Answer: Hardenbergia, the lilac vine, isn't prone to any particular disease, but it seems to be a favorite of spider mites. If what you're finding is simply attached to the leaf, it may be a pouch of insect or spider eggs. Many insects attach their eggs to the undersides of leaves and cover them with protective material so they'll survive the winter season. You can leave them alone, peel them off, or squish them to kill the eggs inside. On the other hand, if what you're finding is a blister or swelling of leaf tissue, it may be caused by gall-making wasps or mites. Since we don't know for sure what's causing the problem, it might be best to just pick off the affected foliage and then watch your plant carefully for signs of insect activity when the weather warms this spring. Gall-making wasps are very small (about mosquito size). Spider mites, the other gall causing insect, generally feed on the undersides of leaves, producing a stippled effect, and they tend to leave webbing between the leaves and the stems of plants. Look for signs of webbing, stippling of the leaves, or tiny little insects on the undersides of leaves. Spider mites can be discouraged by a strong stream of water directed onto the undersides of leaves.
Best wishes with your lilac vine!
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