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 the 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.

Ca deficiency 

As Konstaninos Papoutsus suggests beware of competition from potassium and magnesium inducing a Calcium deficiency. Orchards are often oversupplied with potassium since it promotes brix and yield and this will induce a lowering of calcium and magnesium. Calcium issues can also be related to boron deficiency and the best way to get a handle on the specific issue is to do plant tissue analysis prior to flower to seen the nutrient levels and their ratios. 

Calcium deficiency during growth and development of fruits

Apple cork spot, bitter pit and Jonathan spot are physiological disorders that can affect apple quality and reduce visual appeal. Fruit growers sometimes confuse these disorders with damage resulting from insects or pathological diseases, particularly those involving fungus infections or hail injury.

Cork spot, bitter pit and Jonathan spot are similar in that damage to tissue occurs mostly on the surface and in cell tissue just below the surface. Although apples affected with these disorders are edible, the unattractive external appearance often arouses consumer concern and reduces salability of fruit.

Occurrence of cork spot and bitter pit appears to be related to reduced calcium availability in the developing fruit. Cork spot commonly occurs on York Imperial and occasionally on Delicious and Golden Delicious. Bitter pit can occur on many cultivars, but is most common on Granny Smith, Delicious, Grimes Golden and Northern Spy in Ohio. Low soil pH, light crops and excessively vigorous shoot growth are associated with increased incidence of these disorders.

Jonathan spot, however, appears to result from toxins accumulating in the lenticels of the fruit skin. Although Jonathan is the most susceptible cultivar, Wealthy and Rome Beauty are also moderately susceptible to this disorder. Symptoms, time of occurrence and control measures available at this time are described below for each specific disor

Calcium Deficiency, Water Imbalance, Environmental Factors, Varietal Susceptibility