Answer: I don't think the problems with your rose bush and your rhubarb are related. It sounds as though a herd of hungry caterpillars found the rhubarb leaves irresistable. I'm sure they're gone by now - spinning cocoons so they can emerge as butterflies or moths. Since you can't find any culprits on your rhubarb leaves, there's really nothing you can do now. Why not harvest the leaves and attached rhubarb stems by pulling them up and away from the crown of the plant? You can still eat the rhubarb - you'd have cut the leaves off any way. Harvesting will encourage your plant to produce more leaves.
As for your roses, it sounds as though they have developed a case of black spot disease. Cut off the affected leaves so the disease won't spread to other healthy parts of your plants. There are sprays you can use to help prevent black spot. Check the garden section of your favorite store for black spot control options.
Best wishes with your garden!
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