my roses have a gray powdery substance.
Powdery mildew is common on roses in our pacific northwest climate, though without a photo I'm not certain that is your problem.
OSU Extension has publications and articles about how to deal with this problem on roses. Note that choosing resistant roses is a repeated theme, and some homeowners choose to replace those that have repeated problems.
"Easy ways to fight the ‘big four’ foes of roses" http://extension.oregonstate.edu/gardening/2015/05/easy-ways-fight-%E2%80%98big-four%E2%80%99-foes-roses
"Controlling Diseases and Aphids on Your Roses", EC1520, https://catalog.extension.oregonstate.edu/ec1520
"Disease-resistant roses make gardening life easier", http://extension.oregonstate.edu/gardening/2017/02/disease-resistant-roses-make-gardening-life-easier