Maple Diseases & Insect Pests

Diseases

Leaf Scorch: On maple (Acer species) trees, a number of problems cause symptoms that are generally classified as leaf scorch. Scorch symptoms are light brown or tan dead areas between leaf veins or around the leaf margins. Occasionally the leaf margins are yellow or chlorotic. Scorch symptoms tell us that one or more of the following factors are affecting the tree:

• Physiological leaf scorch is the most common. It is caused by dry weather, combined with wind and high temperatures. When trees do not get enough water, they will scorch. The symptoms are on all parts of the tree or only on the side exposed to sun and wind. Scorching due to dry soil may be overcome by proper watering. Water established trees with one inch of irrigation once per week during periods of heat and drought.
• A response to an undesirable soil contaminant, such as salt (applied to roads for wintertime ice control), accumulation of fluorides in containerized soil mixes with perlite or high levels of phosphate fertilizers.
• A reaction to a vascular pathogen, either fungal or bacterial.

Anthracnose of maple (Aureobasidium apocryptum).
Photo by Paul Bachi, Bugwood.org

Anthracnose: This disease is caused by the fungus Aureobasidium apocryptum. Leaves and buds may turn brown and die, followed by twig and branch dieback. Infected trees may be deformed with crooked and angular branches or witches’ brooms (clusters of shoots growing from one area of a branch). This disease is most severe during wet weather.

Prevention & Treatment: The most effective control is to replant with resistant trees. Spraying three times at two-week intervals with a copper-based fungicide, mancozeb, chlorothalonil, or thiophanate methyl starting when the leaves begin to unfurl in spring will provide control (see Table 1 for specific products). Read and follow all directions on the label.

Bacterial leaf scorch of maple (Xylella fastidiosa).
Photo by John Hartman, Bugwood.org

Bacterial Leaf Scorch: This disease is caused by the bacterium Xylella fastidiosa. Symptoms often start out as marginal chlorosis or yellowing of the leaves before they fade to a drab green or brown color. Presumably, the symptoms develop because of water stress within the water-conducting vessels. Diseased trees lose vigor, and branches or entire trees may eventually die. The symptoms are most noticeable in late summer to early fall, following extended periods of drought. The bacteria are transmitted by leafhopper feeding, through roots grafts between trees, and vegetatively through propagation grafts and cuttings.

Prevention & Treatment: In general, practices that encourage root development and root function are recommended. Incorporate organic soil amendments into the soil to improve aeration and drainage of clay soils or to improve the water holding capacity of sandy soils. Irrigate during periods of drought. The disease has been suppressed by oxytetracycline injections but not cured. A certified arborist should be contacted if chemical control is needed.

Phyllosticta leaf spot of maple (Phyllosticta minima).
Photo by Paul Bachi, Bugwood.org

Phyllosticta Leaf Spot: This disease, also known as purple eye, is caused by the fungus Phyllosticta minima. On the leaves, spots appear with pale yellow centers and purple borders. The spots are irregularly round and ¼ inch in diameter. Black fruiting bodies of the fungus develop in a circle in the centers of the spots. These fruiting bodies occur in greater numbers on the upper leaf surface. Severe infection can result in partial defoliation of the tree. Often the disease goes unnoticed until leaf accumulation occurs under the tree.

Prevention & Treatment: The fungus survives the winter in fallen leaves. In the spring, spores are produced and dispersed to the new leaves of susceptible trees. Rake up all fallen leaves. Fertilization and watering may help to reduce the disease. ‘Autumn Flame’, ‘Tilford’, and ‘Gerling’ red maple varieties are relatively resistant to leaf spot. Spraying three times at two-week intervals with a copper-based fungicide, mancozeb, chlorothalonil, thiophanate methyl, or triadimefon starting when the leaves begin to unfurl in spring will provide control (see Table 1 for specific products). Read and follow all directions on the label.

Tar spot of maple (Rhytisma species).
Photo by Andrej Kunca, Bugwood.org

Tar Spot: Tar spot is caused by the fungus Rhytisma, where any of three species may be involved: R. acerinum, R. americanum, and R. punctatum. Spots arise in late spring or early summer after leaves attain full size. At first, the infected tissue is light green or yellow. Then, during late summer, raised, shining black, tar-like dots develop within the yellow spots on the upper leaf surfaces. The lower surface of a leaf beneath a large tar spot turns brown, but the surface beneath speckled tar spots remains yellow. Leaves with multiple spots may wither and drop prematurely but seldom so early or in such quantities that it threatens the health of the tree.

This disease is more common in the forest but may be seen in some landscape situations. Tar spots are among the showiest and least damaging foliar diseases.

Prevention & Treatment: The fungus survives the winter in fallen leaves. Rake up and discard the leaves in the fall. Spraying three times at two-week intervals with a copper-based fungicide, mancozeb, or triadimefon starting when the leaves begin to unfurl in spring will provide control (see Table 1 for specific products). Read and follow all directions on the label.

Insect Pests

Woolly alder aphid (Paraprociphilus tessellatus).
Bob Lepak, Bugwood.org

Aphids: Aphids are soft-bodied insects that range from ¹/₁₆ to ⅜ inch long. They may be green, yellowish, pink, gray, or black. They feed by piercing plant tissue and sucking plant sap. They prefer feeding on new growth in such areas as shoots, buds, and the undersides of leaves. As they feed on plant sap, they excrete honeydew (a sugary waste material). The sooty mold fungus grows on the honeydew, resulting in unsightly, dark fungal growth.

Woolly alder aphid (Paraprociphilus tessellatus) is gray to black in color. It gets its name from the fluffy, white wax found on its abdomen. It requires alder and silver maple to complete its life cycle. Occasionally, it is found on red maple. Colonies of these pests are obvious because of their white, fuzzy appearance. They are usually seen on leaves, twigs, or bark. Although infested leaves shrivel and drop early, the pests cause little permanent damage. As a result of the honeydew, sidewalks and cars become sticky.

Prevention & Treatment: Several natural enemies, such as ladybird beetles (ladybugs) and lacewings, feed on aphids. These predators should be allowed to reduce aphid populations as much as possible. Controlling this pest on a large tree using chemicals is expensive and often not practical. Since little permanent damage results from woolly alder aphids, tolerating some damage is a good choice. As a result of their phenomenal ability to reproduce, aphids are very difficult to control with insecticides. Leaving one aphid alive can result in the production of a new colony very quickly. In addition, the use of insecticides kills the beneficial insects that normally keep aphid populations under control.

However, if natural predators do not reduce aphid populations sufficiently, the following foliar spray insecticides are recommended: cyfluthrin, cyhalothrin, permethrin, bifenthrin, pyrethrin, and neem oil. Treat when aphids appear and repeat at seven- to 10-day intervals if needed. As an alternative, dinotefuran or imidacloprid can be applied as a drench around the root zone of aphid-infested plants and is systemically taken up by the root system for insect control (see Table 1 for specific products). As with all pesticides, read and follow all label directions and precautions.

Cottony maple leaf scale adults (Pulvinaria acericola).
Photos by Nancy Gregory, University of Delaware

Scale: Many scale species, including Pulvinaria acericola, Pulvinaria innumerabilis, and Melanaspis tenebricosa, are pests of maples. Scales are unusual insects in appearance. They are small and immobile, with no visible legs. Scales vary in appearance depending on age, sex, and species. Some are flat and appear like scales stuck to a plant, while others appear like white cottony masses. They feed on sap by piercing and sucking the leaf, stem, or branch with their mouthparts. Their feeding can weaken or kill branches. Heavily-infested trees are stunted with small flowers and leaves. Leaves may turn yellow and drop early.

Like aphids, soft scales, such as Pulvinaria species, also excrete honeydew. The growth of the sooty mold fungus on the honeydew results in leaves that are dark grayish-black. Armored scales, such as Melanaspis tenebricosa, do not excrete honeydew, as they feed differently than the soft scales.

Prevention & Treatment: A combination of various natural enemies, including ladybird beetles (ladybugs) and parasitic wasps, usually keeps scales under control. On small trees with light infestations, scales can be scraped off, or the infested branches can be removed and destroyed. On a large tree, it is not always practical to control scale chemically. The size of the tree, the need for specialized equipment, and the cost may prohibit this approach. Adult scales are relatively protected from insecticides by their waxy covering. Their immature forms, called crawlers, are susceptible, however. If it is determined that chemical control is necessary, the recommended chemicals include the following: cyfluthrin, cyhalothrin, bifenthrin, or permethrin. Apply materials when crawlers appear and repeat in 10 days. Both soft and armored scales can be controlled by a soil drench with dinotefuran. Soil drench applications are best made in the spring as new plant growth appears. See Table 1 for specific products. As with all pesticides, read and follow all label instructions and precautions.

Ocellate gall midge (Acericecis ocellaris).
Photo by Lacy L. Hyche

Gall Makers: Maples often develop irregular growths or swellings known as galls on their leaves. Gall development is a reaction by the leaf tissue to feeding or egg-laying by various mites (such as Vasates quadripedes and Vasates aceriscrumena) and insects (such as Acericecis ocellaris and Cecidomyia ocellaris). Galls vary greatly in appearance, from wart-like bumps to spindle-shaped protrusions to felt-like patches on the leaf’s surface. Each insect or mite produces its own distinctive gall shape. Often the distinctive shape allows for the identification of the pest. Galls typically develop in spring at about the time that leaves are expanding. Once the gall forms, the pest is protected inside the structure. When homeowners see these growths on the leaves of their maples, they often become quite concerned. It is important to remember that while unsightly, they do not cause permanent injury to a tree.

Prevention & Treatment: Since leaf galls do little, if any, long-term damage to the tree, control efforts are typically not needed or recommended. If a tree is small, the homeowner can handpick and destroy leaves before exit holes form to allow the release of the pest.

Granulate ambrosia beetle (Xylosandrus crassiusculus) larvae.
Photo by Will Hudson, University of Georgia

Asian Ambrosia Beetles: Japanese maples are among the more common hosts of the granulate ambrosia beetle (Xylosandrus crassiusculus), with other hosts including styrax, ornamental cherry (especially Yoshino), pecan, peach, plum, dogwood, persimmon, sweetgum, magnolia, fig, Chinese elm, and azalea. This pest is attracted not only to damaged, stressed, or transplanted trees but to seemingly healthy trees as well. The beetle becomes active in early March (or earlier), and the female beetles bore into trunks or branch wood of thin-barked hardwood trees. Once a tree has been attacked, it becomes more attractive to further attack. Often these trees are less than four inches in diameter.

Granulate ambrosia beetle (Xylosandrus crassiusculus) frass protruding from bark.
G.Keith Douce, University of Georgia, Bugwwod.org

Visible symptoms include wilted foliage, as well as the toothpick-like strands of boring dust (frass) that protrude from these small, pencil-lead size holes. The Asian ambrosia beetle does not feed upon the host’s wood but instead carries with it an ambrosia fungus, which grows within the galleries made by the beetle. This fungus serves as a source of food and may partially be responsible for the death of the host plant.

Prevention & Treatment: Heavily infested plants should be removed. If only a few branches are infested, they may be cut out. The life cycle takes approximately 55 days until the emergence of the next generation of beetles, so prompt removal or burning of the wood is important. Protective sprays on other susceptible plants may reduce their spread. Permethrin may be used as a trunk and scaffold limb spray beginning in March (see Table 1 for specific products). Thoroughly wet the bark. Multiple treatments may be needed during a season. Research indicates that spraying the infested trunks with permethrin may cause the beetles to leave the galleries they have already created. Since the beetles do not consume the host plant material, dinotefuran and imidacloprid systemic soil treatments are ineffective.

Other Problems

Girdling Roots: If a tree shows symptoms of poor vigor such as small leaves, death of small limbs, top dieback, or leaf scorch, the condition could be due to girdling roots. This problem occurs when a root entwines around another large root or the base of the tree and prevents or hinders water and nutrient movement. Often girdling roots occur below ground level, indicated by a lack of root flare at the base of the trunk.

Prevention & Treatment: The portion of the root that is girdling the tree should be removed. The open wound can be treated with wound paint prior to covering with soil. Fertilization of the tree after root removal will aid in recovery.

Table 1. Insecticides & Fungicides for Maple Insect Pest & Disease Control.

Note: Pollinating insects, such as honey bees and bumblebees, can be adversely affected by the use of pesticides. Avoid the use of spray pesticides (both insecticides and fungicides), as well as soil-applied, systemic insecticides unless absolutely necessary. If spraying is required, always spray late in the evening to reduce the direct impact on pollinating insects. Always employ cultural controls first, then use less toxic alternative sprays for the control of insect pests and diseases. For example, sprays with insecticidal soap, horticultural oil, neem oil extract, spinosad, Bacillus thuringiensis (B.t.), or botanical oils can help control many small insect pests and mites that affect garden and landscape plants. Neem oil extract or botanical oil sprays may also reduce plant damage by repelling many insect pests. If soil-applied insecticides are used, make applications immediately after flowering to reduce the amount of insecticide exposure to pollinating insects. For more information, contact the Clemson Home & Garden Information Center.

Pesticides are updated annually. Last updates were done on 8/24 by N. Jordan Franklin.

Originally published 01/01