Yard and Garden: What’s on Those Maple Leaves?

June 23, 2016, 9:42 am | Richard Jauron, Laura Sternweis

AMES, Iowa – As homeowners enjoy the shade from their maple trees, they may notice something strange on the leaves or branches. Horticulturists from Iowa State University Extension and Outreach answer questions about galls, fungi and scale that may appear on maples. 

The leaves on my maple tree have erect, spike-like growths on their upper leaf surfaces. Should I be concerned? 

Maple LeavesThe 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 1/5-inch long. They 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 extremely small mites that are only 1/125-inch long. Adult mites spend the winter under the bark and other protective places on the trees. In early spring, the adults move to 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 emerging leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity continues until mid-summer when it starts to decline. Adult mites leave the foliage in the fall and move to overwintering sites.  

Other galls that are commonly seen on maple leaves include the maple bladder gall (small, globular, wart-like, red or black growths on upper leaf surfaces), gouty vein gall (thickened, green or red, pouch-like swellings along leaf veins on lower leaf surfaces), and velvet or erineum galls (green to red, velvet-like 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. 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 or feasible.

There are small, white objects resembling kernels of popcorn on the branches of my silver maple tree. The tree also is dripping sap. What should I do? 

The small, white, popcorn-like objects are likely cottony maple scale. Cottony maple scale is an insect.  It is most commonly found on silver maple trees. However, it also can be found on other maples, oak, linden, hackberry, honey locust and other trees.  

In June, female scales begin to produce large, white, cottony egg sacs that may grow to the size of dimes (up to ½ inch in diameter). Large numbers of egg sacs look like popcorn strung along the branches and twigs.  

The eggs within the expanded egg sacs (up to 1,000 per sac) begin to hatch in early July. The new scale nymphs crawl to the undersides of leaves where they feed on sap from the leaf tissue. The scales grow to adulthood on the leaves and mate in August or September before the females return to twigs to spend the winter. There is one generation per year.  

Cottony maple scale insects excrete a clear, sticky substance called honeydew. The honeydew drops onto leaves on the lower portions of infested trees. It also drops onto plants or other objects (patio furniture, cars, driveways, etc.) beneath trees. Oftentimes, a sooty mold fungus colonizes the honeydew, resulting in a black sooty appearance on leaves, branches and other objects.  

Cottony maple scale usually causes little harm to healthy trees. Natural predators will control the infestation within one or two years. Treatment of cottony maple scale with insecticides may do more harm than good as the insecticide will kill their insect predators.  

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