Galls on Trees and Other Plants are Common and Need No Treatment

Galls on Trees are Common and Need No Treatment

Bill Sevier: April 26

Q: What are these blister-like things on the leaves of my oak tree? Will it harm them? P.T., Tulsa

A: The structure on your leaves is a common oak leaf gall and will not harm a mature tree.

Galls are abnormal growths on plants and come in many different shapes and colors. Several types of pests cause galls, including insects, mites, bacteria, fungi and viruses. These pests specialize and almost always cause galls on a specific plant type — oak galls on oaks and ash galls on ash trees and so forth.

Galls may affect any part of a plant but most commonly are found on stems and leaves. Any time you see a bizarre, abnormal growth on a plant — round, flat, fuzzy or irregular and of one of many different colors not belonging to the plant — it is safe to assume you have found a gall. There are many hundreds of gall types and have been the subject of much folk lore in the past.

Many types of oak galls, such as oak leaf gall, are produced by a tiny wasp about the size of a mosquito. In spring, the wasp lays eggs as the plant is putting on new growth. The egg-laying process produces a growth hormone, which stimulates the plant’s system to grow a protective shell. This shell is the gall, and once formed, it completely seals off the larvae, protecting them from predators and insecticides.

The galls almost never affect a tree’s growth and survival; most homeowners’ main concern is the appearance of the tree when it is heavily infested.

Some types of galls can be detrimental to fruit trees, and insecticidal sprays may be recommended as the wasp is laying eggs in early spring. However, spraying is not suggested for shade trees in the landscape. If the tree is small enough where one can reach the galls, they may be cut out. The infested leaves should be raked and placed in the trash, but otherwise, galls are best ignored.

One unique problem significant to homeowners, which is associated with oak leaf galls, is the “oak leaf itch mite.” There was an outbreak of this mite in the Midwest two years ago, and Tulsa had its share of the mites. The mite has a bite like a chigger, but the bite area tends to be a bit larger and may contain a pustule. As the name indicates, itching is a prominent feature of the bite.

Oak leaf itch mites are produced by a female mite laying eggs in a “marginal oak leaf gall,” where its larvae feed on the galls larvae and reproduce. They are then released by the many thousands. Because of their size, they are spread widely by wind and are able to pass through screens of windows and doors. At present, these mites have no solution other than letting nature control them, which seems to have happened. They may return in the future, but there is no predicting this.

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Garden tips

·        Lace bugs, aphids, spider mites, bagworms, etc. can start popping up in the landscape and garden later this month. Keep a close eye on all plants and use mechanical, cultural and biological control options first.

·        Remove any winter-damaged branches or plants that have not begun to grow. Prune spring-flowering plants as soon as they are finished blooming.

·        Proper watering of newly planted trees and shrubs often means the difference between success and replacement.

·        Do not spade or till wet soil; it will destroy the soils structure and eliminate air spaces which takes a very long time to recover.