Stippen is a non - parasitic disease, one in which the primary cause is not certainly known, but one for which all parasitic organisms, such as bacteria and fungi, are in no way responsible. The common opinion of those who have given some consideration to this phase of the subject is that the pits result from a disturbed water relationship. As to the nature of this disturbance, however, authorities are not entirely agreed.
Since every grower has his own opinion in regard to the cause of this disease it may be of interest to give some of the theories of those who have most carefully investigated it. In order to make these theories clear it is first necessary to have clearly in mind the general structure of the apple-fruit and the nature of the spots. The flesh of the growing apple is made up of rather large egg-shaped cells within which the living substance of the fruit is enclosed, together with the water and mineral substance brought up as food from the roots, and the starch made by the leaves and green fruit. As the fruit ripens the starch is converted into sugar. Passing from the twig into the fruit, by way of the fruit stem, are large numbers of sap-tubes which, as they enter the base of the fruit, separate, and, branching, permeate the flesh in all directions. These sap-tubes are the channels through which water and mineral food from the soil are carried to the living cells of the fruit. The apple is enclosed in a smooth, water-proof skin made up of a layer of cells, the outer walls of which are thick with a waxy infiltration of a waterproof nature. Scattered here and there, but most abundant toward the blossom - end of the apple, are minute openings through the skin known as lenticels. They are visible to the naked eye on certain varieties as minute brown specks. Through these lenticels air passes into and out of the apple; and water brought from the roots passes out as vapor. This process of water elimination is known as transpiration. Great quantities of water pass out in this way if conditions are favorable. Low humidity, high temperature, rapid movement of the wind, and intense light facilitate the process. If plenty of water is supplied by the roots, the process of transpiration removes the great bulk of it as vapor through the lenticels. If the supply of water is deficient, but at the same time conditions favor transpiration, then too much water is removed and wilting results.
When the brown spots are examined with a compound microscope it is found that the affected cells are collapsed or perhaps broken, and hence the tissue sinks, forming a pit. Whether the cells are merely collapsed or are broken is a question yet undecided. The cell contents are lacking for the most part, only starch grains, which were not converted into sugar during the ripening process, remain. Their presence is regarded as proof that the injury to the cells occurred before the ripening of the fruit. The cells are brown and have every evidence of being dead. Their brown color and their lack of sap, and therefore their dry texture, give the described appearance to the whole diseased area. Careful search will show that the brown spots always arise in close connection with a branch of the sap - tube system.
With these facts in mind the most important theories, which have been championed to explain the killing and drying out of the affected cells, may now be examined.
(1) The injury is due to a rapid transpiration, or loss of water, from the cells, thus bringing about a concentration of the sap in those cells nearest the sap-tubes. This concentration of sap which is chiefly a concentration of the acids, results in the injury and death of the cells. The concentration of the cell-sap is therefore the immediate cause as outlined by this theory, and this concentration is increased by an insufficient water supply as well as by excessive loss of water. Those who cling to this theory believe that it accounts for the development of the disease both in storage and on the trees, and in the latter case for its appearance in wet as well as in dry weather. The following facts form the basis for this proposed explanation: (a) Warm, dry weather favors transpiration. (6) During such weather the soil is deplete in moisture and therefore cannot supply the cells with water sufficient to offset the rapid loss by transpiration, (c) During the wet season the apple-fruit grows very rapidly and the net-work of sap-tubes is not able to keep pace with the growth of the pulp-cells and the proper balance is disturbed. The sap-tube system is then deficient and the cells suffer from a lack of water, (d) The disease is favored by alternating wet and dry weather. Fluctuating temperature and humidity near the ripening period are highly favorable to bitter-pit, and conversely as long as the water is regularly supplied to the fruit, or at least as quickly as it is lost by transpiration, there is no pitting. Dry weather suddenly followed by heavy rainfall results in the development of stippen on the tree. And excessive transpiration, such as occurs in warm, dry weather, with a light crop of fruit is favorable to the disease. In other words, when transpiration is relatively greater than the water supply, stippen is produced. It is seen then that the weather conditions play a prominent role in causing this disease while the fruit is still on the tree. On the other hand, the methods of orchard management are contributing factors not to be overlooked. Heavy pruning is thought to favor the disease, for pruning affects transpiration. When the balance between the portions of a tree above ground and the roots under ground is disturbed, injury is likely to result. The application of manures not only supplies food but modifies transpiration. Green-manures conserve moisture in the soil. Manures of any kind invigorate the tree and enable it to use water from the soil in a more economical fashion. Cultivation conserves moisture and affects transpiration in a manner favorable to the tree. Irrigation must be taken into account. Regular and continuous growth is inimical to the development of bitter-pit. But if water is applied through irrigation in an injudicious fashion, conditions are made favorable to the disease. The disease also develops in storage. It has been found that fruit apparently clean when picked will show stippen after a considerable period in storage. Conditions in storage favorable to the disease are similar to those favoring it on the tree. Fluctuating temperature and humidity combined are evidently responsible for the production of bitter - pit. On the other hand, a uniform, low temperature and a dry air are unfavorable to the disease.
(2) The second theory brought forward to explain the cause of stippen is that the injury is due to excessive transpiration of water from the fruit during warm days followed by reduced transpiration during the cool nights. A warm day results in rapid absorption of water by the roots and in its rapid loss by transpiration. The ensuing cool night checks the elimination process, but the soil remaining warm allows the roots to go on pumping water into the tissues of the fruit. This results in an internal pressure which finally bursts the distended cells. They soon die and turn brown, and give rise to the symptoms of stippen.
(3) A third theory is based on the assumption that the disease is most destructive in dry seasons. The premature dryness of the soil results in a failure of sufficient mineral food to reach the fruit, in consequence of which certain groups of cells starve and perish.
Many other causes have been assigned to stippen. Among the more prominent ones may be noted: local poisoning by spray material, poisons absorbed by the roots, bacteria, fungi, insects, mechanical injury, unfavorable grafting, degeneration from old age, and finally a peculiarity of the variety. A discussion of these various factors as causes of bitter - pit seems out of place in the present state of knowledge regarding the disease. None of them is known to be either direct or contributory causes of stippen.