Issue link: http://read.dmtmag.com/i/291333
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." Nitrogen rate 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 mois- ture, 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 col- laboration 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. Chlorosis 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 man- agement, Cheng said. It is caused by an accumulation of excessive amounts of carbohy- drates—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. Yel- low 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. • www.goodfruit.com GOOD FRUIT GROWER APRIL 15, 2014 35 PHOTO COURTESY INES HANRAHAN, WASHINGTON TREE FRUIT RESEARCH COMMISSION Although the incidence of bitter pit increases with the size of the apple, fruit of any size from lightly cropped trees are more prone to bitter pit than fruit from heavily cropped trees.