In certain of the northeastern states apples and pears are affected with a superficial Bark Canker. In practically every apple or pear orchard of New York and neighboring states there is more or less of this disease. But even in those orchards where every tree is affected there is no evidence that serious damage is being done. Every branch of a tree may be extensively affected, while large, bearing limbs commonly exhibit cankers of considerable extent, but in spite of these facts the disease is probably never injurious. Its superficial nature accounts for this in a satisfactory manner.
The common occurrence of this disease is sufficient in itself to warrant a description. Many growers and even scientists have confused the superficial Bark Canker with the Black Rot canker. The former disease, like the latter, is found chiefly on the older and larger limbs. But there is no striking depression developed in the case of the superficial canker as with the Black Rot canker. At most there is but a slight sinking of the affected bark. The outer bark is killed, and a sharp crevice marks the extent of the lesion (Fig. 34). This line of demarcation is prominent and takes an irregular course on the affected limb. The originally infected areas are small and more or less circular, but large cankers of various shapes finally appear as a result of the coalescence of two or more cankers. Accompanying the pathological changes in the normal bark minute pustules develop in the affected area; these dot the surface and resemble very much those on the Black Rot canker. (Compare Figs. 13 and 34.) If critical notice be taken, however, it will be seen that the pustules on the superficial Bark Canker are of an open, saucer-shaped type, while on the Black Rot canker they are closed, flask - shaped bodies. The dead bark clings tightly to the limb for some time. Later, bits of bark fall from the tree. Old cankers usually show considerable checking of the bark (Fig. 34); these crevices are short and sometimes extend at right angles to the long axis of the affected limb.
The fungus Myxosporium corticolum is responsible for the superficial Bark Canker of the apple and the pear. The known facts connected with its life-history and habits are few. The mycelium of the fungus grows in the outermost bark-tissues only, never reaching the cambium. Scarcely before it penetrates to an appreciable depth its progress is halted by a cork-layer developed by the tree in response to the stimulation induced by the invader. This plate of cork is apparently never penetrated by the fungus, therefore its attacks are confined to the surface cells. The affected tissue is killed and eventually it sloughs away. In this process the cork-layer marks the line of cleavage. During its course of development the fungus forms fruiting bodies, acervuli, just beneath the surface of the bark. At maturity these break through the bark and expose a saucer-shaped interior. Conidia arise from the inner wall of this cavity; and in the moist weather of spring the spores ooze forth in white masses. These spores bring about new infections during the growing - season. In the winter the fungus lives in the cankered bark.
Fig. 34. - Superficial Bark Canker.
The damage caused by Myxosporium corticolum is so slight that little attention need be given to its control. Frequent inquiries are made regarding its nature and treatment, but it is very doubtful whether any sort of remedial measures are ever necessary or profitable.
Edgerton, C. W. Two little known Myxosporiums. Ann. Myc.
6: 47-52. 1908. Morse, W. J., and Lewis, C. E. Maine apple diseases. Myxosporium canker. Maine Agr. Exp. Sta. Bul. 185: 373-374. 1910. Lewis, C. E. Inoculation experiments with fungi associated with apple leaf spot and canker. Phytopath. 2: 49 - 62. 1912.