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 disorder.
Cork Spot Symptoms
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.
Control 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 planting. Calcium chloride at 2 pounds per 100 gallons water (1.5 tablespoons per 1 gallon water) applied in four sprays beginning two weeks after full bloom and continuing at 10 to 14 day intervals thereafter may help to reduce cork spot. Calcium chloride at this rate may be added to pesticide sprays normally used in controlling post-bloom diseases and insects affecting fruit. Do not apply calcium chloride sprays when temperature is above 85 degrees F. Rinse sprayer thoroughly after use because calcium chloride is highly corrosive. Reduce excessive shoot growth by reducing or not applying nitrogen to the soil of apple trees for one to two years. Remove excessive growth and water sprouts with summer pruning in late July or early August.
Bitter Pit Symptoms
Bitter pit is evident as small, brown, soft, dried pits of collapsed tissue, 1/16 to 1/4 inch in diameter. Most of the pitting occurs just beneath the apple skin and typically is concentrated at the blossom half of the fruit. Rarely are pittings found on the shoulder toward the stem end of the apple.
Symptoms of bitter pit are seldom observed much before harvest time. The few that do develop before harvest are sometimes referred to as tree pits, whereas those that develop after harvest are called storage pits. The majority of bitter pit occurs as storage pits. Fruit placed directly in low-temperature storage may not exhibit symptoms for several days or weeks in storage.
One of the distinguishing characteristics of bitter pit is that symptoms will normally develop within a month or two after harvest. Fruit not showing symptoms by that time usually do not develop the disorder. While the pits will darken and become more depressed and more numerous, they remain dry, are relatively superficial and are often corky. They may be associated with the lenticels of the fruit but are not confined to these natural openings or pores.
Control of Bitter Pit
For bitter pit, the best long-term control is soil liming as suggested for control of cork spot. For established planting, foliar spray applications of calcium chloride may help to reduce bitter pit incidence. Use 2 pounds per 100 gallons water (1.5 tablespoons per 1 gallon water). Four to five applications at 10 to 14 day intervals can be made, the last coming about two weeks before harvest. Rinse sprayer thoroughly as calcium chloride is very corrosive. Do not apply at temperatures above 85 degrees F.
Symptoms of Jonathan Spot
Jonathan spot lesions are typically deep brown to black, superficial, slightly depressed, circular with a very definite border and 1/16 to 1/8 inch in diameter. One fruit may have several to many such lesions, depending on severity. Spots are not associated with fruit injury but usually occur at the lenticels.
Jonathan spot may appear either in the field or in storage. In the field the first evidence of spotting is near harvest time as the fruit reaches maturity. This stage of the disease is not serious because fruits are usually picked before they are completely mature. However, mature fruit will develop the disease while on the tree if picking is delayed.
The second phase of the disease occurs as the fruit ripens in storage. It is not affected by stage of maturity at harvest. Regardless of maturity, Jonathan fruits in common storage will develop the disease in about two weeks. In storage where the air is constantly moving and is purified with carbon filters, the development of Jonathan spot is retarded.
Control of Jonathan Spot
No satisfactory control measure has been found for Jonathan spot on susceptible varieties. However, if fruit is harvested at the proper stage of maturity and is stored properly, this disease usually is not serious.