2007 Year in Review

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Galls on Leaves

Bumps or galls are being spotted on leaves, twigs, roots and flowers of many plants. Plant galls often appear as balls, knobs, lumps, or warts of various shapes, sizes and colors. Galls develop from plant tissues that have been irritated and/or stimulated by a parasitic organism, states David Robson, University of Illinois Extension horticulture educator. The growth is similar to a cancer-like growth, with cells rapidly developing in a variety of shapes or forms.

Galls are believed to be caused by powerful plant-growth regulating chemicals produced by the parasite. If the parasite leaves the host or dies, normal cells are again produced. Aphids, midges, wasps or mites cause about 95 percent of the known types of galls. Bacteria, fungi and viruses cause the remaining 5 percent.

One of the most common galls in central Illinois is the maple bladder gall. The small wart-like growths can easily be spotted on silver (soft) maple. Galls are first green, then turn red and finally black.

Hackberry leaf gall forms a tiny nipple on the upper surface of hackberry leaves. So common is this gall, many people believe it is a natural part of hackberry leaves. The gall is first green and then turns a purplish-brown.

A dozen different galls can be found on oak and hickory trees. Hickory galls are usually caused by aphids, while oak galls are caused by wasps. Although the galls are common and easy to see, they are seldom important enough to control. However, there are times when galls can be serious problems, but on stems and twigs instead of leaves.

Gouty and horned oak galls occur on the branches of pin and shingle oaks, as well as a few others, though you seldom find the galls on white oaks and bur oaks. These galls do start out as leaf galls, but a secondary stage starts to invade the twigs and limbs. When that happens, knobs develop, that can be smooth (gouty) or knotted. As the gall grows, it girdles the end of the branches which can cause severe dieback to the trees.

Unfortunately, there is no easy control of these galls. Hand pruning is needed, but may be impractical due to the quantity and size of the tree. Improving the over-all health of the tree is critical with regular fertilizing and watering. In severe cases, the galls may weaken the trees to the point where it’s better to remove the tree. Raking up leaves in the fall and composting (or burning where allowed) might reduce the populations somewhat.

In the other situations, general, insect- and mite-produced galls disfigure twigs and foliage but rarely seriously affect the health and vigor of the host plant. Once gall formations become noticeable, the insect or mite causing the injury is protected from chemical sprays. It is too late to exercise any control other than removing the infested part. Hand removal is only practical when there are a few galls present. Chemical sprays are seldom warranted where gall infestations are light.

Plants with heavy infestation or those that are tremendously unsightly to homeowners can be treated. Spray treatments can be used to prevent future gall formation, but must be correctly timed to eradicate the pest once it emerges and before it has had time to induce the gall. Sprays applied after galls are formed are ineffective.

Some gall producing insect populations can be reduced with a dormant oil spray applied to trunk and branches in early spring, just before buds open. Some insecticides will control gall formation on elm, hickory, and maple leaves when thoroughly applied to foliage as buds are unfolding. To control most galls, foliage should be thoroughly sprayed before the leaves are fully expanded. It is nearly impossible to treat large established trees. This is mainly an aesthetic problem.

Contact your local Extension unit office for the current recommended controls. Make sure to read and follow label directions when applying any chemical.

by  Editor, theCity1.com
May 21, 2007

 

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