I hybridize for rust resistant plants, but have used 1 rusty parent in a cross in hopes of capturing good features that plant has; rusty seedlings will be culled.
Last winter we had enough below freezing days to clear rust out for awhile here in zone 9a, but it now has come back as a different strain, I think.
Here rust flourishes in temperatures below about 82 degrees. When it starts to get hot here, rust subsides, so it is only an issue in spring and fall. Fall rust isn't as bad because humidity is usually not as high as in spring.
I got tired of spraying so no longer do it. I'm sorting through plants to keep only rust resistant ones.
When plants are moved, they are stressed and are more prone to show rust. I'm currently seeing some results from using mirate of potash around plants that tend to rust. This of course, is not a scientific study.
