In common with other species of Prunus the foliage of cherries is attacked by several fungi which produce diseased spots on the leaves, the dead areas usually dropping out leaving holes as if punctured by shot. Thus we have "shot-hole fungus," "leaf-spot" and "leaf-blight" as effects of these diseases. Three fungi are in the main responsible for these leaf troubles; these are _Cylindrosporium padi_ Karsten,[58] _Mycosphærella cerasella_ Aderhold[59] and _Cercospora circumscissa_[60] Saccardo. The ravages of these fungi are prevented by the proper use of bordeaux mixture and lime and sulphur, remedies which, however, must be used with some care to avoid spray injury. With these, as with other fungi, cultivation has a salutary effect as it destroys diseased leaves which harbor the fungi during their resting period. Cherry leaves are often covered with a grayish powder which in severe cases causes them to curl and crinkle and sometimes to drop. This powdery substance consists of the spore-bearing organs of a mildew[61] (_Podosphæra oxyacanthæ_ De Bary). Powdery mildew is much more common on nursery stock than on fruiting trees and in New York is a serious pest on young cherry trees. In the nursery, injury may be prevented by the use of copper sprays or lime and sulphur, either of which is also an efficient preventive in the orchard but the mildew is seldom prevalent enough on orchard plants to require treatment. Wherever cherries are grown in either the nursery or orchard, crown gall[62] (_Bacterium tumefaciens_ Smith and Townsend) has obtained a footing. In the North at least, it seldom greatly injures old trees, but if the galls girdle a nursery plant serious injury results. Therefore, badly infected young trees showing galls should not be planted. However, but little harm is liable to result under most conditions. When infected plants have been planted it has been found that galls vary greatly in duration, sometimes disappearing within a year or two and at other times persisting indefinitely. The tumor-like structures are usually at the collar of the plant and vary from the size of a pea to that of a man's fist, forming at maturity rough, knotty, dark-colored masses. Neither prevention nor cure has been discovered, though it is known that soils may be inoculated with the disease from infected stock and that, therefore, diseased trees should not be planted in soils virgin to the galls. It is probable that there are differences in the susceptibility of Sweet and Sour cherries to the fungus and that the varieties of the two species vary in their resistance but as yet no one seems to have reported on the differences in susceptibility of cherries to the disease.