Tracy ...
Roses that are susceptible to mildew seem to just get mildew when conditions are right. That said, where you plant your roses can make a difference for those that are not as prone to mildew.
One of your best defenses is to make sure that there is excellent air circulation around the plant.
You have your roses sited in a bed with other plants. It's hard to get good air circulation in that situation for a rose.
Another thing you might look at is whether or not cool, moist air pools in that area of your garden. That situation often encourages the development of mildew.
Carly is growing a rose that I grew as a tree rose in San Diego, 'Sweet Chariot'. When I had the rose out in front of my condo, it was always covered in mildew. I finally realized that cold air dropped off of the mesa above the culdesac in front of my condo and decided to move the rose, which was in a container to the back. In back of my condo, there was always a slight breeze, so the air never pooled in that area.
'Sweet Chariot' was always clean once I moved it to a site with good air circulation and had no disease problems. Same rose, but in a different location.
A healthy plant is always more disease resistant. So your cultural practices can also make a difference.
During my rose life, I have always looked at the factors which may cause disease and what I can do as a gardener to reduce disease conditions before I reached for a spray. That's just my style of gardening.
