This is a raspberry and blackberry disease peculiar to the Pacific Northwest. It has been increasingly troublesome in the Puget Sound region since 1904. All blackcap varieties of raspberries are susceptible. It also affects red raspberries.
In general, when raspberries are affected with Blue Stem the plants fail to mature the crop properly and they may die during the summer. The canes usually are first to show signs of the disease. The fruit begins to dry up and harden before maturity. This change in the fruit is accompanied by a discoloration and wilting of the leaves and a darkening of the canes. Symptoms vary with the severity of the attack.
The first discoloration of the shoots occurs a few inches above the ground. A blue-black stripe extends from some point a few inches above the ground upwards on the shoot. The stripe may be narrow, or the whole side of the shoot may be dark-blue. Very commonly shoots are discolored throughout their circumference for three feet from the ground. This discoloration may increase three to six inches in length each day during the growing season. At the ends the dark-blue area is fringed and fades into a reddish brown color. The margin is quite distinct. As a result of this affectation some shoots may die. Death occurs throughout the entire growing season, although most dying takes place during the first half of the summer. Other plants which do not die show a wilted and yellowed condition of the foliage. Canes do not always show external signs of the blue-black discoloration. Internally, however, affected canes exhibit characteristic reddish streaks. The roots also may show this internal discoloration. In a single plantation every hill may show the Blue Stem disease.
The life-history of the causal fungus, Acrostolagmus caulo-phagus, is not fully known. It apparently lives in the soil and in some way gets into the plant at a point below the surface of the ground. There is some evidence that entrance is gained through the roots. The mycelium invades the wood ducts profusely. Progress in the roots is slow; however, in above-ground parts the rate of spread, as already indicated, may be from three to six inches each day. The mycelium at times completely plugs the wood - ducts, thus accounting for the wilting, yellowing and death of affected plants.
As yet no experimental data are at hand on which to base reliable recommendations for the prevention of Blue Stem.
Lawrence, W. H. Bluestem of the black raspberry. Washington Agr. Exp. Sta. Bul. 108: 3 - 29. 1912.