutrition.
The fruit disorder bitter pit, which is associated with low calcium levels, can be a serious problem, especially in young trees, trees with a light crop load, or trees in warm sites, Dr. Lailiang Cheng, horticulturist at Cornell University, New York, reported during a Honeycrisp Fruit School presented by Washington State University in December.
On the other hand, when the tree is heavily cropped, the apples don’t color up, are smaller, and have lower sugar levels.
“To grow good quality Honeycrisp, the crop load has to be right,” Cheng emphasized.
“I think making foliar applications after harvesting the crop is a great way to increase the trees’ nitrogen reserves to strengthen the spurs for next season.”
—Dr. Lailiang Cheng
It’s a similar situation with nitrogen. In low nitrogen conditions, when the leaf nitrogen content is less than 2 percent, trees are more prone to biennial bearing. Increasing the nitrogen supply improves fruit set, fruit size, and yield, and reduces the biennial bearing tendency. However, it also reduces fruit color, firmness, and storability.
“So, the nitrogen supply has to be right in order to balance all these opposite effects,” Cheng said.
This is why Cheng believes the two key aspects of Honeycrisp nutrition are:
—Improving the fruit calcium level to reduce bitter pit; and
—Optimizing the tree’s nitrogen status to balance tree vigor, biennial bearing tendency, and fruit quality.
Tree roots take up very little calcium between bud break and bloom. After bloom, total calcium uptake increases linearly until harvest. Partitioning of calcium between shoots and leaves or fruit is different from other mineral nutrients. Once calcium is taken up by the roots, the partitioning is largely dictated by transpiration. As a result, typically about 85 percent of the calcium in new growth ends up in the foliage and only 13 to 14 percent in fruit. Cheng said that in warm climates, such as Washington, even less calcium might be partitioned to fruit because of the higher evapotranspirative demand on leaves.
Although the incidence of bitter pit increases with fruit size, fruit of any size from lightly cropped trees is more prone to bitter pit than fruit from heavily cropped trees. Cheng said this is because fruit is a major sink for potassium, and so when the crop load is light, the apples have high potassium levels, which inhibit calcium uptake and, in turn, translate into higher bitter pit incidence.
In contrast, fruit magnesium levels seem unaffected by crop load. Generally, high levels of magnesium in the fruit will compete with calcium, so lower levels are preferable.
Calcium management begins with ensuring that there is enough calcium in the soil and that the tree roots are able to take it up. Trees need to have enough boron and zinc to promote root growth because calcium is taken up only by the very tips of the roots, Cheng said. Water stress any time after petal fall will set up the trees to have more bitter pit in the fruit.
Vigor of the tree should be managed to mitigate competition between the foliage and the fruit for calcium, and the crop adjusted so that the trees carry a moderate load of five to six fruit per square centimeter of trunk cross-sectional area, Cheng advised.
Despite such efforts to promote calcium uptake, in many cases fruit calcium can still be too low. Foliar sprays might be needed in addition, particularly in Washington’s warm climate.
Cheng recommends four to five sprays of 1.5 to 2.0 pounds of calcium chloride in 100 gallons of water, applied at two-week intervals, starting shortly after petal fall. The following season, use higher concentrations of perhaps three to four pounds in 100 gallons and apply three times. This program provides about four to six pounds of elemental calcium per acre per season.
He warned that many calcium products contain a low percentage of elemental calcium and need to be applied more often, otherwise the tree will not take up enough calcium to make a difference. For example, calcium chloride in flake form contains only about 28 percent calcium and Stopit (calcium chloride) contains only 12 percent. In some products, the calcium content is even lower. A surfactant can help promote calcium uptake.
Ideally, a Honeycrisp tree will have a high nitrogen status early in the season to promote canopy development and early fruit growth, Cheng said. As the season progresses, trees should have lower nitrogen levels in order to ensure good fruit quality and maturity.
The tree has two sources of nitrogen, even before fertilizer is applied. First of all, it has nitrogen reserves from the previous season that are readily available for fruit growth and development. It also has access to nitrogen released from organic matter in the soil, though uptake depends on soil temperature, soil moisture, and the amount of organic matter. During the summer, this is a significant source of nitrogen for tree growth and development.
After bud break, the tree’s demand for nitrogen rapidly increases, particularly after bloom when tree growth accelerates. After shoot growth stops, demand for nitrogen gradually decreases, and as the season progresses, trees should have lower nitrogen levels in order to ensure good fruit quality and maturity.
Studies in New York show that if fertilizer is applied early in the season, after bud break, nitrogen is taken up by the roots and partitioned equally between spur leaves, shoot leaves, and fruit. When fertilizer is applied later, in May, during active shoot growth, nitrogen is taken up rapidly to support mainly shoot growth.
Therefore, in a first- or second-leaf planting where the grower wants to promote tree growth to fill the space quickly, nitrogen should be applied during the period of rapid shoot growth, Cheng said. But, in a mature orchard, where the grower does not want to promote vegetative growth or affect the fruit at harvest, nitrogen should be applied shortly after bud break and before petal fall.
In sandy soils or soils with low organic matter that don’t retain nitrogen well, fertilizer should be applied throughout the spring and early summer in low amounts. If the trees are fertigated, focus on the period of high demand from bloom to the end of shoot growth.
Cheng said studies show that the later in the season nitrogen is applied, the higher the nitrogen content of the fruit at harvest, and growers should bear in mind that high nitrogen concentrations increase bitter pit and reduce the storability of fruit.
Nitrogen fertilizer can be applied after harvest when the tree will take up calcium and store it for use the following season. At that time of year, it will not stimulate growth, so cold hardiness is not an issue. It is a good time to make a foliar application of 3 percent urea.
“For Honeycrisp trees, especially in Washington, if you harvest the crop in early September, you still have the entire months of September and October,” Cheng said. “I think making foliar applications after harvesting the crop is a great way to increase the trees’ nitrogen reserves to strengthen the spurs for next season.”
The amount of nitrogen a tree needs depends on the nitrogen status of the tree, the existing supply in the soil, and the uptake efficiency. The optimal nitrogen application rate is site specific because each block is different in terms of organic matter, cation exchange capacity, soil moisture, etc., Cheng stressed. As a rule of thumb, every 10 percent increase in the nitrogen application rate results in a 0.1 percent increase in nitrogen in the foliage.
In trials he conducted with Honeycrisp in Washington State, in collaboration with the Washington Tree Fruit Research Commission, Cheng applied nitrogen at rates from 30 to 120 pounds. He found that yields and fruit size improved when rates of up to 30 pounds were applied, but there were no additional benefits of applying more than 30 pounds. For bearing Honeycrisp trees, the optimal leaf nitrogen concentration is between 2.0 and 2.2 percent, he concluded.
Zonal chlorosis (a yellowing of foliage) is a trademark of Honeycrisp. Although yellow leaves generally have lower nitrogen concentrations than green leaves, this disorder has nothing to do with nitrogen management, Cheng said.
It is caused by an accumulation of excessive amounts of carbohydrates—particularly starch—in the leaves, and the severity is related to crop load. Trees with a heavy crop load might have a few yellow leaves, but symptoms can show up earlier and be much more severe on lightly cropped trees. It appears that a light crop can compromise the transport of carbohydrates in the phloem, blocking the export of carbohydrates from leaves to fruit so that excessive amounts build up in the leaves. Yellow leaves have three to five times as much starch as green leaves. The yellowing stops the leaves from producing even more carbohydrates through chlorophyll synthesis, Cheng said. •