Yard and Garden: Maple Trees

AMES, Iowa — Maple trees are a landscape staple valued for their shade and vibrant fall colors. Homeowners may notice growths, spots or sooty areas on the maple leaves during summer. Horticulturists with Iowa State University Extension and Outreach identify the leaf abnormalities and tell how to manage them. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

There are erect, hair-like growths on the upper leaf surface of my maple tree. Should I be concerned?

The hair-like growths are likely galls. Galls are abnormal growths of plant tissue induced to form by mites, insects or other small organisms. The hair-like gall on the maple leaves is probably the maple spindle gall. Maple spindle galls are yellowish green and about one-fifth inch long and are as thick as the lead in a pencil. The galls are somewhat thicker in the middle than at the ends, hence the common name of spindle gall.
Maple spindle galls are caused by tiny mites. Adult mites spend the winter under bark and other protective places on trees. In early spring the adults move to the developing, unfolding leaves and begin feeding. The leaf responds to the small irritation by rapidly producing extra cells that form the abnormal growth at the feeding site. The gall encloses the mite, which continues to feed and lay numerous eggs within the gall.
Reproduction is prolific and as the new mites mature, they leave the gall and move to other newly developing leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity continues until mid-summer when it starts to decline. In the fall, adult mites leave the foliage and move to overwintering sites.
Another gall commonly found on maple leaves is the maple bladder gall. Maple bladder galls are typically found on the upper leaf surface of silver and red maples. The roundish, wart-like growths are initially light green but quickly turn red and finally black. Other galls occasionally seen on maple foliage include the gouty vein gall, a green or red thickened swellings along leaf veins, and maple erineum gall, bright red velvety patches on the undersides of leaves. While galls may be unsightly, they do not cause serious harm to healthy, well-established trees. Galls cannot be “cured” once they have formed. Preventive insecticide treatments are seldom warranted.

There are black spots on my maple leaves. Is this a serious problem?

Tar spot is a common leaf spot on maples in the United States. Several fungi in the genus Rhytisma cause tar spot. Spots are black, slightly raised and up to 3/4 inch in diameter. The black spots resemble blobs of tar, hence the common name. Fortunately, tar spot does not cause serious harm to maple trees; the damage is mainly cosmetic.
The severity of tar spot can be reduced by raking and removing infected leaves from around the base of the maple tree in fall. In most cases, controlling tar spot with a fungicide is not practical.

The leaves on my maple tree are covered with a black, sooty material. What is it and is it harming the tree?

The black, sooty material is likely sooty mold. Sooty mold is caused by several different fungi. The fungi don’t infect plants, but grow on the sugary honeydew excreted by aphids, scales and other insects. In Iowa, sooty mold is most common on maple, pine, linden and elm trees. While sooty mold can reduce plant vigor by blocking sunlight and interfering with photosynthesis, the damage is mainly aesthetic. It is not necessary to control sooty mold as it does not cause serious harm to healthy, well established trees.  
It’s usually not necessary to control aphid and scale infestations on trees with insectides. Damage to healthy trees is seldom serious. Plus, natural enemies and weather usually provide adequate control of scale and aphids.