Rose rosette disease is caused by an organism similar to a virus that’s transmitted by mites. Although you usually find it in weedy multiflora rose, all roses are susceptible. And it’s becoming more common in cultivated ones.
Symptoms include increased thorniness, small red or purple leaves and rapid, dense growth of the shoots in “witches brooms.”
Unfortunately, there’s no cure for rose rosette disease. If you think your rose has it, remove and send it away in the trash to prevent infecting others. Get rid of any nearby stands of multiflora rose, as well.