Answer: Without more description, I can't recommend a remedy. Roses in the low desert seldom suffer from fungal diseases the way they do in more humid areas of the country. Powdery mildew is the most common problem here. Powdery mildew appears as a whitish/greyish powder on foliage. It overwinters on living plants, and can be difficult to eradicate. There are some things you can do to minimize its effects.
Powdery mildew is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather. The first line of defense is to grow resistant varieties.
Here are some general rules for helping control fungal diseases. Start by making sure that your plants are getting enough direct sunlight. (Eight to ten hours a day is generally the minimum for plants that flower or bear fruit.)
You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread.
The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, you can actually inhibit infection with periodic strong sprays of water (not so strong as to damage the
plant.) After the new growth of affected plants reaches about six inches long and UNTIL THE TEMPERATURE REACHES 90 degrees F, you may apply dusting sulfur to foliage every 14 days. So, it is too late to use that remedy now. If this doesn't describe the problem you are experiencing, send another email to the Q&A site with more detail, so we can help you determine what it is.
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