Raised Brown Mounds on the Backs of Leaves

Gall Basics

• Galls are the results of attacks by living organisms on the leaves. Insects and mites are often responsible for galls on leaves, but fungi, bacteria, viruses and nematodes can also cause the abnormal growths. Galls go through three stages: initiation, growth and maturation. Galls only occur during active growth, meaning they can't form after the leaves stop growing. Gallmakers, or insects or mites that cause galls, vary considerably and use the galls in different ways.

Types of Gallmakers

• Common gallmaker insects and mites include various aphids, mites, psyllids, midges and wasps. The plants form galls in response to either feeding or egg-laying activity by the gallmaker. These gallmakers often develop inside the galls, using the plant-generated structures as a form of protection from the elements, predators and parasites. Several galls often appear on each leaf in close proximity to each other.

Types of Galls

• Not all leaf galls look the same, although many are brown, grayish-brown or tan raised mounds on the upper or lower surface of the leaf. Different types of galls include blister galls, leaf galls, pocket galls, cone galls and nipple galls -- each term is representative of the gall's appearance. The galls may form on the petiole or veins of leaves, or on other areas of the leaf surface. Common brown galls on undersides of leaves include jumping oak galls, woolly oak leaf galls, which resemble tight mounds of brown wool, and globular hickory galls.

To Treat or Not to Treat

• Overall, galls are purely cosmetic and don't cause damage to your trees or ornamental plants. Treatment is not only unwarranted, but also often difficult because of timing issues. Before choosing any chemical treatment, you must first know which insect, mite, fungus, bacteria or virus is causing the galls. For insects and mites, the primary culprits of leaf galls, you must apply the chosen insecticide or miticide when the adults are laying eggs or the feeding stage is present -- whichever life stage is causing the galls. Once the plant has initiated gall formation, the gallmakers will be protected within the gall and insecticides will not reach them. Insecticide applications may also damage the beneficial parasites; a large number of parasitic wasps parasitize gallmakers inside the galls and naturally help keep numbers in check.

Other Explanations

• While the raised brown mounds you're seeing are likely galls, other explanations also exist. Several insects and spiders lay their eggs on leaves or otherwise attach them to leaves; the protected undersides of leaves are ideal areas for an egg sac or individual eggs. You may also be encountering small cocoons or pupating capsules of various arthropods.