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Saturday, April 4, 2015

Mildew on Roses Have Common Link

Q. My roses are covered with mildew.  I have sprayed them and hope they will recover.  I have not had this problem in years.  What causes mildew and how do you keep it from coming back?

Powdery mildew of roses, severe
A. Diseases develop on plants when the environment is just right for that disease, disease organisms come in contact with the plant and the plant must be susceptible to it.

Let’s break each of these factors down. Remember, when dealing with diseases it is most important to prevent diseases from happening rather than cure them with a pesticide after the fact.
Powdery mildew of roses, or powdery mildew of any plant for that matter, develops best in shady spots during cooler times of the year and when it is moist. It does not like to develop in full sun and when it is hot and dry.

We see powdery mildew during cool weather of spring and fall and times just after it rains. It is common when roses are grown in shady spots and watered with sprinklers.

Powdery mildew of weeds
Powdery mildew can be spread by splashing water. This can be from rain or sprinklers. It can also be spread by hands, pruning shears and wind.

Powdery mildew likes some roses better than others. The variety of rose and how healthy it is has a lot to do with how often you will see powdery mildew on it.

So if you want powdery mildew on roses then grow a variety susceptible to powdery mildew, don’t fertilize it very often, prune it incorrectly, place it in the shade and water it with sprinklers.

The flip side is best for disease free roses: select a variety resistant to powdery mildew, plant it in at least 6 to 8 hours of full sun each day, apply compost and wood mulch to the soil, keep it healthy with at least one application of a rose fertilizer every year and irrigate using drip irrigation.

These links may help you with some general information on powdery mildew.


  1. Could you please address the question. She DOES HAVE mildew on roses, she wants to know how to get rid of it. You have given an arrogant non-answer: 'You should have prevented it'. C'mon Bob, get off your high horse. Stop breaking down and answer the question.

    1. My philosophy on the use of pesticides is that they should be used as a last resort, not a first resort. If this person does not address the reasons why the disease occurred in the first place they will continually apply a pesticide whenever these environmental conditions occur again.

      You can go to any box store and pick up a fungicide for controlling powdery mildew on roses. What you will never get from a box store or chemical company is how to prevent the disease from occurring in the first place. The more sustainable way of controlling powdery mildew on roses is to prevent, not try to cure it after it is already started. I stand by my answer. And I ride a very short horse.