Answer: If the rose leaves have a powdery coating, I'd suspect powdery mildew is the cause of your rose troubles. Powdery mildew 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 control. Start by making sure that your plants are getting enough direct sunlight. (Eight to ten hours a day is generally the minimum.) 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.) Good cultural practices and spraying with water is about the only organic method for controlling powdery mildew. Spraying with triforine (Ortho Funginex) will help stop the spread of the disease.
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