Delphiniums are striking border plants but can be attacked by black blotch, an unsightly disease that can affect all aerial parts of the plant.
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.
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
- 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
- There are no fungicides available for the treatment of black blotch
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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