On the leaves of many stone-fruits is produced a rust disease known as plum Leaf Rust, prune rust and rust of stone-fruit trees. The disease is widely distributed in North America, Europe and Asia, and is common in South America, Africa and Australia. In the United States it is reported from most of the states east of the Mississippi River, and from Iowa, Missouri, Oklahoma, Texas, Louisiana, California and Oregon. It doubtless occurs in other states of the Union. In Europe it is found in Germany, France, England, Italy, Austria, Belgium and Switzerland. These statements indicate the cosmopolitan occurrence of plum Leaf Rust throughout the world.
Outbreaks of this disease are not uncommon. In 1886 the troublesome nature of plum rust attracted attention in Australia; in 1889 it was serious in Iowa; and in recent years it has been the object of experimentation in the far West. In severe cases defoliation may occur; this is true of the prune and peach in California and of the peach in Texas. It is more destructive in warm, moist climates where crops are seriously impaired and where trees are said to die within three or four years after the attack. In the United States most damage is done in the southern portion, and of all trees affected the plum suffers most. Chippewa plums are said to be particularly susceptible. Likewise the Imperial Ottoman is badly affected.
The disease appears in midsummer but is most abundant in fall. Only the leaves are commonly affected; fruits or other parts of the plum rarely show rust-lesions. On the lower surfaces rust-pustules are found (Fig. 110); these are light-brown, small, round and somewhat powdery. They are scattered, or they may be so numerous as to practically cover the leaf (Fig. 110). The development of these pustules is preceded by the formation of yellowish spots. Later the pustules, or sori, to be observed on the leaves are dark - brown or almost black, but they still retain their powdery nature. Cause.
The pustules or sori just described are fruit-bodies of the causal fungus, Puccinia Pruni-spinosce. Within the affected leaves of the plum the mycelium of this fungus is found. It comes to the surface and there forms sori, within which are developed first a crop of spores known as uredospores. These are capable of infecting other plum leaves as well as the foliage, and in some cases the fruit of the peach, almond, cherry, apricot and nectarine. On all these hosts, too, uredospores are developed which are capable of infecting the plum. These spores may winter - over, are viable in the spring, and can cause new infections after several months' rest.
Fig. 110. - Plum rust, on lower surface of leaf.
After the uredospores have been developed they are replaced in the sori by other spores, known as teliospores. These follow closely the development of uredospores; they are said to appear in five to twenty days after the uredospores. The teliospores germinate in the spring, producing small, light sporidia which do not infect the stone-fruits, but must go to species of Anemone, Thalictrum and Hepatica, all common wild flowers. On any of these flowers germination and infection occur. The mycelium developed in these hosts lives over until spring, when fruiting sori-bearing seciospores are produced. Just preceding these bodies sterile structures known as pycnia are developed on the upper surface of the leaves as black dots. The mycelium in the leaves may live over in these wild flowering plants for several years in succession, so that the stone-fruits are not absolutely necessary to the perpetuation of the fungus on these wild flowers. In this connection, however, it should be noted that the wild flowers already enumerated are necessary to the perpetuation of the fungus on stone-fruits. The seciospores developed on Anemone and others are wind-borne to the plum, where infection occurs on the leaves. Within three weeks after inoculation uredospores are mature and ready for summer dissemination of the fungus. Plum - trees are not generally attacked until sometime after the month of June, except in Australia, where infection occurs in February and March.
In those sections where plum rust is troublesome it is probable that spraying will reduce the injury. The treatment as advised for peach Leaf Curl, that is, spraying before the buds open with some standard fungicide, is recommended. In southern California it appears that early fall pruning performed to such an extent that fall growth is stimulated is not advisable. The difficulty involved lies in the fact that such foliage remains alive through the winter, and rust developed thereon permits early spring infections which bring about considerable damage to the orchard. In Oregon it is thought that bordeaux spraying the last of August or the first of September will prove profitable.
Scribner, F. L. Leaf rust of the cherry, peach, plum, etc. U. S.
Agr. Comm. Rept. 1887: 353 - 355. 1888. Cobb, N. A. Peach rust in our orchards. New South Wales Agr.
Gaz. 1: 93. 1890. Pierce, N. B. Prune rust. Journ. Myc. 7: 354-363. 1894. McAlpine, D. Peach- and plum leaf rust. Victoria Agr. Dept.
Guides to Growers, 5: 1 - 8. 1891. Hedrick, U. P. Prunes in Oregon. Prune rust. Oregon Agr. Exp.
Sta. Bul. 45: 67 - 68. 1897.