Violet Bramble Rust, this rust fungus thrives on blackberries in wet and cool summers.
Also known as:
Violet Bramble Rust (Phragmidium violaceum). This rust fungus thrives in wet and cool summers. The fungus overwinters on affected leaves.
Violet Bramble Rust (Phragmidium violaceum) is related to Rose rust (Phragmidium mucronatum).
Remove infested leaves and dispose of them (not in the compost pile).