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Market Diseases of Apples, Pears, and Quinces
Pear Cork Spot |
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Cork spot, so far as known, occurs only on the Anjou variety and has been reported only in California and the Pacific Northwest. Certain trees have a higher percentage of cork spot each year than others. The amount of disease varies widely from year to year.
In general the losses are taken at the orchard or packinghouse, but occasionally seriously affected fruits have been seen on the market. Some writers use the term "Anjou cork-spot" for this disease while others have used the term "bitter pit". Cork spot is characterized chiefly by a bumpy, uneven appearance of the pear surface as the fruits approach maturity (top photo). The appearance of the uneven or bumpy surface of cork spot areas is distinctly different from the well-defined pits of stony pit and boron deficiency. Affected areas usually have a more yellow color than the rest of the surface. When diseased fruits are peeled or cut, large masses of brown or grayish necrotic tissues are seen to underlie the spots; unlike the masses of stone cells of stony pit, these masses offer little or no resistance to the knife, although the fruits containing them sometimes become gritty, because of the production of stone cells in the affected tissues. The discolored masses are much larger than those in stony pit or in boron deficiency (middle photo). The cause of cork spot is unknown. Heavy dormant pruning increases the percentage of fruits showing cork spots in the Wenatchee, Washington, district, whereas in the vicinity of Hood River, Oregon, the disease seems to be associated with black end, which occurs only when the trees are on Japanese pear rootstocks. Cork spot is thought to be initiated during periods of high transpiration in trees whose root systems are inadequate to supply moisture during the critical period. It is not known whether moisture stress in the fruits brings about a complex imbalance of mineral nutrients as a causal factor of cork spot or not. Because cork spot does not always show external symptoms, it is advisable not to harvest and ship fruits from trees that have affected fruits. Recommendations of the State agricultural extension service or experiment station should be followed in an effort to control cork spot. |
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