Sycamore Diseases & Insect Pests

Prepared by Marjan Kluepfel and J. McLeod Scott, HGIC Information Specialists, James H. Blake, Extension Plant Pathologist; and Clyde S. Gorsuch, Extension Entomologist. Revised by J. McLeod Scott, HGIC Horticulture Extension Agent, Clemson University. (New 01/01. Images added 03/09. Revised 05/09.)

HGIC 2011

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Of the 10 species of sycamore (Platanus species), one species, American sycamore (Platanus occidentalis) and one hybrid, London plane tree (Platanus x acerifolia) are common in South Carolina. Both of these trees are well adapted to all areas of the state; however, they are susceptible to some diseases and insect pests. As always, the first line of defense against pest problems is to follow recommended cultural practices in order to maintain healthy plants. The healthier a plant is, the better it is able to withstand disease and insect pests. For information on cultural practices that favor the growth of healthy sycamores, see HGIC 1022, Sycamore.

Diseases

Anthracnose (Leaf & Twig Blight): Anthracnose is the most serious disease of American sycamore, but generally causes only minor damage to the London plane tree. It is caused by the fungus Apiognomonia venata (asexual stage - Discula platani). This disease may appear as four distinct phases:

Twig Blight: This phase occurs in the spring before leaf emergence, killing tips of small, 1-year-old twigs.
Bud Blight: This phase occurs in April and May. The expanding buds die because of the girdling action of the canker on the branch.
Shoot Blight: New shoots and immature leaves on infected branches suddenly die.
Leaf Blight: The most characteristic symptoms are crinkling and browning of the leaves. Entire leaves may be killed and then fall. These symptoms are very similar to those caused by late spring frost injury.

Sycamore anthracnose symptom on leaves and twigs
Sycamore anthracnose symptoms on leaves and twigs.
Robert L. Anderson, USDA Forest Service, Bugwood.org

Sycamore anthracnose symptom on leaf
Sycamore anthracnose symptoms on leaf
Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Sycamore anthracnose is favored by cool, wet weather (temperatures under 60 °F) during budbreak and the few weeks of growth that follow. A severely infected tree may be completely defoliated (lose all of its leaves) multiple times in a single season. This defoliation will not kill an otherwise healthy tree unless it occurs repeatedly over several years. When a small twig or branch is affected, cankers may girdle and eventually kill the branch. The death of a branch will often result in the production of many small shoots in the area just below the girdled portion of the branch, giving that part of the tree a bushy appearance that is referred to as “witches broom”. Spores (reproductive structures) from the fungus are spread by rain and wind to healthy leaves, buds and twigs. The fungus survives the winter on fallen leaves and twigs as well as on cankers present on twigs that remain on the tree.

Prevention & Treatment: Apply adequate fertilizer and water to maintain the vigor of the tree(s) as per recommendations in HGIC 1000, Fertilizing Trees & Shrubs. When planting a new tree, select a cultivar that exhibits resistance. ‘Bloodgood,’ ‘Columbia’ and ‘Liberty’ (London plane tree cultivars) are moderately resistant to anthracnose. To limit the number of spores that are present to start new infections, remove and destroy fallen leaves and twigs. In addition, prune out and destroy diseased branches. Care should be taken to disinfect pruning shears in rubbing alcohol or 10% bleach solution after each pruning cut. For small trees, spring applications of chlorothalonil, thiophanate-methyl or copper fungicides may protect the tree from the disease. It is often impractical to spray large trees. Read and follow all directions on the label. For valuable trees, certified arborists can perform root flare injections using thiabendazole (Arbortect Macro-Infusion Fungicide), which may aid in tree protection for three years.

Powdery Mildew: Microsphaera species cause powdery mildew disease on sycamores. This disease is common on plants growing in shady areas with high humidity. Powdery mildew typically begins as circular or irregular patches of white to gray powdery material (spores and threadlike strands of the fungus) on leaves and twigs. These patches may coalesce (grow together) to form a thin powdery layer on the plant surface. Typically, young shoots and leaves are more severely affected than old parts of a tree. Infected leaves become distorted, and may turn yellow and drop. New growth is often stunted. In late summer, small, pinhead-sized black structures may develop on the whitish gray patches. These structures are for surviving the winter. The following spring, they produce a second kind of spore that causes more infection.

Prevention & Treatment: For large trees, it is usually not practical to spray, and control is often not warranted. When spraying is feasible and necessary, several fungicides (myclobutanil*, triforine*, thiophanate methyl*, propiconazole, triadimefon, chlorothalonil, sulfur, copper fungicides, and horticultural oil) are available. Read and follow all directions on the label.
*Note: Powdery mildew fungi can develop resistance to these fungicides if they are applied exclusively.

Bacterial Leaf Scorch: Bacterial leaf scorch (BLS) is a problem on several species of shade trees, including American sycamore and London planetree, oaks, maples, sweetgum, dogwood and American elm. It is caused by Xylella fastidiosa, a bacterial pathogen (disease-causing organism) with a wide host range. This bacterium lives in the host plant’s xylem (water-carrying tubes). Many ornamentals and weeds are hosts to this organism, but show no disease symptoms. To enter a new plant, the bacteria are generally carried by insects such as leafhoppers, sharpshooters and spittlebugs that have fed on xylem fluid from an infected plant. The bacteria may also spread from tree to tree by root grafts.

Characteristic symptoms of BLS in sycamore include irregularly shaped areas of scorch (dry and brown tissue) on leaf margins and between leaf veins. These symptoms are generally evident in late summer/early fall. Oftentimes, there will be a band of reddish color between the scorched portion of the leaf and the rest of the leaf. Affected leaves may curl upward, but often remain on the tree.

Sycamore leaves showing the scorched appearance and upward curling characteristic of BLS
Sycamore leaves showing the scorched appearance and upward curling characteristic of BLS.
Edward L. Barnard, Florida Department of Agriculture and Consumer Services

Initially, leaves on only one or a few branches may be affected. With time, symptoms may appear on other branches of the tree. Symptoms tend to begin with older leaves on a branch and then develop in younger leaves. As a result, younger leaves at the ends of sycamore branches may appear healthy. Over several years, entire branches may die. The resulting tree decline may require removal of the infected tree.

Prevention & Treatment: There is no way to prevent BLS, but if affected limbs are pruned out early in the infection and well below the scorched leaves, a cure may be possible. Providing irrigation, mulch and fertilizer to an infected tree can extend the life of the tree. Trees with extensive dieback should be removed and replaced with non-susceptible trees. Currently, spray treatments are not available. However, certified arborists can perform annual root flare injections of antibiotic treatments, using oxytetracycline (such as Bacastat), which can reduce symptoms by suppressing the pathogen.

Insect Pests

Sycamore Lace Bug: Sycamore lace bug (Corythucha ciliata) gets its name from the lacy pattern seen on the adult’s wings, head, and thorax (chest region). The adult is small, 1/8 inch in length and brown to black in color.

Sycamore lace bugs
Sycamore lace bugs.
Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Adults survive the winter under peeling bark of the sycamore tree. They become active in the spring at about the same time as leaves begin to develop. The adult female lays eggs on the leaf’s undersurface, attaching them with a brown sticky material. In a few days, the nymphs (immature stage) hatch. The nymphs are wingless, darker than the adults and covered with spines. Both adults and nymphs feed on the undersides of leaves. They have piercing/sucking mouthparts that they use to remove sap from the leaves. Around the feeding sites, the leaf tissue turns yellow. These yellow flecks are visible from the leaf’s upper surface. Late in the summer, the lower surface of the leaf will typically be covered with dark spots of waste material and the cast off skins of immature lace bugs. Damage to trees is typically not serious, but heavy infestations can reduce growth.

Prevention & Treatment: The sycamore lace bug has several natural enemies, including assassin bugs, minute pirate bugs, lacewings, spiders and predaceous mites. As much as possible, these predators should be allowed to reduce lace bug populations. To control sycamore lace bugs, it is important to inspect leaves every week for signs of the insect. If the tree is small enough, spraying with a strong stream of water can control a light infestation. Controlling this pest on a large tree using chemicals is expensive and often not practical. Since little serious damage results from lace bug infestation of sycamore trees, tolerating some damage is a good choice. If insecticide application becomes necessary and is feasible, acephate (sprays) and imidacloprid (soil drench) are recommended for homeowner use. As with all pesticides, read and follow all label directions and precautions.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.