Pears with cork spot have bumpy surfaces and brown corky lesions in the flesh.
Cork spot, a disorder of pears caused by a deficiency of calcium, was more prevalent than usual in Washington last season, Tim Smith, Washington State University Extension educator for north central Washington reported at the annual Pear Day.
"We’re probably losing more fruit to calcium deficiency this year than all of the psylla and fruit rots combined," he said. "We’re obsessed about psylla and spend a lot of money on it, but we’re losing more fruit to calcium problems than anything."
Cork spot in d’Anjou pears is the equivalent of bitter pit in apples. The fruit surface is bumpy, and affected areas are usually more yellow than the rest of the skin. The flesh has brown or grayish corky lesions.
Many nutritionists emphasize the importance of the soil in supplying nutrients, Smith said, but nutrition is more complicated in perennial crops than in annual crops. A tree—particularly a big pear tree—is a vast reservoir of nutrients, and, generally, all the grower has to do is top off the nutrients and correct imbalances by fertilizing.
The tree gets most of its nutrients from soil, and roots pick up nutrients from soil over a long period of time. However, roots go dormant in the fall, just as the foliage does. When the tree leafs out in the spring, the roots start to grow. A pear tree’s roots are not very deep and can be affected by competition from weeds or by tilling in organic orchards.
"Healthy roots are going to do a lot better in nutrition than sick roots," Smith said. "You have to do everything you can to help the roots remain healthy." During cell division and growth, the tree has a strong need for nutrients, including nitrogen, phosphorus, and calcium, but it does not draw them from the soil. In the spring, most of the important nutrients that will feed the developing fruit clusters are already stored in the wood and bark, he explained. They come from fertilizer applied in previous seasons that is stored in the tree.
Fertilizer applied during this time will not make any difference, Smith warned. "There’s not much you can do about this that you haven’t already done last year or maybe the year before."
One of the greatest misconceptions is that if a tree has a lot of fruit on it, it needs fertilizer to make the fruit grow better. "That’s absolutely not true," he emphasized. "You cannot fertilize your fruit better for this year’s crop. Fertilizing makes the foliage grow but makes the fruit worse."
If the tree has a heavy crop, the fruit and leaves compete for carbohydrates, and there is less growth of foliage. And when there is less foliage, all the nutrients stored in the tree are less spread out, so the nutrient level in the foliage goes up. "When you add more nitrogen, you’re just adding nitrogen to a situation where nitrogen is high anyway," Smith warned.
If the tree has a light crop, that triggers a problem with cork spot because all the foliage steals the calcium away from the fruit.
Water stress also lowers fruit calcium, he said. Water stress tends to occur in the middle of the summer when calcium is needed for fruit expansion, and that leads to cork spot.
Smith said it’s difficult to prevent cork spot, but it can be reduced with calcium sprays. He also cautioned against applying large amounts of potassium, which competes with calcium and induces calcium-related disorders.
Nitrogen: Years ago, nitrogen deficiencies were unheard of. When you stop or cut back on your use of nitrogen, it can take years for the nitrogen level in the tree to drop. But Smith said he’s now seeing trees that are slightly deficient, especially in organic orchards. Symptoms are small limbs, poor shoot growth, and small fruit. When you shift from conventional to organic, you still have years of conventional fertilizer you can draw upon, Smith said, but after a time you might have to make sure that it’s not running out.
Boron: Boron, an important mineral, is not found in soils in Washington. So, unless you add at little every year, the trees will become deficient. Symptoms include boat-shaped leaves, withered shoot tips, and blossom blast. Boron needs to be in the soil in low amounts constantly.
Copper: Occasionally, Smith sees copper deficiencies in pear trees in parts of the Wenatchee Valley where there is no copper in the soil. For years, growers applied copper to control diseases, but use of copper sulfate has become less common.
Sometimes, in young orchards, the trees don’t grow well. Once the deficiency is corrected, however, growth takes off.