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.
 |
| Ash anthracnose |
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.
 |
| Maple anthracnose. |
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
