091113-trees-02 swamp white oak two

Twelve oak trees are native to Iowa. This is the bark of a swamp white oak in Vander Veer Botanical Park, Davenport.

Oak trees provide shelter, food and nesting cover for a variety of wildlife as well as shade and beauty for urban landscapes.

Here are some questions about oak trees with answers from horticulturists with Iowa State University Extension and Outreach. To have additional questions answered, contact Hortline at hortline@iastate.edu or 515-294-3108.

Q: Why are the leaves on my pin oak yellow-green?

A: In Iowa and Illinois, the foliage of the pin oak (Quercus palustris) often turns a sickly yellow-green because of an iron deficiency, a problem called iron chlorosis.

Most soils in Iowa and Illinois contain sufficient amounts of iron, but in alkaline soils (those with a pH above 7.0), pin oaks are unable to absorb adequate amounts. Because many soils in Iowa and Illinois are slightly alkaline, chlorotic pin oaks are rather common.

Correcting an iron chlorosis problem is difficult. Applying additional iron to the soil usually does not help because the soil already contains sufficient amounts of iron. The problem is absorbing it. Lowering the soil pH to 6.0 to 6.5 would allow the roots of the pin oak to more readily absorb iron in the soil. Unfortunately, lowering the soil pH is extremely difficult, if not impossible.

An injection of an iron-containing compound directly into the trunks of chlorotic trees by professional arborists is often effective. The effects of a trunk injection may last two or three years.

Q: There are brown leafy twigs scattered throughout the canopy of my red oak. What is the problem?

A: Botryosphaeria twig canker, a fungal disease, is probably responsible. It impairs the flow of water and nutrients in the branch and eventually girdles the twig, causing it to wilt and die. Dieback usually extends only 4 to 6 inches inward from the branch tips. The dead twigs typically appear in mid to late summer and are scattered throughout the tree.

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Botryosphaeria twig canker is most commonly seen in red and pin oaks, but also occurs in others. In general, it causes minimal damage to otherwise healthy oak trees. Sound cultural practices, which promote tree health, are the best defense against Botryosphaeria twig canker.

Another possible cause for the problem are two long-horned beetles called the twig pruner and twig girdler. Branch tips destroyed by these beetles eventually break off and fall from trees.

In contrast, the dead twigs produced by Botryosphaeria twig canker remain in the tree through fall.

Botryosphaeria twig canker differs from oak wilt in that only the tips of branches are affected. Oak wilt affects whole branches.

Q: There are round growths on several leaves on my oak tree. What are they?

A: These are likely oak apple galls, abnormal growths of plant tissue induced to form by mites, insects or other gallmakers. The gallmaker responsible for oak apple galls is a small wasp.

Oak apple galls are green, up to 2 inches in diameter, and are named for their resemblance to small apples. Female wasps inject a gall-inducing chemical into plant tissue along with her eggs. The resulting galls provide protective homes and food for the wasp larvae. The galls eventually become light brown upon the emergence of the adult wasps.

Oaks are hosts to numerous types of galls. The galls vary in size, shape and color. They may be found on leaves, leaf petioles, twigs and branches. Wasps, midges and other gallmakers are responsible for the various galls.

Fortunately, most galls do not cause serious harm to healthy trees. Galls cannot be “cured” after they have formed. Preventive insecticide treatments are seldom warranted and usually ineffective.

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