The scab disease of the apple is universally the best known of all fungous troubles affecting this fruit. While it attacks only the apple and certain closely related species, its ubiquitous nature accounts for its prominent rank among the diseases of its kind. This familiar trouble is commonly thought to be the same as pear scab, but these two diseases, while similar in their symptomatic and causal aspects, are not identical. Nor should the scab diseases of the peach, cherry, and citrus be confused with the scab of apple.
All varieties of apples are affected, some more severely than others. A variety may be resistant in one year and susceptible in another, under conditions which in both cases are apparently favorable for scab on the average susceptible variety. For example, the Baldwin is usually listed among the resistant sorts; yet in 1910 this variety showed in certain localities 98 per cent scab on unsprayed trees. The Ben Davis, also said to be resistant, often shows as much scab as the average variety. A suggested explanation of these conflicting observations is found in such factors as the color of the fruit, the relation of weather conditions to the development of the fruit, and the adaptation of the pathogene to new conditions. Scab lesions are not so conspicuous on dark - colored fruit as on lighter varieties, and thus the Baldwin bears the reputation of being resistant, while the Rhode Island, which perhaps is really the more resistant of the two, is regarded as very susceptible. Further, in connection with the weather, in a given year the Baldwin may be at its most susceptible stage at the time when weather favorable to infection prevails; that year the Baldwin would appear susceptible while in another year it escapes the disease and is then classed as resistant. This suggestion may well apply to other varieties. And finally, the evolution of the pathogene often keeps pace with that of the host. With the development of resistant or immune strains or varieties come eventually strains of the pathogene capable of attacking them.
While this disease is generally known to American growers as scab or apple scab, it is frequently called the fungus. In Australia and South Africa it is spoken of as Black Spot. The disease is unquestionably of foreign origin and probably has been peculiar to the apple since that fruit was brought under cultivation. The first records came from Europe in 1819. It was subsequently reported from France in 1829, having since attracted attention in nearly all countries where apples are grown. The disease was first recorded in America from Pennsylvania and New York in 1834. For a time apple scab was reported to be absent in the apple-growing valleys of the Pacific Coast and Rocky Mountain states, but within the last ten years it has in those regions become an important factor in apple-culture. While now having a general geographical range over the United States wherever the apple is grown, it is most destructive in the cooler regions of eastern and northern United States, the northern Mississippi Valley, the northern Pacific Coast regions, in the apple - sections of Idaho and Montana, and in the mountainous regions of Virginia, Arkansas, and certain other southern states.
Apple scab may be said to be the most important disease of this fruit in northern United States and in southern Canada. In the Mississippi Valley, north to central Illinois, Indiana and Ohio, other diseases, especially Bitter Rot and apple blotch, are close competitors, and in many seasons are far more important than scab. It has been estimated that the average annual loss in New York State due to failure to spray the apple - crop is not less than three millions of dollars, and for the United States there is a corresponding loss of over forty millions. Not infrequently there is a total loss from failure of fruit to set due to this disease.
The losses are greatest in epiphytotic years, but these occur with sufficient frequency to make apple-growing unprofitable unless preventive measures are taken. The nature of the losses may be indicated as follows: (1) Reduction in or destruction of the set of fruit. Heavy losses of fruit, in some cases total, are incurred under conditions favorable to scab at blossoming-time. (2) Impairing the efficiency of the foliage. Affected leaves are often smaller than normal ones and they may fall prematurely. (3) Reduction in size of the fruit. Scabby apples are almost always smaller than healthy ones. (4) Reduction in quality of the fruit. This is usually regarded as the chief consideration, but obviously other types of losses are nearly of equal importance. (5) The keeping qualities of the fruit are diminished. Pink Rot and other storage troubles commonly follow scab on stored apples. (6) The number of windfalls is increased just before picking time. Scabby fruit does not cling well to the trees.
The disease affects the leaves, flowers and fruit. On certain varieties, such as the Lady, scab is found on the twigs; but this form of the disease is rare in America.
On the leaves the disease usually makes its first appearance in the spring on the lower surface, since that side is first exposed as the leaf emerges from the bud. These hypophyllous spots are as a rule smaller and less prominent than those on the upper surface. The former lesions are brownish or olivaceous, of a webby appearance, with margins indefinite, fimbriate, and commonly tending to follow the veins of the leaf (Fig. 1). The spots on the upper surface (Fig. 1) are similar to those below, but are more definite in outline, darker, more velvety and larger. When a great number of lesions occur on a leaf, each spot is smaller than where fewer lesions prevail. The spots on the upper surface often cause the affected area to become convex above and concave below. Near the end of the season the spots may fuse and frequently the leaf is killed. In case of severe infection defoliation may result. On the blossoms the lesions are found chiefly on the pedicels and calyx. The olivaceous spots may encircle the slender stalk as a result of which it withers and ultimately falls, thus reducing the set of fruit. Blossoms which show even a single lesion seldom hang to the tree. The petals are never affected. Mature scab spots on the fruit are very similar in size, shape and color to those on the upper leaf-surface, although at first they are smaller and more sharply defined. They are usually most numerous about the blossom-end (Figs. 2 and 3) of the fruit, due to the fact that infection occurs largely in the spring while the young apples still remain in an upright position (Fig. 4). The lesions on the fruit show a whitish, papery margin, often very wide; this is the undissolved cuticle which has been slightly uplifted by the pathogene advancing at the edge of the lesion (Fig. 2).
Fig. 1. - Apple scab on leaves; types of infection on upper (left) and lower (right) surfaces.
Fig. 2. - Apple scab, margins of spots showing uplifted cuticle. Lesions about blossom - end.
Fig. 3. - Cluster of scabby apples. Many scab - lesions near calyx end.
Fig. 4. - Scab on young apples. Lesions on both the fruit and pedicels.