Psyllids can be confused with aphids, and unfortunately, they are more difficult to contain. They feed on the sap from your plants, and they inject a toxic saliva that can cause leaves to turn yellow and veins to turn purple. Check under your leaves for nymphs, which are about the size of an aphid, though they later develop wings. While they're eating, they're still, and they secrete small, white granules that look sort of like sugar. To stop them, dust your leaves with sulfur, particularly the undersides.