There are several diseases that can hinder blackberries, including Orange Rust, Anthracnose, Botrysphaeria, Botrytis, etc. The list can be long, or relatively short, depending on the area of the county where the blackberries are being grown. One common disease in the southern U.S. is Double Blossom aka Rosette (caused by the fungus Cercosporella rubi). Dr. Barbara Smith (USDA-ARS, Poplarville MS) has spent a great deal of time studying this particular disease. She writes on it extensively here (http://www.ars.usda.gov/SP2UserFiles/person/5276/BlackberryRosetteInformation.pdf). The control measures are to plant resistant cultivars (genetic control), eradicate the nearby host plants (wild blackberries), prune out infected plant tissue, apply appropriate fungicides (see link and your local county extension office), and, in severe cases, mow down the plants. Thorny cultivars tend to be the most susceptible and thornless types less susceptible, but variation exists within those categories. Essentially, rosette is a fungus that infects primocanes, but the symptoms are not manifested until the floricane begins to flower. Symptoms include bunching of growth, off-colors, and no fruit production. Below are some photos to help you identify the symptoms:
A less common disease also goes by a couple of different names — Orange Felt (or Orange Cane Blotch or Algal Cane Spot). Not a lot is known about this disease, but it is caused by a parasitic alga species (Cephaleuros virescens). It occurs primarily in hot, humid regions of the country. The overall ramifications of having this disease are largely unknown, but it may eventually girdle canes and cause decline or death. Dr. Phil Brannen of the University of Georgia provides and excellent summary here (http://www.smallfruits.org/Bramble/pestinformation/OrangeFelt.pdf). Below is a photo I took this afternoon.
As with any disease you wish to control, first consult in expert in the area for recommended fungicides. Also, read the label prior to application, because THE LABEL IS THE LAW! Sorry for the caps, but it cannot be said strongly enough.