Delphinium black blotch

Delphiniums are striking border plants but can be attacked by black blotch which is an unsightly disease that can affect all aerial parts of delphinium.

Delphinium black blotch

Quick facts

Common name Black blotch of Delphinium
Scientific name Pseudomonas syringae pv. delphinii
Plants affected Delphinium species, monkshood (Aconitum napellus)
Main symptoms Dark blotches on leaves, stems and flower parts
Caused by Bacterium
Timing During wet summers

What is delphinium black blotch?

Black blotch is a bacterial disease that can affect Delphinium species and monkshood. It is not fatal, but reduces the quality and vigour of affected plants. Wet summer weather favours disease development.

Symptoms

You may see the following symptoms:

  • Rounded to irregular brown-black blotches, sometimes quite shiny, develop on leaf surfaces (particularly the upper surface)
  • The blotches may enlarge or merge together to affect a large surface area, in which case the leaf may turn brown and shrivel
  • Leaf stalks, stems and flowers may also be affected
  • Heavily infected plants are stunted and lack vigour

Control

Non-chemical control

  • Dispose of affected plants as soon as symptoms are seen
  • Do not save seed from affected plants
  • If replacing diseased plants with new stock, plant on a fresh site
  • Do not purchase plants with any signs of leaf blotching

Chemical control

  • There are no fungicides available for the treatment of black blotch

Biology

The bacterium Pseudomonas syringae exists as a number of different strains, known as pathovars and abbreviated to 'pv'. Most of these strains, including P. syringae pv. delphinii, have a restricted host range.

Black blotch of Delphinium has not been studied extensively, but it is thought that the bacterium can overwinter on old plant debris in the soil. Bacteria are then splashed up by rain to affect the current season's foliage through the air pores (stomata) on the lower leaf surface. Prolonged spells of wet weather will allow further spread of the disease over the plant.

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