This spring has been cool and wet, leading to slow emergence of leaves on many shade trees. This combination of favorable weather conditions and slow leaf maturity created ideal conditions for development of anthracnose disease on several common shade trees.
In short, the term anthracnose refers to a symptom that causes dark blotching and often leaf distortion. Defoliation (leaf drop) often occurs during severe infections. The disease often is not fatal, and a new flush of foliar growth immediately follows. Causal fungi may also infect twigs and branches, which develop into cankers and girdle stems.
Anthracnose diseases are not caused by the same fungi. In fact, causal pathogens are quite host specific, meaning that the anthracnose pathogen on dogwood will not infect ash, etc. Symptom appearance and severity differ with each host and with climatic conditions.
The fungal pathogens that cause anthracnose diseases have similar life cycles. Spore production occurs in spring during periods of rain; without rain, sporulation (spore production) is reduced and spore dissemination (spread) is not possible. Anthracnose fungi sporulate in spring as deciduous tree leaves emerge. Mature leaves are resistant to infection, but slow emergence in spring exposes tender leaves to fungal spores for an extended period of time.
Ash anthracnose. Common symptoms include brown blotches (Fig1) along leaf edges. Leaf drop often results, and then new leaves soon emerge. Causal fungus, Discula umbrinella.
Dogwood anthracnose. Leaf spots, leaf blight, and lower branch dieback may occur. The disease is most commonly observed on trees growing in shaded locations. This disease can lead to severe cankering, tree decline, and ultimately tree death. Causal fungus, Discula destructiva.
Maple anthracnose. Symptoms begin as leaf spots (Fig 2) and may progress into shoot blight and shoot cankers. Leaf spots with brown, somewhat angular symptoms may be confused with tar spot (spots are round and black). Symptom development and susceptibility vary with tree species, but lesions often follow veins. Causal fungi, Discula sp. and Kabatiella apocrypta.
Oak anthracnose. Not commonly observed in Kentucky. Irregular brown spots develop on leaf tips and along veins. Causal fungus, Apiognomonia quercina.
Sycamore anthracnose. Young, expanding leaves develop irregular dark, necrotic blotching centered along leaf veins or edges. These dark blotches may turn tan-colored as the diseased areas of the leaves dry out. Blighting of twigs or shoots may follow. Trees produce new foliage rather quickly, but affected branches may remain crooked (lateral shoots became dominant when terminals were killed). Also affects London plane tree. Causal fungus, Apiognomonia veneta.
For most trees, with the exception of dogwood, anthracnose disease is not lethal. However, repeated defoliation can be stressful to trees. Additionally, persistent rains and disease spread can lead to infection of twigs and branches. Good cultural practices are important to reduce disease:
- Anthracnose is favored by a moist environment. Select a planting site with a sunny eastern exposure to promote rapid foliage drying early in the day.
- Rake and destroy fallen leaves, as they can be a source of inoculum (fungal spores). Do not compost.
- Remove dead twigs and branches, as fungi can overwinter in dead wood.
- Reduce plant stress when possible.
- Avoid wounding, such as bumping with mowing equipment and making jagged pruning cuts. The dogwood anthracnose pathogen can enter trees through wounds to branches or trunks.
- Maintain mulch 2-3 inches thick over the root zone and beyond the drip line (not against the trunk) to help maintain soil moisture and to protect trees from lawnmower injury.
- Protect trees from drought. Water at least once a week during hot dry months using soakers or drip irrigation. Avoid overhead sprinklers; wet foliage favors sporulation and infection.
- Do not transplant dogwood trees from the wild, as they may be infected with anthracnose fungi. Purchase healthy trees from a reputable nursery.
- Diagnose and treat insect and disease problems as soon as possible.
- Plant disease resistant dogwoods such as C. florida 'Appalachian Spring' or oriental dogwoods (Cornus kousa) for high risk sites, such as those with heavy shade and nearby diseased trees.
- Fungicides are often not recommended. They can be costly and it is difficult to effectively cover large trees. Commercial nurseries, on the other hand, should protect trees with fungicides. Dogwood that are threatened by anthracnose may benefit from early spring fungicide applications.
For more specifics on these anthracnose diseases, see
Dogwood Anthracnose http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-OR-W-6.pdf
Anthracnose Diseases of Shade Trees http://www.ca.uky.edu/agc/pubs/ppa/ppa17/ppa17.pdf