Downy mildew of pansies is a fungal disease causing pale blotches and fuzzy grey mould on leaves, leading to these patches, and eventually the whole leaf, dying. It is particularly a problem on winter-flowering pansies in cool, wet weather.
What is downy mildew?
Downy mildew is a disease of pansies caused by the fungus-like organism Peronospora violae.
This fungus only attacks pansies, particularly winter-flowering types in cool, wet weather.
You may see the following symptoms:
- On leaves: Pale blotches on the upper leaf surfaces and corresponding patches of fuzzy grey growth on the underside. The margins of severely affected leaves may curl under, and these leaves often shrivel and die, sometimes, becoming colonised by the grey mould fungus Botrytis cinerea
- On flowers: Severe infections lead to stunting and a reduction in the number of flowers
Pansies can also suffer from other leaf problems, particularly a fungal disease known as pansy black spot which causes black discolouration on leaves rather than the pale blotches caused by downy mildew.
Fuzzy grey growth on the underside of the leaves.
- Remove infected leaves as soon as the discolouration on the upper surface is seen
- Do not allow infected leaves to contaminate the soil
- Do not grow pansies on the same site in consecutive years
No fungicides are available to gardeners to control pansy downy mildew. The fungicides available for other problems on ornamental plants, such as rusts and powdery mildews, are ineffective in controlling downy mildews.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Downy mildews are a large group of plant diseases caused by microscopic fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.
Like other downy mildews, Peronospora violae is described as a biotroph; a pathogen that penetrates into host plant tissues over an extended period without killing them, while it extracts nutrients from the living host cells. During this period it releases airborne spores from the fuzzy fungus-like growth on the underside of leaves, which disperse to initiate new infections.
When the infected leaf tissues eventually die, it forms resting spores in the dead material which will then contaminate the soil. The pathogen can not grow in the absence of living plant cells.
There is little specific information on Peronospora violae, but it is likely that infections from resting spores are responsible for damage if pansies are replanted on sites previously affected. Infections are also caused by spores blowing in from other plants, possibly 1-2km (½-1 mile) away.
There is also a risk that commercially raised plants may have been treated with fungicides, which suppress the symptoms but do not kill the pathogen, resulting in the disease breaking out after purchase when the effect of these fungicides eventually wears off.