Common tree pests (diseases and insects) and public concerns on City of Toronto trees. For more information on forest health care and pests, please visit the City of Toronto website.

Diseases:    

  1. Anthracnose (fungal leaf disease)
    Symptoms: Tree leaves will often have brown patches or turn brown completely. Leaves may also curl, and/or drop early in the season.
    Anthracnose affects mainly white oaks, but it is common on ash, maple and other tree species. It may appear serious, but it is not life threatening and does not cause permanent damage. Severe infections may result in the loss of leaves in spring, however the trees usually produce a new flush of leaves in summer.
    The fungus overwinters on infected twigs and fallen leaves and is activated in the spring by cool and wet weather. Raking leaves in the fall and pruning dead or dying branches helps reduce the new infection the following year. Leaves may be composted by City composting programs.
  2. Apple Scab (fungal leaf disease)
    Symptoms: Begins in the spring as small olive-green spots. Later these spots can darken to black. Infected leaves become distorted in shape, turn yellow and drop early in the summer.
  3. Black Knot (fungal disease) Black Knot. Black Knot is a fungal disease of cherries, plums and other stone fruit trees. It causes twig and branch swelling and discoloration, resulting in girdling and dieback of branches and sometimes the main stem. Infection can occur on both healthy and mechanically injured woody tissue of the current season's growth. The infecting spores arise from the black knots and spread by wind and rain. Where possible, prune out and destroy newly forming galls to reduce their spread in the future. Avoid pruning when the spores are out between March and June. When removing woody galls, employ good pruning practices and make branch cuts at a distance from the gall. Disinfect pruning tools between each cut.
    Symptoms: Brown, black woody swelling, knots on the branches of cherry.
  4. Cytospora canker of spruces (fungal disease)
    Symptoms: The needles on the spruce turned purple, than brown and drop, leaving the infected branches bare. Branches are covered with white resin.
    Most susceptible hosts are Colorado blue spruce, Norway spruce, but it also affects black, red and white spruces, Douglas-fir, balsam fir, larches, and red and white pines. Trees weakened by drought, mechanical injury or planted in unfavourable sites are more susceptible to the disease.
    The fungus enters through wounds. The infection starts on the lower branches and spreads upwards as spores of the fungus are dispersed by rain-splash. The disease does not kill the tree instantly. Several years or sometimes decades may pass before the trunk or large limb is completely girdled.
    Avoid stem and bark injuries, since the fungus establishes through wounds. Wounds made by improper pruning of lower branches to open space around the base of the tree are the most common entering points of the fungus. Water your trees thoroughly in extended dry periods. Infected branches cannot be saved and should be pruned back to main stem. There is no effective chemical control of this disease.
  5. Diplodia tip blight
    Symptoms:
    The tips of the branches on the (Austrian, Scots) pine tree turn brown and die off. By the end of the summer, shoots became brown and dead. Needles turn brown and entire branches die.
    Diplodia tip blight is a fungal disease that mainly affects older trees that over years become stressed by other environmental factors, like drought, soil compaction and mechanical injuries. Thorough weekly watering during extended dry periods of the growing season will result in a tree that is more vigorous and more resistant to tip blight. Rake up all blighted needles, twigs, and especially cones which harbour the fungus and destroy or discard them.
  6. Dutch Elm Disease
    Symptoms:
    The leaves on the elm tree begin to wilt, curl, shrivel and turn brown on one or more branches. The foliage throughout the entire crown wilts and the tree dies.
    All the native elm species are susceptible to the disease. Please note that the Siberian elm is resistant to this disease. If the infection is not controlled at early stage the fungus will spread rapidly and the tree will probably die. For City owned tree call Urban Forestry and request an inspection. For a private tree hire an arborist.
  7. Fruiting body of fungus, mushroom on a part of the tree
    Symptoms:
    a mushroom growth (fungus fruiting body) or a conk attached to the main stem or/and large branches.
    Presence of fruiting bodies of fungi are usually indicators of a wood decay. Wood decay is a process of wood disintegration that is caused by fungi or other micro-organisms. The process begins through wounds, where the wood becomes exposed to pathogens. Many species of fungi cause wood decay. Most of them produce characteristic types of fruiting bodies known as conks or mushrooms. Presence of conks and mushrooms indicates advanced decay inside the tree. It is difficult to know the extent of decay without further investigation. The tree may be dead, or have a structural deficiency and may need to be pruned or removed in order to avoid an accident. In the urban environment, these trees can be hazardous and need to be evaluated for structural integrity and managed appropriately for inspection and tree assessment.
  8. Pear Trellis Rust (fungal leaf disease)
    Symptoms: leaves show small orange spots. In early summer, the spots enlarge and become bright orange and later red lesions. In late summer, the underside of these lesions begin to swell and form a number of blisters.
    The damage on ornamental pears is usually cosmetic character, however, heavy and repeated infections may cause early leaf drop and dieback of the branches. There is no registered pesticide in Ontario for the control of this disease.
  9. Powdery Mildew (fungal leaf disease)
    Symptoms: White powdery spots or patches may cover the entire leaf surface. Leaves that are heavily infected may become chlorotic and senesce early. Although unsightly, the disease does not cause serious damage to established trees.
  10. Tar Spot (fungal leaf disease)
    Symptoms: The first symptoms of infection show up in early summer as small yellow spots on leaves. The spots become larger and darker and by late August, they look like spots of tar. Tar spots do not cause serious damage to established trees.

Insects/Pests:

To view more Fact Sheets on other tree pests, please visit the City of Toronto website.

  1. Aphids
    Aphids are soft-body insects that suck sap from the soft issues of many tree species. Sticky honeydew from aphids drops on branches, cars and anything else beneath the tree. Generally, aphids do not cause significant damage to trees. No inspection is required.
    Aphids can produce sticky liquid sap called honeydew. The excess sap drips onto leaves, tree stems, branches, and objects beneath the tree. A black sooty mould fungus may grow on honeydew. The mould does not injure the tree, but by covering the leaves, it can reduce photosynthesis. Honeydew attracts ants, wasps and flies. Ants that feed on honeydew protect and spread the aphid colony to maintain the food source.
    The natural enemies of the aphids, such as ladybeetles and lacewings, usually control most of the population. A heavy rainfall usually reduces aphid populations. You can help reduce the population by using a hard jet of water from a hose to wash them from the leaves of the tree.
  2. *Asian Long-horned Beetle (ALHB)
  3. *Carpenter Ants
  4. Box Elder Bug
  5. Eastern Tent Caterpillar 
    "Eastern Tent Caterpillar (Malacosoma Americana) is a native insect, that is present in Toronto and affects native and ornamental crab apple, apple and cherry trees and will occasionally feed on peach and plum. Outbreaks of ETC are cyclical and usually come every 10-12 years and last 2-3 years. While the tents and caterpillars can be unsightly and disturbing to some homeowners, the effects of this insect are considered to be a nuisance and ETC does not normally result in tree mortality. Urban Forestry will continue to monitor ETC populations in the coming years.
    If you feel it necessary to control the nuisance, Urban Forestry can advise you environmentally friendly control options such as a pressure washer or hose to remove the tents and/or caterpillars, mechanically destroy the crawling caterpillars by scraping them off the tree and sweeping caterpillars from the ground and objects around the house.
    You may wish to submit a Pruning Inspection request.
    (Crab Apple Replacement Policy)
  6. Elm Leaf Miner
    Symptoms:
    White spots, or later in the season large blotches on elm leaves. Leaves lose green tissue and turn brown.
    This is most likely an Elm leaf miner feeding damage. The larvae of this insect feed on the inner green leaf tissue between the upper and lower leaf surface. Elm leaf miners feed on most elm species, but Scots and Camperdown (a cultivar of Scots) and Siberian elms are the preferred. Although unsightly the damage from this insect is in most cases cosmetic by nature.
  7. *Emerald Ash Borer Question on Emerald Ash Borer, and the Tree Azin injection program should be directed to Urban Forestry or to FHC Inspectors.
    Emerald Ash Borer (EAB) Note: City trees that have a small aluminum tag and are sprayed with a green slash approximately 2 metres above the ground, have been identified as suitable candidates for injection against EAB.
  8. Fall Cankerworm
    Fall Cankerwork (Alsophila pometaria), a native insect, can be one of the most damaging defoliators of Toronto's urban forest. Members of this family are often called inchworms, loopers or spanworms. Caterpillars have a very peculiar means of movement. They loop when they walk. There are two forms; one green theother brown in colour.
  9. *LDD Moth (European Gypsy Moth)
  10. Honey Locust plant bug
    Symptoms:
    The leaves on the honey locust tree are distorted, discoloured (yellow) and dwarfed. Part of the branches or the entire crown has no leaves developed. There are a lot of tiny green insects on and around the tree.
    Honey locust plant bugs suck sap from honey-locust leaflets in the spring, causing them to be distorted and fall from the tree when the bugs are numerous.
    Tolerate the damage because the tree's health is unlikely to be harmed. The plant bug completes its development and stops feeding by the end of June. Control with insecticidal soap can effective, although seldom necessary. It is also not practical for large trees.
  11. Ladybeetles (beneficial insects)
    Symptoms: There are red or orange insects with black markings on the tree. There are alligator-like orange-blue bugs on the leaves.
    These are either adult lady beetles or their larvae. Both adults and larvae, feed primarily on aphids and mites. Encourage their presence since lady beetles, ladybugs, or ladybird beetles are beneficial predatory insects. Over 450 species are found in North America. Some are native and some have been introduced from other countries.
  12. Scales
  13. Box Tree Moth
  14. *Termites: Termites (Eastern Subterranean) can occasionally feed on trees, but it is usually only dead trees or dead parts of live trees that are colonized by termites as a food source. Termites do not kill trees, but attack dead parts of live trees.  The City does not use pesticides or treat termites in trees with pesticides.  Subterranean termites nest in the ground and come above ground to attack dead wood or other cellulose materials. They must have constant contact with their nest underground, as they require a certain level of moisture to survive. Once disconnected from their nest and the ground, with no moisture, they will soon perish. If the food source does not contact the soil, they can build mud tunnels or tube shelters to reach it several feet above the ground. Termites are quite difficult to detect. The presence of shelter tubes along the sides of buildings or trees is the best indication of termite presence.
  15. There is no evidence that trees are a point source for termite infestation. Removing trees which have termites feeding in them does not remove termites from the area of infestation as they nest in the soil. Living and structurally sound trees with the presence of termites do not need to be removed. Removing a living tree will also kill the remaining roots and the dead wood tissue in the soil creates more favourable conditions for termites. The tree should be removed only if it is structurally unsound. Eliminating the food source by pruning dead wood, removing dead tissue and wood debris from a tree is recommended. Shelter tubes built on the tree can be scraped off if noticed. There is presently no safe, effective and environmentally compatible method for protecting trees from termites.

    The best defence for homes is the breaking of the wood: soil contact. There are chemical control options as well, which is a task for a registered pest control company.

Other tree health problems:

  1. Galls
    Galls grow in the presence of certain bacteria, fungi, insects, mites or virus that cause plant tissue to swell. The galls do not represent any structural hazard or a serious health threat to the tree. The problem is more to do with appearance. You can have the tree checked by an arborist to prune away old galls, but make sure the pruning does not disfigure the tree.

  2. Girdling Roots

  3. Lichens
    Symptoms: green, blue cover on bark of the tree trunk and branches
    Lichens do not damage trees. Lichens can be found on various surfaces, often on tree barks. Lichens are not parasites and they do not harm trees. They grow in various forms and may be flat, leafy, and hair-like. Colour may vary from white to gray, red, brown, green, yellow, and black.
    Lichens are organisms consisting of a fungus and a blue green alga. The alga supplies the fungus with carbohydrates and vitamins, and the fungus provides water and minerals. They are good indicators of a clean air. Most lichens will not grow in a polluted atmosphere.

  4. Caterpillars
    Some caterpillars, such as the European Gypsy Moth, may cause considerable damage to a tree while others are more of a nuisance than a treat to overall tree health.
    Tent caterpillars can cause partial or complete defoliation of host trees. These trees usually flush a new set of leaves later in the summer. Most trees can withstand more than two consecutive years of complete defoliation.

  5. *European Starling

Related Information: