Apple cork spot is a physiological disorder that can affect apple quality and reduce visual appeal. Cork spot generally appears in the outer portion of the fruit flesh as small green dimples or depressions. This disorder may begin developing in June and continue throughout the initial stages of growth and enlargement. The green spots eventually enlarge to corky, discolored areas 1/4 to 1/2 inch into the flesh of the apple. The corky spots may occur anywhere on the fruit flesh.
Fruit growers sometimes confuse this disorder with damage resulting from insects or pathological diseases, particularly those involving fungus infections or hail injury. Although apples affected with this disorder are edible, the unattractive external appearance often arouses consumer concern and reduces salability of fruit. Cork spot commonly occurs on York Imperial and occasionally on Delicious and Golden Delicious. Low soil pH, light crops and excessively vigorous shoot growth are associated with increased incidence of cork spot.
The best long-term control for cork spot is the addition of agricultural ground limestone to the soil at planting, according to soil test recommendations. Limestone should be added at three to five year intervals after planting, based on soil tests. In addition to soil liming, calcium sprays may help to reduce the incidence of cork spot in established apple plantings, although they may not be effective in every situation. Recommendations on how much calcium to apply and how often to apply it may vary by geographic location, so it essential to obtain a local recommendation. Do not apply calcium chloride sprays when temperature is above 85 degrees F or leaf and/or fruit injury may occur. Rinse the sprayer thoroughly after use because calcium chloride is highly corrosive to metal.
Horticultural practices, such as preventing excessive shoot growth by reducing or not applying nitrogen to the soil of apple trees for one to two years, and removing excessive growth and water sprouts with summer pruning in late July or early August can reduce the incidence of cork spot.
For more information, go to http://ohioline.osu.edu/hyg-fact/1000/1403.html.
This article was adapted from an article by Drs. Richard C. Funt and Michael A. Ellis, Ohio State University.