From the foregoing statements the following facts regarding bitter - pit are to be considered in discussing control:
(1) The disease has a very general geographical range.
(2) Some varieties of apples are more susceptible than others, although probably none is immune.
(3) The history of the disease indicates that it frequently becomes a serious problem to the apple - grower.
(4) The fruit only is affected and it usually shows the disease at the approach of maturity on the tree, or in storage.
(5) The disease develops beneath the skin at points near the sap tubes. This close proximity has suggested to the minds of those investigating the disease that stippen is in some way connected with the water supply of the fruit. Prominent authorities agree that a deficiency in the water - supply gives rise to the difficulty. Either the water is lacking in the soil, or it is eliminated from the fruit before certain cells have received their necessary quota. This results in sap concentration, chiefly an increased acidity within the cell, and the living substance is killed.
(6) Factors having to do with transpiration are concerned in the cause of stippen.
(7) Intermittent weather conditions favor the development of the disease. A light crop of poorly distributed fruit is most liable to bitter - pit.
(8) Fluctuating temperature and humidity in storage favors the disease. Uniform, low temperature and dry air are inimical to it.
It is then apparent that growers should everywhere be on guard against this disease. No one can safely assume that his fruit will always escape by virtue of the variety which he grows or of the locality in which it is produced. It is very important that everything possible be done which will tend toward the production of good crops evenly distributed over the tree year after year; for herein lies a possible solution of the control of stippen. Most growers know that vegetative growth is unfavorable to fruitfulness. On the other hand, injudicious, heavy pruning to eliminate the vegetative growth also favors bitter-pit. It is believed that above all other orchard operations pruning has the most direct effect on the development of stippen. For it may regulate the amount, and to a certain extent the size, and it may determine the distribution of the fruit on the tree. The system adopted should be carefully planned, it should aim at the production of a regular crop of evenly distributed fruit, and should therefore be light. The lateral system of pruning, that is, the retention of laterals, as far as practicable, is to be adopted as a part of the general pruning-scheme, particularly in the case of varieties most susceptible to the disease. For there is some evidence that bitter-pit is much reduced on trees when the fruit is borne on lateral rather than on main branches. Frost at setting is also unfavorable to fruitfulness, therefore the question of frost-protection should receive attention in localities where experience has proved its necessity. Regulated sap-flow favors fruitfulness, and is also unfavorable to bitter-pit. Most alert commercial fruit growers practice thinning. It is an operation essential to regularity of bearing. It makes yields regular and bitter-pit is consequently reduced. Trees with a light crop and unusually large-sized fruit are most subject to pit. Again, overcrowding favors the disease. Manuring has a bearing on the development of stippen. The practice is commendable in that it not only supplies food, but neutralizes toxic substances in the soil. It also affects the moisture content of the soil and thus transpiration is modified. The water-elimination process in its relation to bitter-pit has been emphasized. Although long years of experimentation are necessary to determine the effect, if green-manuring were practiced in non-irrigated orchards the disease would probably be reduced. Cover-crops are planted by many of the leading apple-growers. These crops aid in regulating the water-supply of the apple, and when plowed under will conserve the soil moisture. The question of drainage should not be overlooked. This disposes of excessive water during the days of heavy rainfall, and conserves moisture during drought-periods. The disease is worst in low, wet portions of an undrained orchard. Moderation should be practiced in the matter of irrigation. The heavy application of water following a drought-period is favorable to stippen development. Excess should be avoided, and the aim should be to maintain uniformity of moisture conditions. Thorough orchard cultivation is advised. This operation affords proper aeration of the soil and consequent normal and efficient root - action. It conserves moisture during periods of dry weather, and as a result transpiration is properly regulated. Spraying or dusting operations have an important bearing on the problem; for, if neglected, the foliage is almost sure to be diseased and therefore less efficient both in the manufacture of food and in transpiration. It has been found that diseased leaves transpire less than healthy ones.
The prevention of stippen in transit or in storage is relatively simple. Shipments should be made under cool conditions. The temperatures must be low (30 to 32 degrees Fahr.) and constant. In storage the same uniform, low temperatures must be provided. It has been shown conclusively that the development of stippen may be prevented by storing the apples at 30 to 32 degrees Fahr. under dry air conditions. The ordinary cellar or cool storage is not reliable. An increase in the temperature to 34 degrees Fahr. allows the disease to develop. The storage room should be properly ventilated; good ventilation in conjunction with refrigeration is of prime importance for successful storage of fruit.
It is thus evident that those practices which tend to a uniform, normal growth throughout the season, with fewest sudden stimulations or checks on growth, are, in the long run, most inimical to bitter - pit.
McAlpine, D. Bitter pit investigation. The past history and present position of the bitter pit question. First progress report, pp. 5-197. 1911 - 12. (Extended bibliography.)
McAlpine, D. Bitter pit investigation. The cause of bitter pit: its contributing factors, together with an investigation of susceptibility and immunity in apple varieties. Second progress report, pp. 5-224. 1912 - 13.
McAlpine, D. Bitter pit investigation. The control of bitter pit in the growing fruit. Third progress report, pp. 5-176. 1913 - 14.
McAlpine, D. Bitter pit investigation. The experimental results in their relation to bitter pit and a general summary of the investigation. Fourth report, pp. 5-178. 1914 - 15.
Whetzel, H. H. Baldwin spot or stippen. New York State Fruit Growers Assoc. Proc. 11: 28 - 34. 1912.
Evans, I. B. Pole. Bitter pit of the apple. Transvaal Agr. Dept. Tech. bul. 1: 3 - 18. 1909.
Jackson, H. S. Diseases of pomaceous fruits. Fruit pit. Oregon Crop Pest and Hort. Bienn. rept. 1911-1912: 234 - 236. 1913.
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