The disease on the leaves is noticeable from a time shortly following their unfolding to the end of the summer. One to several spots occur on a single leaf; in the latter case the lesions are scattered or grouped (Fig. 12). On the upper surface the first evidence of this Leaf Spot is a minute, purplish speck, which soon enlarges until it attains a diameter of about one-eighth of an inch. The spot later becomes yellowish brown, circular, and definite; in this stage the margin is elevated, and the diseased portion is sunken. Older spots become lobed, due to the secondary extension of the pathogene from one or more points in the leaf. This activity results in a series of more or less concentric areas making up an irregular spot, but in which the outline of the original diseased portion can still be recognized (Fig. 12). The center of the lesion becomes grayish brown and the entire affected area presents an appearance which has given rise to the name frog-eye (Fig. 12). Frequently the small black fruiting bodies of the pathogene develop on the Leaf Spot, although they are not always readily noticed on account of their minuteness. The spots are not so conspicuous on the lower surface of the baf. In severe cases the foliage turns yellow and may fall six or eight weeks prematurely. Bordeaux injury is very similar to this Leaf Spot.
Fig. 12. Leaf Spot of apple.
Fig. 13. - Black Rot canker.
The cankers (Fig. 13) are found more often on the uppermost side of the larger limbs. At first the bark is discolored and sunken. The discoloration is for a short time reddish brown but very soon is darker, the diseased portion, upon close examination, being easily distinguished from the surrounding healthy bark. Sometimes the cankers remain small and the pathogene dies at the end of the year. In other lesions the pathogene spreads from year to year until the canker is several feet in length. Very early in the formation of the canker a crevice is developed at its margin, on the healthy side of which corky tissue originates. This plate of cork limits temporarily the extent of the lesion. Further spread results in an increase of the diseased area with the formation of a second marginal crevice. Repetition of this process proceeds from one or more points at the edge of the canker until a series of more or less concentric crevices is developed (Fig. 13, near top). This phenomenon is very similar to that described for frog - eye of the leaves. Affected bark remains closely appressed to the wood for a year or more, but finally cracks and falls away, exposing the wood and a callus about the margin of the wound (Fig. 13). Before the bark falls there appears over its surface the same sort of black fruiting bodies of the pathogene as described for the other affected organs (Fig. 13). Girdling of affected limbs commonly occurs, as a result of which the parts above the lesion ultimately die. This is evidenced by a yellowing and browning of the leaves and the shriveling of the bark and fruit. Sometimes there is an hypertrophy of the limb at the upper and lower ends of the canker.
The above description constitutes external symptoms in which the bark appears to be the only part affected. But upon removal of the healthy bark immediately above and below the canker the sap wood is found to be stained brownish, the discoloration appearing in a long slender streak, continuous from the canker to a point several inches distant.
This disease is of fungous nature, and the causal pathogene is Physalospora Cydonice. The fungus passes the winter in the old cankers as mycelium and as pycnospores. It is not only found on fruit-trees other than the apple, but also on a number of other plants such as alder, ash, basswood, dogwood, elder, hawthorn, hop-hornbeam, lilac, maple, mulberry, oak, pine, rose, sumac, witch-hazel and others. In the spring the pycnospores emerge from the pycnidia beginning in April; this discharge goes on throughout the growing-season. The spores ooze from their pycnidia in coil-like masses; the rain and wind carrying them to the leaves, fruits and bark. Insects are not known to disseminate the fungus. Germination of the spores occurs within six hours, the tubes entering the various susceptible parts through wounds, except in the case of young leaves which are penetrated through the uninjured surface. Within a few days symptoms of the disease appear as already described. Pycnidia, in which spores develop, are soon formed on the affected organs of the apple. These spores are carried to other susceptible organs throughout the growing-season. Some of the spores may be disseminated to any of the many plants previously listed. With the advent of winter the fungus ceases its activities, and lives through the dormant season in the form of mycelium and spores in the cankers. A perithecial stage is known, but it apparently plays no prominent role in the life - history of the pathogene.
The treatment of this disease consists in the eradication of cankers and in protecting the susceptible parts by the use of fungicides.
The treatment of cankers may be followed along one of two lines. The affected limb may be cut from the tree, or the diseased bark may be removed. The grower must study the problem and must proceed in accordance with the conditions. The size and value of the limb and the extent of the infection will be criteria. Small limbs, that is, those with a diameter less than one inch, should in all cases be removed. It would rarely be profitable to attempt to treat a canker on a branch of this size. If the larger cankered limb is valuable because of its producing powers, then the size of the canker must finally determine the course to pursue. Smaller cankers on large productive limbs can be profitably removed. If the lesion extends more than half around the limb, no attempt should be made at removing the canker; the entire branch should be removed.