This is a well - known raspberry and blackberry disease over the eastern United States and southern Canada. It also occurs as far south as Florida and California. The disease is found in Europe and Asia commonly.

The attacks are limited to wild and cultivated blackberries and raspberries. It is extremely common on black raspberries in New York. In Illinois the Snyder is said to be resistant. On all of these forms the disease is variously referred to as Orange Rust, spring rust, red rust, bramble rust, and erroneously as yellows. The disease destroys the usefulness of the leaves and they finally fall. Annual recurrence of this phenomenon results in rendering the plants worthless. Rarely do affected plants recover. Ten per cent rusty plants are frequently reported. Twenty - five per cent or more are recorded. In some localities the orange rust has so seriously affected raspberries and blackberries as to make their cultivation unprofitable.

Symptoms

First signs of the disease are observed in the spring - whence the name spring rust. In April and May, even before the leaves are entirely unfolded, evidences of the disease may be seen. On the upper surface of the leaves glandular bodies are developed. In their full maturity they appear as black specks. The tissue about them is yellowish. These bodies are not found on all leaves nor on all leaflets of a given leaf. Two or three weeks after the appearance of these structures evidence of rust is seen on the lower surface of the leaves. When mature, these bodies (sori) break open and expose an orange-colored mass of spores; whence the name Orange Rust. Ordinarily these sori with their spore-masses practically cover the lower surface of the leaf. Affected leaves are dwarfed and rolled, somewhat exposing the orange-colored lower surfaces. Rust is rarely found on the canes. This stage gradually disappears during the latter part of June, until by July it is difficult to find in northern United States. Farther south, on a parallel with Maryland, Orange Rust may be seen in late summer and sometimes in the fall. Affected plants are noticeably stunted (Fig. 117) but are not killed. Plants once affected are almost certain to be diseased again the following year. Some rusted plants show a reduced number of prickles. Cause.

The bramble fungus, Gymnoconia interstitialis, is the cause of this disease. Its mycelium lives from year to year in the affected plant. In the roots the mycelium is found between the cells of the cortex, near the cambium. As it proceeds upward the pith of the canes is invaded. In the leaves the spongy parenchyma is infected. In young stems the hyphae may be found in any of the tissues. In all these attacked organs the mycelium develops haustoria. These arise as side branches from the mycelium. They pierce the cells to get food. As an infected cane grows the fungus follows the growing-tip. In the spring on the upper surface of the opening leaves sterile black bodies appear. These are called spermagonia, or pycnia. Two or three weeks later the orange-colored cushions develop on the lower side of the leaf. Many spores are formed, which are capable of germinating at once. From each spore a short promycelium bearing four sporidia is protruded. These sporidia cause other infections on raspberries and blackberries during the growing - season. Mycelium develops from the sporidia. In the winter the fungus is again dormant as mycelium in the canes, crown and roots.

Fig. 117.   Orange Rust; healthy plant on right, diseased on left.

Fig. 117. - Orange Rust; healthy plant on right, diseased on left.

Control

The perennial nature of this rust fungus makes it difficult to control. The only known remedy for diseased plants is to dig and destroy them. All wild blackberries and raspberries known to be affected should be destroyed; they may be regarded as weeds in this connection. Spraying to protect healthy plants from infection should be beneficial, although no experimental data on which to base specific recommendations are available. The fungus has definite enemies which destroy the spores in large numbers. Certain insects eat quantities of spores. A certain fungus, Tuberculina persicina, is parasitic on the rust pathogene.

References

Clinton, G. P. Orange rust of raspberry and blackberry. Illinois Agr. Exp. Sta. Bul. 29: 273-296. 1893. 2d Kunkel, Otto. The production of a promycelium by the aecidiospores of Caeoma nitens Burrill. Bul. Torr. Bot. Club, 40: 361 - 366.

1913. Wilson, G. W. Rusts of blackberries, dewberries, and raspberries.

North Carolina Agr. Exp. Sta. Rept. 35: 57-61. 1912. Newcombe, F. C. Perennial mycelium of the fungus of blackberry rust. Jour. Myc. 6: 106-107. 1890. Stewart, F. C, and Blodgett, F. H. A fruit disease survey of the Hudson Valley in 1899. Blackberry diseases. Orange rust.

New York (Geneva) Agr. Exp. Sta. Bul. 167: 286 - 287. 1899.