Apples, wherever they grow and of whatever variety, are subject to diseases and injuries of one kind or another. Very few varieties are famous because of their marked resistance to all, or even one, important disease. Growers are fully aware that many varieties, both standard and fancy, are by no means scab-proof. In the warmer states, the Yellow Newtown and Ben Davis are highly susceptible to Bitter Rot. Stippen, or bitter-pit, one of the most important of apple diseases, is most common on the Baldwin and others of the best varieties of apples. In the northeastern United States, the Twenty Ounce has come to be known as a canker variety; it is invariably affected when other neighboring varieties stand free from this type of trouble. Likewise the Tompkins King, Grimes and others suffer unusually from collar Rot. But some varieties are markedly less sensitive to certain troubles than others. The York Imperial and Grimes are relatively resistant to Bitter Rot, and many other prominent examples might be cited.
There are no less than a dozen very important diseases and injuries to which apple-trees are subject, any one of which may bring about considerable annual loss. Many minor diseases, often overlooked, bring the total dollar loss far above the fancy of fruit-growers. It would be difficult to set a figure which would represent, accurately or even approximately, the losses incurred each year. A few well-known examples may be tabulated in order that some notion of the gravity of this source of waste may be gained. One authority estimates from reliable figures that there is an average annual loss of more than $40,000,000 in the United States due to the failure to spray apples. Such devastation, of course, is brought about both by diseases, chiefly caused by fungi, and by insects, and it would be quite impossible to assign the losses to the proper offender in every case. Individual examples of losses follow: (1) Bitter Rot wrought damage to the apple-crop of the United States in 1900 amounting to $10,000,000; (2) Black Rot canker induces an annual loss of $750,000 in New York; (3) blotch is said to have caused in one county in Arkansas, in 1906, a loss of $950,000; (4) rust, in 1912, was so destructive in West Virginia that actual fruit - losses ranged from $2000 to $3000 an orchard.
Apple diseases are important because of the nature of the injury inflicted. Losses do not stop with damage to, or destruction of, the fruit. Blossoms may be killed; the set of fruit may be dropped; woody parts including twigs, limbs, trunks and roots may be blighted, cankered or rotted; leaves may be spotted and even dropped prematurely. It is indeed fortunate that most of these types of troubles may be reduced to a point where fruit - growing may still be carried on with pleasure and profit.
Most apple diseases are caused by fungi, but a few result from the detrimental action of bacteria, and certain environmental factors. The best-known, the most cosmopolitan, and the most important of apple troubles generally is without question the fungous disease known as apple scab. Other diseases and injuries affecting this fruit are discussed somewhat in order of their general prevalence and importance.