Similar to plants and trees, hedges are not immune to diseases and pests. Some of the most common ones to watch out for include myrtle rust, psyllids, and aphids. Myrtle rust is an airborne fungus that can appear as yellow powdery spots on leaves while psyllids are insects that suck the sap on Lilly pillies leading to damaged and spoiled leaves. Aphids, on the other hand, feed on most plants and trees and can be spotted through browning leaves.