Rosette or Double-Blossom

A fungal (maybe, bacterial) disease that prevents normal blackberry growth

Rosette, also known as double-blossom or witch's broom, is another fungal (or phytoplasma bacterial as another version) disease that prevents normal fruit development. Both wild and cultivated blackberries are susceptible. Observations made at the Overton Research and Extension Center indicate varieties differ in their susceptibility to rosette. Humble has high resistance, whereas Comanche and Cheyenne are very susceptible. Cherokee, Bryson and Womack are moderately susceptible. Brazos and Rosborough are moderately resistant. Varietal resistance may not be permanent.
Symptoms of rosette are short, broom-like clusters of foliage arising from infected canes. Flower buds on infected canes may appear somewhat larger and redder than normal. At flowering, petals may appear wrinkled or distorted, and sepals of the bract much elongated. Blossoms on infected canes, or blossoms recently infected by aerial spores of the fungus, fail to make fruit. On damp mornings, close examination will reveal a network of light gray fungal growth on affected flower parts.
Control of rosette requires removing affected canes as soon as they become noticeable. Destroy all wild berry plants in the vicinity. Remove and burn all fruiting canes soon after harvest and keep plants adequately spaced for good air circulation. Where heavy infection has occurred, mowing all canes to the ground may be necessary. Do this soon after harvest to allow production of new canes for the next growing season. Fungicide sprays for anthracnose treatment can reduce rosette infection in wet areas. Pristine with Switch, Benlate or Mildue Cure (organic), a fungicides used for fruit rots and powdery mildew, has also been effective in controlling rosette if sprayed regularly during the bloom period.