Market Diseases of Apples, Pears, and Quinces
Occurrence and Importance
Bitter pit occurs in all the important apple-growing regions of the world. The disease has been recognized for nearly a century. Perhaps, with the exception of scald, there has been more research on bitter pit than on any other disease of apples, and yet the problem is not thoroughly understood nor completely solved.
In Germany the disease is called "stippen". In the United States, bitter pit occurs on most varieties, but it is not of commercial importance on all. In New England, the disease is frequently called "Baldwin spot" because of its seriousness on that variety. The Baldwin, Northern Spy, Rhode Island Greening, Grimes Golden, Yellow Newtown, Stayman, Arkansas (Mammoth Black Twig), Delicious, Gravenstein, and Rambo are among the more susceptible varieties. In 1965 and 1967 several instances of bitter pit were observed on Golden Delicious, a variety usually not listed among the more susceptible apples.
Apples are predisposed to bitter pit while they are on the tree. Apples are either healthy or subject to bitter pit at harvest. Both kinds may come from the same tree. The disease may begin to appear before the apples are harvested, especially on those grown in New England, but bitter pit is primarily a postharvest disease. Apparently sound susceptible fruits, especially those of early maturing varieties, can be expected to develop bitter pit during the transit and marketing period immediately after harvest. Bitter pit develops more rapidly at 50 °F than at 32° and more rapidly at both these temperatures than at 70°. Considerable bitter pit may develop within 7 to 10 days on susceptible varieties at 50°. In some susceptible fruits, however, bitter pit will not have developed by the end of that time. The disease often develops in a month or weeks at cold-storage temperature, and develops further, especially on immature fruits, after removal from such storage. It does not spread from one fruit to another, but pits may enlarge and new pits may develop during transit on apples from cold storage.
As the affected cells die, they lose moisture and the skin over the area sinks in a round or slightly angular pattern like a small bruise. The skin over the pits gradually becomes gray, brown, or sometimes black (top two photos). Typical pits are 1/16 to 1/8 inch in diameter and are usually distributed over the calyx half of the apple. In highly susceptible varieties, however, such as Rambo and Arkansas, the pits may extend to the shoulder area. When the apple is peeled, a small round or oval mass of dry, brown spongy tissue is found below each surface pit. Often the loss of moisture from the spongy tissue causes it to shrink from the normal tissue and form a cavity (third photo). Bitter pit spots, though closely associated with the terminal branches of the vascular bundles of the apple, are not confined to the region immediately beneath the skin, but may occur deeper in the flesh, especially in certain varieties. If present, the deeper spots can be demonstrated by starting at the calyx end and making thin slices across the fruit.
Pits on certain varieties, such as Yellow Newtown, Winter Banana, and Baldwin, are larger than those described above. In addition, they are more nearly circular in outline and are usually rather sharply sunken. On apples of the Yellow Newtown variety, the spots may be more prevalent on the cheek than on the blossom end. Pits on the Winter Banana sometimes coalesce and form irregular shapes.
The imbalance of water between fruits and leaves, due to excessive transpiration, was recognized early as a basic factor in the cause of bitter pit. Early explanations, such as the pits being caused by ruptured cells, direct desiccation bringing about a toxic concentrate in the cells as they lost moisture, and the starvation of certain cells as the result of soil moisture deficiency, have long been discarded as over-simplifications of the disorder.
Investigators in Australia about 1928 advanced the explanation that the disease is due to the killing of immature, starch-filled tissues of rapidly growing apples or of fruit in storage, probably resulting from excessive transpiration that induces osmotic action between the starch-filled cells and those in which most, possibly all, of the starch has been changed to sugar. According to this explanation, the starchy areas are killed by excessive desiccation. In light of present information, it appears that the presence of starch grains in bitter-pit tissues indicates that this is a result of the disorder and not its cause. The presence of starch in the dead tissues appears as evidence that the normal physiology of the affected cells ceased to function before the starch was converted to sugar. Histological studies have demonstrated that in early stages of bitter pit development, starch can be observed in surrounding healthy cells the same as in the pitted cells.
As early as 1918 it was found in the Pacific Northwest that bitter pit increased following the use of irrigation water containing a high percent of epsom salt (magnesium sulfate). It was many years later, however, before studies were started on the influence of nutrients or imbalance of nutrients on bitter pit.
The influence of competition between fruits and leaves for water and its solutes has been carefully studied in relation to the occurrence of bitter pit. For example, heavy thinning of the fruits in relation to the normal number of leaves increased the incidence of bitter pit. However, if the leaves were thinned to a point where the fruits had the advantage in obtaining water and nutrients, the incidence of bitter pit was reduced. No attempt was made to explain the physicochemical changes in affected fruits as a result of the imbalance that induced the development of bitter pit.
Since 1956, worldwide research has been conducted on this problem. It was found for example that magnesium nitrate increased the incidence of bitter pit, whereas calcium nitrate reduced it. Many elements and combinations have been tested. Investigators now generally agree that bitter pit is induced in the orchard late in the season by a deficiency of calcium in the fruits. Some take the position that bitter pit is caused in the orchard by competition between leaves and fruits for calcium. The problem is not one of simple calcium deficiency, however, but a complex relationship between elements like calcium, magnesium, potassium, and nitrogen. Other investigators lay less emphasis on the competition between leaves and fruits. They consider calcium deficiency in the fruits to be due to its inability to move freely or to the interference with its movement by certain other elements.
The recommendations of the State agricultural extension service or experiment station should be followed in setting up a program for controlling this disease.