Description and control of downy mildew on roses. Peronospora sparsa is the fungus that causes this type of rose mildew. Powdery mildew on roses is caused by Sphaerotheca pannosa.
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The leaf spots caused by Peronospora sparsa are sometimes confused with black spot but the circles are smaller and purple or brown in color.
Eventually, infected leaves will turn brown and crisp.
It is important that you know which mildew (downy or powdery) your rose is infected with as different fungicides are used to treat them.
The fungus that causes downy mildew on roses is plant specific and will not spread to other types of plants.
Like powdery mildew on roses, it is encouraged by cool, humid nights which follow warm days. 64 degrees F. is the temperature at which the disease is most likely to flare up.
Rake up all infected leaves and bag them. Never compost infected plant parts as this is a good way to spread the spores to other parts of the garden.
Copper based or dithiocarbamate fungicides provide good control when used regularly. So will sulfur dust. These are non-systemic products which need to be applied to the leaves and stems.
These products work better when used to prevent mildew than when used to cure an established infection. If the disease was present the season before, begin spraying against it soon after new growth begins in the spring. Reapply every 7-10 days or in accordance with the label on the product you are using.
Return to Preventing Common Rose Diseases
Preventing common rose diseases. Treatment of rose bush diseases. Rose plant diseases in Florida. Downy mildew, powdery mildew on roses, rose rust disease, black spot rose bush disease, rose rosette disease.
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