It is difficult to estimate the importance of this disease. Apples are not destroyed, but their appearance and quality are affected in a manner not easily measured. Figures representing reliable investigation into the amount of losses are not available. Not infrequently, however, growers state dogmatically that 50 per cent or more of their crop is affected. Rarely is a crop entirely lost; the fruits are not reduced to absolute worthlessness although in severe cases an apple affected with bitter - pit, or stippen, is extremely objectionable. From the standpoint of losses through reduced quality, the past history of the disease indicates clearly that stippen ranks high among the most important apple diseases of the world. In New York state, at least during certain years, the disease is the most serious of all apple troubles.
Although pears and quinces are known to be affected, yet the apple is by far the most seriously injured. It would not be safe to say that any variety of apple in the United States is immune to bitter-pit. Observations and records show, however, that the Baldwin, above all others, is the most susceptible variety grown in North America. Many growers, in fact, have come to believe that stippen is a disease peculiar to this variety, whence the name Baldwin spot. But this name is objectionable since the Baldwin is liable to other spot diseases of the fruit. Other varieties prominent in American apple - culture that are commonly affected with stippen are Northern Spy, Rhode Island and Tompkins King. It is a noteworthy fact that the disease under consideration affects most commonly, and often to a grave extent, some of the most valuable commercial varieties, while, as intimated above, a number of other varieties are subject to it.
Like many other diseases of plants, stippen was known to growers long before it was the subject of published writings. There are good grounds for believing that the trouble may have existed in the apple from the time when it was first generally cultivated. In 1869 a pitting of apples was described in German literature. In that country it is now called Stippen, Stippflecke, and Stippigwerden, all of these names referring to a pitting or stippling. In 1886 it was reported from Australia and about ten years later the name bitter-pit was given it in that country. In the early nineties it caused alarm among Australian growers because of its obscure and mysterious nature, and finally an extensive investigation was initiated as a result of action on the part of the growers. First authoritative records of stippen in the United States date back to 1891; it was then common on Baldwins in Vermont. Subsequently it was found to occur in Canada, England, France, Russia, South Africa and New Zealand. In fact the geographical range of bitter - pit seems to be coincident with that of the cultivated apple. Most damage is done in America and Australia.
The fruit only is affected by stippen. The disease may be found toward the end of the growing-season, at least after the fruit is half grown; or it may not be seen until after the fruit has been placed in storage. Fruits approaching maturity seem to be in the most critical stage of their development so far as bitter - pit is concerned. Its appearance amongst the fruit on a tree is very erratic. Sometimes only one apple on a twig is affected. Again, all the fruits of a cluster are pitted.
Fig. 7. - Stippen, or bitter - pit.
It has, beyond reasonable doubt, been demonstrated by means of the X-rays that the disease develops internally before there are any external signs of it. The first outward symptom of the disease is the appearance of slightly sunken spots here and there over the surface, usually most numerous toward the blossom-end (Fig. 7). These spots or pits tend to a circular form and vary from mere dots to depressions measuring a quarter of an inch or more in diameter. They look like hail or sand bruises (Fig. 7). On a red variety the color is at first darker red, while on a green variety the spots are darker green. Finally the depressed areas are brown. As the fruits mature the pits become more numerous and deeper, but the skin covering them is not broken nor ruptured in any way. Another form of the disease is found in what is known in the western United States as hollow-apple. This form is sometimes called confluent bitter - pit or crinkle because the upper surface of the fruit develops rough folds giving it a crinkled appearance.
If an affected apple is cut open, an area of the flesh directly beneath a pit is found to be dead, brown, dry and spongy, or corky. On account of these characters various names have arisen: Dry Rot, apple brown spot, leige (cork disease), and others. Throughout the pulp of the apple there are usually found brown streaks or spots of dry, spongy flesh which do not extend to the surface of the apple, and which are not connected with the surface lesions. Where the pits and internal brown spots are numerous the apple may have a bitter taste, whence the name bitter - pit.
There are many appearances similar to, or mistaken for, bitter-pit. The apple is subject to several Fruit Spot diseases some of which may be regarded as identical with stippen by the casual observer. The New England Fruit Spot is distinguished from stippen by its spots being decidedly less sunken, and by the lack of semblance to a bruise. In the Fruit Spot one or more black pimples develop on the affected area; in the case of stippen there is no evidence of such structures.
Bitter-pit, or stippen, is frequently confused with hail marks, and bruises of various kinds. In the case of any such injuries, however, the skin has the appearance of having been pushed in rather than shrunken, and also mechanical injuries often show a broken skin, a feature not characteristic of stippen. Scab is distinguished as follows: it shows at first as a velvety, then as a corky, lesion, not depressed. The various apple Rot lesions are not depressed until much larger than stippen spots. San Jose scale marks are sometimes regarded as bitter-pit lesions, but the former does not show the plain depression of the latter. Most growers will not mistake other diseases for bitter - pit.