Thornless blackberry is a potential alternative crop for Central Florida. There are several pest problems a potential grower will encounter, especially diseases because of the warm, wet, humid conditions we experience. Profitable yields will not be achieved in the long run without control of these diseases. The Southeast Regional Caneberries Integrated Management Guide provides control measures for all pests. Some of the most common diseases are below.
The disease I have seen most often affecting blackberries is Orange Felt, a parasitic alga. The advanced stage symptoms are fuzzy orange growth on the canes as seen at the top of the post. Very hot and humid conditions favor the growth of this alga that will girdle canes or stress the canes so that other diseases are made worse. Various practices can reduce the incidence of this disease: pruning to open up the canopy and permit faster leaf drying, a weed-free strip or plastic mulch under the canopy, avoiding plant stress, and avoiding poorly drained sites. Even with all these practices you are likely to see this disease, so be ready. Copper products do not show consistent or sufficient management of this disease. Phosphonate fungicides are the only ones which consistently suppress this disease on blackberries.
This fungal disease, Leptosphaeria coniothyrium, also causes stem canker on roses and other ornamentals. The fungus overwinters on dead tissue like old floricanes or pruning debris. Spores produced spring through fall then infect wounded primocanes with the assistance of rainfall or irrigation. The pathogen grows in the primocane through the season and results in floricane bud failure and dieback in the spring. Watch for cane lesions that are dark red to purple with irregular purple borders. The fruiting structures look like small, black, pimple-like bumps buried in the tissue. Practices to reduce this disease include: avoid wounding primocanes, prune when at least four days of dry weather is expected, and pinch off the tips of primocanes rather than making severe cuts with shears. This may require more frequent pruning of primocanes, but the smaller wounds heal more quickly. Also remove infected canes and all old floricanes after harvest each year, cutting as close to the ground as possible and removing debris from the field. Practices similar to the ones to avoid Orange Felt are also useful. Apply fungicides after pruning to provide a protective barrier on wounds. More information.
Various fungal leaf spots, Cercospora, Pseudocercospora, and Septoria, attack blackberry. Be on the watch for these and apply fungicides to protect as soon as you see signs. See The Southeast Regional Caneberries Integrated Management Guide for more control information.