I am not certain if this is a fungus or not as it doesn't appear to match the symptoms of rust, black spot or mildew fungus that I have read about. The blooms of my yellow knock out roses have a scaley appearance, almost as if the green surface of the leaves has peeled off, leaving a brown substructure . Blooming is currently vigorous . Any thoughts? A
Rose Bush Leaves Turning Brown and Scaley
Here is a link to an article about Scale Control as well for you:https://www.gardeningknowhow.com/plant-problems/pests/insects/control-plant-scale.htm Also Neem Oil treatments work very well in treating scale problems.
First of all make sure that your rosebushes are well hydrated (watered) and also check them to make sure they have not become too tight with foliage and canes. If the bushes have grown so tight that air cannot get through them and around them, the door is opened for fungus problems and other diseases to get started. Not to mention the hiding places provided for insects, mites and little slugs or worms that will cause problems. You may well have a scale problem. I would recommend treating the soils around the rosebushes with a product called Bayer Tree & Shrub Granules. This is a systemic product that will go up through the rosebushes in their natural juices such that even those insects that hide well will be eliminated. It also contains a bit of fertilizer that helps with overall bush health and vigor. I do spray all of my rosebushes with an earth friendly fungicide called Green Cure as well, at the Cure Rate noted on the label instructions. I have had great success with Green Cure preventing and curing fungus issues. Here is a link to a articles on rose diseases for you as well: https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-bush-diseases.htm