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Archive - September 2021

What are Plant Galls?
Plant galls are abnormal growths on the outside of a plant.

What Causes Plants to Form Galls?
Galls can be caused by viruses, bacteria, fungi, insects, mites and other plants.

Some insects can make a plant produce a gall so that they can live in it, receiving protection and food from the plant. The larval form of the insect may inject chemicals into the plant and/or the damage from the insect larva eating the plant causes the growth to form. 

This process happens when the plant is growing quickly, particularly in spring. Once the insect has reached the adult stage, it leaves the plant. You can often see an exit hole from a gall that is caused by an insect. Some of the types of insects that cause galls to form are gall wasps, gall midges, gall flies and aphids. Gall insects often attack only a certain type or types of plant and may only attack a specific place on the plant.

Cedar-apple rust (Gymnosporangium spp.) is a fungus that causes galls on cedar trees and members of the plant group that contains apple trees, pear trees, hawthorn trees and their relatives. The fungus works in an alternating pattern between the two groups of trees and must have a host from each group to complete its life cycle. Cedar trees are usually not harmed by the galls, but damage can occur to the fruit and leaves of the other trees in the cycle.

Small roundworms in the soil can cause galls on the roots of plants.

Bacteria can cause galls on walnut, peach, cherry, plum, apricot and other related trees.

Mistletoe (Phoradendron leucarpum) is a parasitic plant that can cause galls on its host.

Where Do You find Plant Galls?
Galls can form on stems, leaves, buds, roots, fruits and flowers and are found on plants throughout the state.

What Else Should I Know about Them?
Some galls have been used throughout history for making ink.

Most galls do little or no damage to the plant.

Oak trees are host to more than 500 different wasps, aphids, mites and midges that cause galls on leaves and twigs.

What Do Plant Galls Look Like?
Plant galls take many different shapes and sizes. You can see many different plant galls from Illinois on this document produced by the Field Museum.

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