Note the red and dark purple colour splotches, and general distortion. There are miticides available for the treatment of this pest. Destroying plants by burning is also a good option. North Carolina State University has a good discussion of Broad Mite Bacterial blackspot. Unsightly but rarely a major problem in the garden.
Copper is a useful preventive.
Collapse of the plant is rapid with this disease and irreversible. Grow the delphs elsewhere. This disease is often spread by dispersing infected soil via garden boots and implements. More specific advice for treatment of some of the above and other problems, may be found on our commercial growing and spray regimes pages.