AMES — 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 firstname.lastname@example.org
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.