3 August 2005
Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580 fax: 434.872.4578
Anthracnose is a name for a group of diseases caused by several closely related fungi that attack many of our finest shade trees. It occurs most commonly and severely on sycamore, white oak, elm, dogwood, and maple. Other host plants that are usually only slightly affected include linden (basswood), tulip tree, hickory, birch, and walnut. Each species of anthracnose fungus attacks only a limited number of tree species. For example, the fungus that causes sycamore anthracnose infects only sycamore and not other tree species. Other anthracnose-causing fungi have similar life cycles, but require slightly different moisture and temperature conditions for infection.
Anthracnose fungi may cause defoliation of most maple, oak, elm, walnut, birch, sycamore, and hickory species and, occasionally, of ash and linden trees. Damage of this type usually occurs after unusually cool, wet weather during bud break. Single attacks are seldom harmful to the tree, but yearly infections will cause reduced growth and may predispose the tree to other stresses. Damage may be in the form of:
- killing of buds, which stimulates the development of many short twigs or "witches' brooms;" these may spoil the shape of the tree
- girdling and killing of small twigs, leaves, and branches up to an inch in diameter
- repeated early loss of leaves, which over several successive years weakens the tree and predisposes it to borer attack and winter injury
- premature leaf drop, which lessens the shade and ornamental value of the tree
Specific symptoms of anthracnose vary somewhat depending on the tree species infected. On dogwood, two different anthracnose diseases may occur. Symptoms of spot anthracnose (Elsinoe corni) include tiny leaf and bract spots, about the size of a pinhead, with whitish centers and purplish borders. Symptoms of Discula anthracnose (Discula destructiva) include irregular small to large brown blotches with purplish borders on leaves and bracts, lower branch dieback, and trunk cankers that culminate in death of the tree.
Anthracnose fungi overwinter in infected leaves on the ground. Some canker-causing anthracnose fungi, such as the sycamore anthracnose fungus, also overwinter in twigs on the ground or in cankered twigs that remain on the tree. Microscopic spores of most anthracnose fungi are produced in infected tissues during April and May. The spores are blown and splashed to the buds and young leaves and, with favorable moisture conditions, penetrate and infect the swelling buds and unfolding leaves. Long rainy periods help the fungus to spread rapidly.
Disease control measures for different trees vary slightly because the period of infection is different depending on the fungal species involved. If fungicides are used, sprays must be applied on a preventative basis, beginning before infection takes place. Spraying large trees for many anthracnose diseases may be impractical and unnecessary, especially in dry springs. Sanitation is important in reducing the amount of fungal inoculum available for new infections. For large, high-value sycamore trees, injection with a fungicide, on a 3-year basis is also an option.
For effective control of most anthracnose diseases:
- Rake up and remove infected leaves in the fall. Leaves may be shredded and composted or burned.
- Prune out and burn or bury dead twigs and small branches. Prune to thin the crown. Thinning will improve air movement and promote faster drying of the leaves.
- If fertilizer is needed, fertilize in the fall about a month after the average date of the first frost or in early spring about a month before the date of the last frost to increase tree vigor.
- If chemical control is desired, spray with a fungicide when buds begin to swell and twice again during leaf expansion (in most years, this would be at 14 day intervals).
Because Discula anthracnose of dogwoods causes cankers, this disease is difficult to control. A combination of cultural and chemical methods is recommended for most effective control. Once cankering is severe on a tree, the tree cannot be saved.
Avoid planting dogwoods in sites where leaves will remain wet for long periods of time. Sites along streams, lakes, or ponds, or areas where fog tends to collect are prime sites for disease development and should be avoided. The disease is more of a problem in shaded locations; thus, planting in sunny locations can help to prevent disease. Although dogwood is naturally an understory tree, trees that are properly cared for can do well in full sun.
Trees should be mulched to a depth of 2-4 inches to help conserve soil moisture. Place mulch in a flattened ring around the tree instead of piling it against the base of the trunk like a volcano. Placing mulch in contact with the trunk causes bark to remain moist and become more susceptible to decay. Water trees during drought and fertilize as needed to help trees remain vigorous. Avoid using fertilizers high in nitrogen, however, as they can stimulate succulent growth that is more susceptible to infection by Discula. Water trees with a soaker hose rather than a sprinkler to avoid prolonged leaf wetness.
Remove cankered branches and destroy them as soon as they are noticed. Make cuts well below cankers through healthy wood. Sterilize pruning tools between cuts with rubbing alcohol or a 10% solution of household bleach. Remove any plant debris that falls to the ground from infected trees. Also, avoid collecting and planting seed from wild dogwoods or transplanting dogwood seedlings from the woods because these plant materials may harbor the fungus.
Fungicide recommendations include Banner Maxx at label rates every 14-28 days, OR mancozeb (Dithane WF, Mancozeb DG, or Dithane T/O), OR chlorothalonil (Daconil WeatherStik, Ortho Daconil 2787, or Daconil ZN) at label rates with a wetting agent (few drops of liquid dish soap) at two week intervals. It is important to begin treatment before or during bract or foliar infection but before the fungus invades the twigs. Once cankers have formed, fungicide treatment may not be effective.
To date only one cultivar of flowering dogwood has been developed with resistance to Discula anthracnose (Appalachian Spring). However, this cultivar does not have resistance to powdery mildew. The kousa dogwood (Cornus kousa, Stellar series) is resistant but not immune to dogwood anthracnose but may harbor fungal inoculum that can perpetuate the disease.
This fall Virginia Cooperative Extension will be offering Master Gardener training in Albemarle & Greene Counties. For more information, call (434) 872-4580.
For more information about landscape topics contact your local Virginia Cooperative Extension office. Many county Extension offices have a help desk that is staffed by Master Gardener volunteers. These volunteers are trained to answer questions about home and landscape pest problems. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.
Leave a Reply
You must be logged in to post a comment.