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.

The leaf-rust[63] (Puccinia pruni-spinosæ Persoon) of stone-fruits, occurring rarely on the fruit, sometimes attacks cultivated cherries and is a rather common disease of the wild Prunus serotina. This rust is troublesome only, however, in warm, moist climates. It is most apparent in the fall and is easily recognized through its numerous rust-colored sori on the underside of the leaves. Defoliation takes place in severe infestations. Either bordeaux mixture or lime and sulphur may be used as a preventive.

Old cherry trees are often attacked by a fleshy fungus or "toadstool"[64] (Polyporus sulphureus (Bulliard) Fries). This fungus is said to be world-wide in its distribution and to occur upon a large variety of trees. It is very striking in appearance, the clusters appearing during late summer or early autumn in large, shelving branches, the sporophores fleshy and of cheese-like consistency when young but becoming hard and woody with age. At first the "toadstools" are all yellow but later only the under surfaces are yellow while the upper surface is orange-red. The plants are more or less odoriferous, the odor increasing with age. Happily, the fungus is not very virulent but is often the cause of decay in the tree-trunk—the brown-rot of the wood of this and other orchard and forest plants. In localities where the fungus thrives it may usually be controlled by covering all wounds with tar or other antiseptic materials.

At least two other fleshy fungi have been found injuring cherries. These are Clitocybe parasitica Wilcox[65] and Armillaria mellea Vahl.,[66] the latter the honey agaric, more or less abundant in both Europe and America. Both are associated with and are probably a cause of the root-rot of the cherry and other orchard fruits. Neither is a common enough pest in this country, however, to receive extensive description in texts on diseases of plants. Control measures are different in localities where fungi occur, consisting in the main of getting rid of stumps and roots in orchard lands and planting to field crops before using for orchard purposes. Infected trees should be removed or isolated by trenching about them.

All stone-fruits suffer more or less from an excessive flow of gum. The name gummosis[67] is generally applied to these troubles. Gumming is much more prevalent in the far West than in the East but is to be found wherever stone-fruits are grown. This excessive gumming is a secondary effect of injuries caused by fungi, bacteria, insects, frost, sunscald, and mechanical agencies. There is a good deal of difference in the susceptibilities of varieties and species to this trouble, the Sweet Cherry suffering much more than the Sour sorts and varieties of other species having hard wood suffering less than those having softer wood. There is less gummosis, too, on trees in soils favoring the maturity of wood; under conditions where sun and frost are not injurious; and, obviously, in orchards where by good care the primary causes of the diseases are kept out.

A number of diseases of the trunk arise from mechanical injuries from wind, sun, frost and hail. Few, indeed, are the fruit-growers whose trees are not occasionally damaged in one way or another in the vicissitudes of a trying climate. Very often these mechanical injuries are followed by fungal parasites or insects so as to make it difficult to distinguish the primary from the secondary trouble. There is a wide difference in the susceptibility of Prunus avium and Prunus cerasus to such injuries, the Sweet Cherry, with its softer wood, being much more easily injured by any and all stresses of weather than the Sour Cherry. In the main the elements cannot be combated but low heading of the trees is a preventive from sunscald, at least, and sometimes may have a favorable effect in preventing wind and frost injuries.

CHERRY INSECTS

Insects troubling cherries are numerous but hardly as destructive as with other tree-fruits. Entomologists list about 40 species of insects attacking cherries and about as many more occasionally attack the varieties of one or the other of the two cultivated species. The majority of these pests came with the tree from its habitat over the sea but several have come from the wild cherries of this continent.