Issue link: http://read.dmtmag.com/i/291333
34 APRIL 15, 2014 GOOD FRUIT GROWER www.goodfruit.com 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 potas- sium levels, which inhibit calcium uptake and, in turn, translate into higher bitter pit incidence. Magnesium 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 com- petition between the foliage and the fruit for calcium, and the crop adjusted so that the trees carry a moder- ate 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 pro- gram provides about four to six pounds of elemental calcium per acre per season. He warned that many calcium prod- ucts contain a low percentage of elemen- tal 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 (cal- cium chloride) contains only 12 percent. In some prod- ucts, the calcium content is even lower. A surfactant can help promote calcium uptake. Nitrogen management Ideally, a Honeycrisp tree will have a high nitrogen status early in the season to promote canopy develop- ment 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 nitro- gen 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 rap- idly increases, particularly after bloom when tree growth accelerates. After shoot growth stops, demand for nitro- gen 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 fol- lowing 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 "To grow good-quality Honeycrisp, the crop load has to be right." —Dr. Lailiang Cheng