One of the commonest and most striking of the diseases of the plum is black-knot[156] (_Plowrightia morbosa_ (Schw.) Saccardo) characterized by wartlike excrescences on shoots and branches. In early summer these knots are dark green, soft and velvety, but as the fungus ripens in the fall the color changes to a carbon-like black and the knots become hard and brittle. The disease is usually confined to one side of the twig or branch so that death of the affected part does not ensue at once. Black-knot is an American malady, at one time confined to the eastern part of the continent where in some localities its ravages forced the abandonment of plum-growing. The fungus is now endemic to wild or cultivated plants in practically all the plum-growing regions of the continent, but it is still epidemic only in the East, the South and West being practically free from the disease. Unless especially virulent black-knot is controlled by cutting out the diseased wood. Usually eradication is not possible without several prunings during a season. Much has been made of the supposed immunity of some varieties of plums to black-knot. In the vicinity of this Station, where the disease is always present and often rampant, the differences in immunity are not very marked in varieties of the same species. The Trifloras are less attacked, however, than any other group of plums, and the Insititias rank next in immunity. No variety of the Domesticas has yet proved to be free from the disease but strong claims are made that Middleburg and Palatine are relatively free.