Thoughts On Powdery Mildew
When we moved to Pasadena from Connecticut in 2012 I was thrilled at the thought of never having to deal with black spot again but I soon discovered that SoCal has its' own scourge and that is powdery mildew. Most roses are susceptible to PM ranging from hardly at all to very vulnerable and in extreme cases the rose shrivels up. It can be as bad a problem as BS was back east.
How to deal with it? Here is what I have found out. First, I noticed that some areas of the gardens had a lot of PM and others none at all and that air circulation and amount of sun were probably the most important factors. The roses are bigger and thicker this year than last so there is more shade and less air circulation.
I also noticed that PM spreads from one rose to the next first appearing as spots on new growth and quickly covering infected areas. No single rose had it that didn't at least partially infect its' neighbors. I also saw that certain vulnerable roses were free of it in some areas and had it bad in others. For example, Oklahoma in one area was entirely free of it but Mister Lincoln which should be similarly vulnerable had it bad.
I've sprayed with Green Cure which is potassium bicarbonate and is absolutely safe and recommended for PM but it really doesn't help if the case is severe. It says on the label to spray at the first sign of PM which I didn't do but will in the future. If you are going to spray for PM you probably have to do it as a preventative as you have to for BS. Like BS, I doubt if you can do much about a severe infection of PM that is already present.
And then I found the answer (I think). I noticed that roses that were being doused every other day by the lawn sprinkler system had no sign of PM. Since the roses all are on drip systems most of them don't ever get wet except from the morning dew that doesn't always get dried off by the sun giving the PM spores a chance to take hold. Rain would wash them off but of course it almost never rains here. The spores which unlike BS seem to do their damage only on the outside of the rose spread from rose to rose needing only the moisture provided by the morning dew.
So the answer may be as simple as hosing roses down frequently something I know they enjoy anyway.. I'm hoping so. So far so good.