Do you find that a rose that is resistant to one is more resistant to the other. Or is there little correlation?
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In my very limited experience I’ve noticed that roses marketed as disease resistant are prone to both but they don’t appear at the same time or in the same quantities. Some types may be a bit more resistant to powdery mildew or blackspot, but from what I’ve experienced it seems negligible. In the last year I have noticed that climatic conditions/plant stress affect and almost cause these diseases. When I lived in Cape Town with the coastal climate my Pure Magic suffered with powdery mildew, and my other roses had maybe a few leaves with it as-well, but now that I’ve moved to the less humid highveld here in Johannesburg the powdery mildew is totally gone, and with the winter all of my roses suddenly got blackspot. Maybe these diseases temp/humidity requirements don’t overlap and this makes certain types seem more resistant?
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I don’t believe there is a correlation.
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No, in these parts, the original permutations of Knock Out are highly spot resistant. They do, however rust and mildew with wild abandon. IMHO, if a rose resists any fungal attack it’s primarily because the fungus simply hasn’t tried hard enough…yet.