Cork spot occurs during the growing season, and is characterized by localized green to brown sunken spots on the fruit or in the flesh. The spots in the flesh, which may or may not develop just under the skin, are brown, corky, dead tissue, and are more prominent toward the calyx half of the apple. The spots in the flesh have a bitter taste. Under severe conditions, fruit cracking can occur.
Bitter pit develops late in the growing season or in storage. It is very similar to cork spot, but the areas of brown, corky, dead tissue are confined to areas just under the skin.
Jonathan spot occurs late in the growing season or in storage. It is characterized as small, brown to black spots on the skin that may or may not be sunken. The spots develop most frequently on the sun-exposed side of the fruit, and are often associated with the lenticels (dots on the skin's surface). The flesh under the spot has a water-soaked appearance, but does not develop an off-flavor.
Water core occurs before harvest, and is characterized as translucent, water-soaked areas in the flesh. These water-soaked areas commonly develop near the core, but in severe cases, can radiate out to the skin. Fruit with water core are edible, and may have a sweeter flavor, but their storage life is reduced. Mild cases of water core can disappear in storage.
Internal breakdown is a storage disorder that is characterized by a browning of the flesh and eventual softening and disintegration of the fruit. All fruit will eventually develop internal breakdown. The cultivar and storage conditions determine the onset of normal internal breakdown. However, its premature development can be a serious problem.
Storage scald is a disorder that results in brown discoloration of the skin during storage. It is usually only a problem when fruit are stored for a long period of time. Fruit that develop storage scald are edible, but not very attractive.
Apple cultivars exhibit differences in susceptibility to each of these disorders. Three of the disorders; cork spot, bitter pit, and Jonathan spot, appear to be cultivar-related expressions of the same basic disorder. Although the symptoms of these disorders differ, they have two common characteristics. 1) Larger sized fruit are more prone to these disorders than smaller sized fruit. 2) They have all been associated with low levels of calcium in the fruit.
Calcium is an important mineral nutrient affecting fruit quality. One of its roles is in maintaining cell wall integrity and bonding between cells by combining with soluble pectin to form insoluble calcium pectate. As a result, fruit with high levels of calcium are firmer and have a longer potential storage life. A second role appears to be as a "safening agent" that de-activates undesirable by-products of fruit metabolism.
Although most Iowa soils are high in calcium, its uptake by plants is closely related to the soil moisture supply, with roots being unable to take it up whenever the soil becomes either too wet or too dry. Since almost all the calcium transport into the fruit occurs in the 4 to 6 week period immediately following bloom, wet or dry weather during this period greatly increases the risk of the development of these disorders.
Commercial apple growers have adopted the use of foliar calcium sprays as a standard practice to reduce the incidence of these disorders using either calcium chloride (70% flake) which can burn the foliage and is corrosive to application equipment, or other safer calcium-containing products such as Nutri-Cal and STOP-IT. To be effective these calcium-containing products must be deposited directly on the fruit since very little calcium is translocated from the leaves to the fruit. These products are applied beginning with the first or second spray after petal fall and are continued through the growing season until sufficient calcium has been applied (generally 5 to 8 applications per year). Up until now, these calcium products have not been packaged in small quantities suitable for home gardeners. However, we have been informed that Nutri-Cal will be available at Earl May Garden Centers in 16 ounce containers this spring.
Although low levels of calcium have been associated with these physiological disorders, calcium sprays alone will not eliminate the problem. What is necessary is a total cultural management program. Such practices include: 1) Planting apple trees on well-drained sites and irrigating during periods of drought. 2) Controlling nitrogen fertilization to avoid excessive vegetative growth and over-sized fruit. 3) Moderate pruning to maintain a proper balance between vegetative growth and fruiting. 4) Promoting normal crop yields from year-to-year by supplying bees for pollination and thinning fruit to avoid biennial bearing. 5) Harvesting at the proper time since apples harvested too early are more prone to storage scald, while those harvested late are more prone to bitter pit, Jonathan spot, water core, and internal breakdown. 6) Applying foliar calcium sprays in years when unfavorable soil moisture conditions develop in the spring, or whenever the trees have had a history of one or more of the disorders.
This article originally appeared in the April 11, 1997 issue, pp. 45-46.
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