Powdery Mildew. Here's what I posted on a different thread. :)
Did some quick research on powdery mildew. Here's why watering makes the white go away:
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http://scvrs.homestead.com/Mar...
Powdery mildew affects roses grown in dry climates and is common in much of California, including the Santa Clarita Valley. Caused by Sphaerotheca pannosa var. rosae, it forms a white, powdery film on the leaves, stems, and flowers of infected roses. Young peduncles, sepals, petals, and stems become twisted and distorted and new growth buds and growing tips may be killed.
Powdery mildew is common in the spring and fall when warm days are followed by cool, damp nights. Spore maturation and release usually occurs during the day when the relative humidity is low. At night when temperatures drop and the humidity increases, spores germinate and the fungus penetrates the plant's surfaces. Spores need low humidity, warm temperatures, and dry leaves to mature and be released, hence the advice to hose off the leaves of infected plants during the day. The only caveat being that the plant must be dry by nightfall.
Again, prevention is the best cure:
Plant resistant rose varieties
Plant them far enough apart to allow lots of air circulation
Plant in full sun
After pruning, strip the leaves from your roses and discard to the trash
After pruning, use a dormant season horticultural oil spray
During optimal growth conditions, wash all surfaces of the plant with hose water.
Do so early enough in the day that the leaves dry out before nightfall.
One of the easiest ways to control powdery mildew is to give your roses a thorough wash at least two mornings a week. A morning bath raises the humidity so spores can't mature. Free water left on the plants also discourages spore maturation. Plants must be dry before nightfall so residual moisture does not provide favorable conditions for spore germination.
Powdery mildew is attracted to lush, new growth. Rosarians growing for spring shows feed their plants heavily early in the year in order to have roses ready to cut. If you are not pushing your plants for a show, try cutting back or postponing the early spring feeding.
