Another serious disease of the cherry, and probably the most striking one in appearance, is the black-knot[56] (_Plowrightia morbosa_ (Schweinitz) Saccardo), characterized by wart-like excrescences on shoots and branches. Black-knot looks more like the work of an insect than a fungus and was long supposed to be such even by those who were studying the trouble. The knots begin to form early in the summer and are of characteristic color and texture--dark green, soft and velvety, but in the fall, as the fungus ripens, the color changes to coal-black and the knots become hard and more or less brittle. The excrescences usually form on one side of a twig or branch so that death seldom follows quickly. The disease attacks both wild and cultivated plants in every part of this continent where cherries are grown but is epidemic only in the East, the cherry regions of the West being practically free from the disease. Up to the present time the fungus has not been found elsewhere than in America. Happily, black-knot may be controlled by cutting out the diseased wood. To completely eradicate the fungus, if it is especially virulent, however, the orchard must be gone over several times during a season. In New York the removal of black-knot is ordered by law, the results showing that when the law is obeyed, especially if there be hearty co-operation among growers, eradication is usually possible. Sweet Cherries are much less attacked by black-knot than the Sour sorts but the differences in immunity between varieties in either of the two species are not very marked--at least such is the case on the grounds of this Station where the disease is always present and is often very prevalent. _Exoascus cerasi_ Fuckel[57] is the cause of a very striking deformity of the cherry in Europe, both _Prunus avium_ and _Prunus cerasus_ being attacked. The disease has been reported in America but has not yet become virulent. The fungus attacks the branches, causing a clustering of the twigs in the form of a broom, giving it the name witches' broom. The leaves on the diseased twigs usually take on a crinkled shape and a reddish color. The malady may be readily prevented by the destruction of affected branches.