Clematis leaves turn yellow due to: fungal diseases such as rust and clematis wilt; insects such as scales, whiteflies and aphids; and environmental stress. Looking for additional symptoms aids in identifying the exact problem.
Clematis wilt affects the stem, causing the foliage to wilt, whereas rust affects the leaves, causing orange or yellowish spots to appear on them. Both whiteflies and aphids feed on clematis leaves, causing them to curl towards the stem, but scales feed on branches, leading to leaf wilting and leaf drop. Over-watering causes overall yellowing of the leaves, and under-watering causes browning of the leaf tips. Lastly, over-fertilization burns both the roots and leaves of the clematis.