I am posting for my mom, who is seeing something odd on a few of her Knock-Outs.
A bud forms, grows, and before it opens it sends out leaves on the bud, then they dry and drop.
She writes: "Sometimes a branch on one of the knock-out rose bushes will get flower buds that look like they are going to open up, but they start getting covered with dark red, frilly, leaf-like growths. They never do open and eventually just turn rusty and dry up. Sometimes all the branches on the bush do this. Sometimes just one or two branches do it. The branch in the picture below was the only branch on the bush that was affected. Note the fading, normal looking blooms."