This disease is best known as the New York apple-tree canker, in those regions bordering the Great Lakes, because of the prominence of this form of the trouble in that section of the country. In New England, Virginia and the Ozark portions of the United States, Black Rot of the fruit and spots of the foliage are the more common forms with which the grower has to contend. Speaking generally for northeastern America, the Middle West and southeastern Canada, this apple disease, in one or more of its three forms, stands second only to apple scab in importance. In other regions where it occurs it attracts considerable attention, but is plainly of less consequence than Bitter Rot or apple blotch. In the Middle Atlantic states it is less important than apple rust. The disease, while present in most European countries, does little damage outside of North America.
Affected fruit is rotted and, as in the case of apple scab, apple blotch and Bitter Rot, is rendered practically worthless so far as the market value is concerned. In New England considerable damage is done in storage. In Kentucky, Indiana, Missouri, Illinois, Alabama and Massachusetts the fruit is often seriously affected in the orchard. In Massachusetts 80 to 90 per cent of the apple Rots are of this kind, while in the other states just referred to Black Rot is, next to Bitter Rot, the most common type of apple fruit-decay encountered in the orchard. The losses incurred through leaf-infections depend on the extent of the infection, that is, the size and number of spots on each leaf. In New York, only mild cases occur and the injury is not appreciable. In New Hampshire, Virginia and the Ozark region defoliation often results from the attacks of this Leaf Spot pathogene. The Ben Davis, Black Twig, Chenango, Baldwin, Rhode Island and Twenty Ounce are most susceptible to Leaf Spot.
The damage done to limbs is rarely appreciated. The largest limbs of mature trees are most subject to this disease, and while to most orchardists the loss of these limbs seems momentous, a great many are inclined to forget the cost of growing such limbs to bearing age as well as the expense of treating the same when thus diseased. Whole trees are sometimes killed. A case is on record where the trees on thirty acres of an eighty-acre orchard were ruined, and those on the remaining fifty acres were rendered almost worthless. The dollar loss incurred by the Black Rot canker would be difficult to estimate. However, reckoning the amount of fruit lost as a direct result of canker on bearing limbs is a simpler matter, and for New York State the figures representing the annual loss through this channel have been conservatively put at $750,000. In those apple - orchards bordering Lake Ontario, the Twenty Ounce variety is by far the most susceptible. But it is difficult to point to a variety which in general is second in this respect. The Esopus, Baldwin, Wagener, Rhode Island and Tompkins King are sometimes so grouped. A peculiar and interesting case was called to the authors' attention in a New York State orchard where old trees of the Baldwin variety had been grafted to Twenty Ounce some twenty years ago. The point of union is about midway distant between the crotch and the highest tip of the branches. The canker shows abundantly on the Twenty Ounce portions of the tree, but the Baldwin stock is entirely free from the disease. In most cases the disease extends down the Twenty Ounce bark to the point of union, there stopping very abruptly. This indicates that the Baldwin is at least not to be classed with the Twenty Ounce and others of the most susceptible kinds. In Ontario, Canada, the Ben Davis and Northern Spy are said to be susceptible varieties.
Fig. 10. - Early stages in the development of Black Rot of apple.
Ripe fruits are more commonly attacked than green fruits. Signs of the disease may show anywhere on the surface or at the blossom-end. In the latter case, there is produced what is called blossom-end rot. Worm-holes are commonly the centers of rotted areas (Fig. 10). Ordinarily only one spot occurs in each fruit. Such a lesion is at first brown, and is frequently referred to as Brown Rot. Very often concentric zones of light and dark colors of uniform width appear about the center of the lesion; in these cases the disease is called ring Rot. The margin of the diseased portion is distinct from that of the healthy (Fig. 10), and the rotted tissues are not of unpleasant taste as in the case of many fruit decays. Later stages of the disease exhibit a black color, whence the name Black Rot. Usually by the time the spot is two inches in diameter small black fruiting bodies of the pathogene are seen on the lesion. Finally, a mummy is produced (Fig. 11), which is at first waxy, then dry and hard. Black Rot has been confused with Brown Rot (Fig. 37, page 141), Bitter Rot (Fig. 5, page 16), and Soft rot (Fig. 25, page 92). Brown Rot results in a browning of the affected tissues which is difficult in this stage to distinguish from Black Rot. Later, the Brown Rot mummy may be like the Black Rot mummy only in that it is also black in the case of certain varieties; it differs, however, in that the Brown Rot mummy is sometimes coal black, glossy, smooth, having no black fruiting bodies, and is much less wrinkled than the Black Rot mummy. Bitter Rot, besides having an unpleasant taste, often shows pinkish spore masses on the lesion. (Compare Figs. 5, 10, and 11.) Soft rot has an extremely softening effect on the tissues and also is attended by an offensive odor. These two characters serve to distinguish Soft rot from Black Rot and other apple rots.
Fig. 11. - Apple Black Rot mummy.