It is to be remembered that the spores of the Soft rot fungus are everywhere in the air, and on objects of all sorts. Rotten apples cannot be cured. But obviously there are many things which the grower and dealer can do or can refrain from doing which will reduce the disease to a profitable minimum. These facts should be borne in mind: (1) the fungus is omnipresent; (2) it gains entrance to apples only through wounds; (3) a rotten apple is a menace to its neighbors in the barrel or bin. Therefore, precautionary measures should be used in handling the fruit. Certainly the modern commercial grower expects to raise apples free from scab; at least there are two reasons why he should: to avoid scab itself, and to indirectly prevent storage rots. A scabby apple is not a number one when it goes into the barrel, and no miraculous handling of the fruit can bring it out in better condition than it was in when barreled. The chances are it will be rendered worthless through the agency of the Soft rot organism which finds easy entrance at the scab spot. For most manual labor a careful manicuring is not essential; but for picking apples it is highly important that the finger-nails be short or that smooth gloves be worn. No careful grower will approve of having the fruit bruised in any way during the picking and packing operations. Every precaution should be taken to avoid bruising the tissue or breaking the skin of the apple. Cold storage cannot mend these wounds, nor does it seem to wholly prevent Soft rot. Doubtless cold storage does a great deal toward lessening the amount of Soft rot from year to year. Some refer to the process as refrigeration; but in any case the principle involved is briefly this: the temperature is reduced to a point where the fungus can develop no further, but at which point the apple is not injured. In proper storage the fruit is not only not injured, but benefits, other than by the prevention of decay, are derived. The period for distribution and for consumption is increased. When apples are stored in small containers, the fruits are cooled more easily than in larger ones. In a barrel, for example, those fruits in the center remain warm for several days after being placed in cold storage. Thus if wounds of any sort are present on the apples, the fungus gets a good start. Moreover, if apples are allowed to stand in a shed or railroad car for several hours, they become warm; likewise those picked in warm weather and allowed to remain in the orchard will also become warm, so that the temperature in the center of the barrel is reduced but slowly in storage. If the temperature in the center of the barrel is 75° to 80° Fahr. when stored, it will require about one week for the temperature of the air in that part of the barrel to be reduced to a point equal to that of the cold storage room. It has been found that apples held at 32° Fahr. for two months showed Soft rot on removal. At a higher temperature, of course, the fungus develops even more rapidly and the destruction is greater. At an average temperature of 47° Fahr., ranging from 35° to 56° Fahr., the fruits may be three-fourths rotted in five weeks; at an average temperature of 60°, ranging from 48° to 69° Fahr., the whole of each attacked fruit is involved within three weeks. It is important to note that fruit carelessly handled before being stored is likely to become affected with Soft rot, even if stored at the freezing point for sixty days. Where the storage temperature is higher than 32° Fahr. and the duration of storage longer than two months, heavy losses from decay may be expected.
The question of applying sulfur fumes to stored fruit for the destruction of Penicillium spores has been studied. The conidia can be destroyed by using sulfur at the rate of one ounce to each twenty - five cubic feet of space, but when the fumes come in contact with ripe apples their commercial value is greatly lessened. The sulfur dioxid passes through the lenticels, bleaching the fruit.
Spraying has no direct effect on Soft rot; but the practice is indirectly valuable in the prevention of scab and the elimination of insects that injure the fruit. If the stored fruit is fancy, and it is desired to keep it for a long time, wrapping each fruit is of decided advantage in preventing decay. It appears after all that the secret of controlling this disease lies in the careful handling of the fruits throughout all operations necessary to marketing and consumption.
Eustace, H. J. Investigations on some fruit diseases. New York (Geneva) Agr. Exp. Sta. Bul. 297: 32 - 48. 1908.
Kinney, L. F. The blue mold. Rhode Island Agr. Exp. Sta. Rept. 1894:195 - 198. 1895.