Tips for preventing rose diseases.
Plant roses that are know to be hardy and disease resistant. Plant the roses with proper spacing so that there is good air movement around the rosebushes, do a thinning pruning on roses that become very dense with foliage. Not alot is needed, just enough to allow air movement through the rosebushes. The proper air movement through and around a rosebush helps keep some diseases away as they have no calm, dark and moist areas to get going. I spray my rosebushes at 10 to 14 day intervals with an earth friendly fungicide called Green Cure at the cure rate and have not had any problems with powdery mildew or black spot for several years now. Spend time in your rose beds really looking them over, check the top and underside of leaves for insect damage or insect eggs. Catching a problem early on goes a long way to gaining control over it. Here is a link to an article on spacing of rosebushes for you: https://www.gardeningknowhow.com/ornamental/flowers/roses/spacing-roses.htm