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Delphiniums are striking border plants but can be attacked by black blotch, an unsightly disease that can affect all aerial parts of the plant.
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:
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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