Advice
Black blotch of Delphinium (Pseudomonas syringae pv. delphinii)
Symptoms
Irregular, tar-like spots, especially on the upper leaf surfaces, but also on stems and flowers. On the undersides of the leaves the spots are browner. The spots may coalesce and the leaves slowly turn black and then die.
Other plants affected
Aconitum napellus (monkshood)
Cause
A bacterium is spread from old infected plant debris in the soil by water splash to infect plants through the natural openings on the lower leaf surface (stomata).
Non-chemical control
Destroy affected delphiniums and replace with new stock on a fresh site.
Chemical control
Copper-based fungicides can give control of bacterial diseases, and are labelled for some specific diseases in the UK, but not for black blotch of delphinium.