This is primarily a disease of the dewberry, although the cultivated blackberry is affected. The trouble occurs to some extent on the high - bush blackberry. Among the dewberries the Lucretia and Rathbone are notably susceptible, while the Black Diamond and other varieties are less liable to attack.
Double Blossom is most common in the Delaware-Maryland peninsula. It has been observed, however, in Illinois, Tennessee, Texas, New Jersey, North Carolina and Alabama. The first record of the disease comes from Illinois about 1885, although the important writings on the subject of Double Blossom are recent.
The history of the disease shows that it does not vary in abundance in a given region from year to year. But, as would be expected, it continues to increase in severity until affected plants are worthless. If diseased plants are allowed to go untreated, their life is shortened by two or more years. In the Delaware-Maryland peninsula Double Blossom has been so severe on Lucretias that half of the growers have discontinued dewberry culture.
Evidences of Double Blossom appear early in the spring just as the leaf-buds are opening. The trouble may be detected previous to this by the enlargement of the diseased buds. When affected leaf-buds open there is produced, in place of normal leaves and shoots, a Witches Broom. This abnormal development may consist of short, slender twigs. Sometimes one healthy shoot is found among diseased ones. Shoots of brooms will frequently remain green after old canes are dead. When flower-buds open, they display various malformations: sometimes the deformity is slight, again extensive, depending somewhat on the extent of infection. Blossoms are usually affected, the sepals and petals being thickened. Some growers erroneously regard these diseased blossoms as male flowers. At times the blossoms are increased in numbers; this is especially true of the petals. Diseased petals have the. appearance of being doubled, whence the name Double Blossom. Other parts of the flower may show abnormal growth, due to the double blossom pathogene. The stamens and ovaries are affected. It happens at times that certain flowers appear healthy, but such flowers may have diseased ovaries. Every bud in an affected plant may be diseased. Sometimes late blossoming is induced in plants attacked by the Double Blossom parasite. In such cases the blossoms are smaller than normal.
The abnormal bud-development just described is due to the fungus Fusarium Rubi. In the early spring its mycelium may be found between the parts of affected buds. With the advent of proper conditions growth is resumed. The mycelium does not pass from the bud into the stem. It does, however, enter certain parts of the bud. The ovaries are penetrated by way of the stigma and the style. An abundant growth of mycelium takes place within these organs, but the carpels and ovules are not penetrated. Neither are the stamens penetrated, although the hyphae may be found abundantly between them. Wherever the mycelium enters the tissues of a bud, haustoria are sent into the cells. Thus the fungus feeds on its host, stimulating the leaf-buds to Witches Broom formation, and dwarfing the ovaries of the flowers. This influence is also felt by other parts of the flower, as noted under Symptoms.
Within a few hours after the opening of a flower-bud, the ovaries become more or less whitish, due to the presence of mycelium and spores which develop abundantly thereon. Spores may be produced within forty-eight hours after a flower - bud opens. These are carried chiefly by the wind; it is believed that the clothing of man, tools of various kinds, and insects are also important carriers of the spores.
The spores perchance alight on young buds that are being formed for the following year. In the presence of moisture the spores germinate and produce mycelium which lies dormant in the buds throughout the winter. In the spring growth is again resumed.
Spraying has not proved to be a satisfactory means of combating this disease. The spores are produced at flowering and during the period of growth of the fruit. Spraying at blossoming might be objectionable, and any spraying done later in the summer must not result in injury to, nor discoloration of, the fruit.
Handpicking deformed buds seems to be satisfactory. The operation should begin as soon as the buds begin to open. As a precautionary measure these buds should be destroyed, preferably by fire. This method is very effective, on account of the fact that the fungus does not go beyond the bud. The grower cannot hope to completely eradicate the Double Blossom pathogene, yet the operation is said to be profitable.
In southern regions all canes are cut at the ground immediately after the crop is harvested. This is considered an important operation in dewberry-culture on account of its value in the control of anthracnose and Double Blossom.
Cook, M. T. The double blossom of the dewberry. Delaware Agr. Exp. Sta. Bul. 93:1 - 12. 1911.
Reimer, F. C, and Detjen, L. R. Double blossom of the dewberry and the blackberry. North Carolina Agr. Exp. Sta. Ann. Rept. 34: 41 - 50. 1912.
Cook, M. T. Some diseases of nursery stock. Double blossom of the dewberry. New Jersey Agr. Exp. Sta. Circ. 35: 24. 1914.
The shoe - string fungus attacks the blackberry among other plants. It is said that considerable injury is done in the Pacific Northwest. (See Apple, page 96.)