The destruction of fallen leaves would appear to lessen the primary infection. But in actual practice that method alone is not reliable and at best is only to supplement spraying or dusting. Spraying is at present the chief method of control. The fungicide used is lime sulfur solution, diluted one gallon of the concentrated solution (testing 32° Baume) to forty gallons of water. To this is added arsenate of lead at the rate of two pounds (paste; or one pound, powdered) to each fifty gallons of the diluted lime sulfur. Aside from its insecticidal value, the arsenate of lead increases the fungicidal value of the lime sulfur. Applications should be made as follows: (1) Just before the blossoms open, but after the individuals of the cluster have separated (Fig. 125, page 440). The period for effective spraying at this time is from one to three days. This application should hold the set of fruit, if the fruit-pedicels are thoroughly coated. This is the most important application of all in a generally wet season. (2) Spray just after the blossoms fall, beginning when they are two-thirds off (Fig. 126, page 441). This application gives a clean crop of fruit. The period for effective spraying is from two to five days. (3) Ten days to two weeks after the second application, depending on the weather conditions. In a dry spring this application may be omitted. (4) The latter part of July or the first of August. Again the grower is to be guided by weather conditions, although ordinarily this application should not be omitted, as a relatively clean crop may, be badly damaged by late infections, if this application is not made. Sprayings in all cases should be done before rain-periods, since the fungicide must be on the susceptible part before the pathogene is. Effective fungicides do not wash off sufficiently to destroy their efficiency. By studying the low barometric areas indicated on the daily weather map the orchardist should be able to predict weather conditions for two or three days in advance. A nozzle giving a fine driving mist should be used. The material should be applied at a pressure of 200 pounds. Spraying against the wind instead of with it saves materials and labor and gives a more effective distribution. Spray from a tower of sufficient height to enable the operator to spray the highest parts. Spray every season, never omitting the first, second, and rarely the fourth applications. Bordeaux mixture has been used widely for the control of apple diseases, and particularly for scab. It has been replaced with lime sulfur spray and a sulfur - lead dust on account of the injury or russeting of fruit which results from the use of bordeaux (Fig. 124, page 434).
It has been shown that dusting with properly powdered materials is fully as effective as spraying. The operation is much more rapid in covering the orchard; an orchard may be dusted in one-fifth to one-tenth the time required for spraying the same. This dusting process not only saves time but an orchard of large acreage may be protected at critical times, a thing not always possible with the slower liquid process. Apply a mixture of ninety parts finely powdered sulfur and ten parts arsenate of lead (powdered), using about one and one-fourth to two and one - half pounds a tree at each application. The smaller dusting machines may be operated by hand, the larger ones by a gasoline engine. The time of application of dust mixtures does not differ from that of the application of sprays.
Wallace, E. The scab disease of apples. Cornell Univ. Agr. Exp.
Sta. Bul. 335: 545-624. 1913. Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bul. 67:109 - 156.
1901. Reddick, D., and Crosby, C. R. Dusting and spraying experiments with apples. Cornell Univ. Agr. Exp. Sta. Bul. 369:308 - 356.
1916. Jackson, H. S. Diseases of pomaceous fruits. Scab. Oregon Crop.
Pest and Hort. Bienn. rept. 1911-1912: 238-241. 1913. Whetzel, H. H., and Hesler, Lex R. Some of our most common apple diseases. Apple scab. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79: 855 - 861. 1916. Aderhold, R. Die Fusicladien unserer Obstbaume. Landw. Jahrb.
25: 875 - 914. 1896. Aderhold, R. Die Perithecienform von Fusicladium dendriticum (Venturia chlorospora f. Mali). Deut. bot. Ges. Ber. 12:338342. 1894.