Answer: What you describe sounds more like leaf-miner damage than a disease. The adult is a flylike insect and the female lays her eggs just beneath the surface of a leaf. The eggs hatch into wormlike larvae that feed between the upper and lower surfaces of the leaf. When they're mature they exit the leaf by spinning a little web that transports them down to the soil surface. That's where they pupate and emerge as adults to repeat the cycle. The larvae are hard to control because they're protected within the leaf. Your best course of action would be to pick off the affected leaves before the larvae have a chance to mature, and add a few inches of mulch over the soil to discourage the adults from emerging. Best wishes with your clematis!
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