Another disease of plum foliage, occurring rarely on the fruit, is the plum-leaf rust[163] (_Puccinia pruni-spinosae_ Persoon) which produces so considerable a number of spore cases on the underside of the leaves as to give the foliage a brownish cast and to cause defoliation in severe infections. The fungus is most apparent in the fall and most troublesome in warm, moist climates. Bordeaux is used as a preventive. Stewart and Rolfs have shown that trunks and branches of plums affected by sunscald in New York are almost invariably infested by a fungus[164] (_Valsa leucostoma_ Persoon) which in the Old World is known as the “die back” of the peach. The disease manifests itself on plums chiefly by affected areas much depressed at the boundary between the living and the dead bark, these areas usually, not always, having connection with sunscald injuries on the trunk. The disease is accompanied by more or less gumming. In common with nearly all rosaceous plants, in nearly all countries, the plum is sometimes seriously injured by the powdery mildew[165] (_Podosphaera oxyacanthae_ DeBary). The affected leaves have a grayish appearance caused by the parts of the fungus which project beyond the leaf tissue; when badly diseased the leaves are more or less arched and curled. Mildew is seldom prevalent enough on plums to require treatment. The crown gall,[166] (_Bacterium tumefaciens_ Smith and Townsend) is a parasite on all of the fruits of the order Rosaceae and is especially common on nursery stock, attacking plums in many soils but rarely, however, to the great injury of the plant. These galls are perennial structures of very varying duration. They are to be found on the roots, 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. Means of prevention or cure are not established though all agree that soils may be inoculated with the disease from infected stock; hence the necessity of discarding diseased trees at transplanting time. Smith found in Michigan and Clinton in Connecticut a disease of the fruit called bacterial black spot[167] (_Pseudomonas pruni_ Smith) of the same generic origin as the crown gall but widely different in nature. The writers and the growers who found the infected fruit, saw the disease only on the Triflora plums. It attacks the green fruits which show conspicuous, black-purple, sunken spots sometimes as large as half an inch in diameter. The injuries are usually isolated and quite superficial but nevertheless, spoil the fruit.