What is Apple Cork Spot?
Apple cork spot disease affects an apple’s quality and visual appeal. It is a physiological disorder like that of other apple fruit disorders, such as bitter pit and Jonathan spot. While it renders the look of the fruit less than appealing, cork spot in apples does not affect their flavor. Cork spot in apples afflicts York Imperial and less often Delicious and Golden Delicious cultivars. It is often mistaken for damage from insects, fungal disease, or hail injury. The disorder begins to appear in June and continues through the development of the fruit. Small green depressions in the skin will enlarge to discolored, corky areas of between ¼ and ½ inch (6 mm. to 1 cm.) on the exterior skin of the apples as they grow. Reduced calcium availability in developing fruit is the cause of apple cork spot disease. Low soil pH, light crops, and excessively vigorous shoot growth coincide with an increased prevalence not only of cork spot but other apple fruit disorders.
Treating Apple Cork Spot
Treating apple cork spot requires a multi-control approach. Ideally, depending upon soil test results, the site should be amended with agricultural ground limestone at planting. Additional limestone should be added at three- to five-year intervals post planting. Again, rely on a soil test each year to determine if and how much limestone should be added. Calcium sprays may also help reduce the incidence of cork spot. Mix 2 pounds (1 kg.) of calcium chloride per 100 gallons of water (454.5 L.) or 1.5 tablespoons (22 mL.) per 1 gallon (4.5 L.) of water. Apply in four separate sprays starting two weeks after full bloom. Continue at 10 to 14 day intervals. Don’t apply calcium chloride when temps are over 85 degrees F. (29 C.). Calcium chloride is corrosive, so be sure to rinse the sprayer thoroughly after using. Lastly, remove any excessive growth and water sprouts in late July or early August. To reduce excessive growth, reduce or cease applying nitrogen to the soil for one to two years. If all this sounds like too much trouble, be assured that apples afflicted with apple cork spot may be less than perfect visually, but they are still suited for eating out of hand, drying, baking, freezing, and canning. If the corky spots bother you, just pare them out and discard.
