Jerry Vitalis, Chisago County Master Gardener
Anthracnose is the generic name for a group of diseases caused by several different fungi that cause similar symptoms.
In Minnesota, anthracnose occurs most commonly and severely on ash, sycamore, white oak, maple and walnut trees, all of which can be severely defoliated.
Anthracnose fungi overwinter in small branches and fallen leaves. In the cool, rainy spring, fruiting bodies form that release spores, which are disseminated by wind. The progress is halted by hot, dry weather.
It is believed that the fungi produce hormones that cause the leaves to fall off prematurely. As the fungus grows, lesions expand and grow together, creating a more characteristic anthracnose lesion.
The lesion works its way down the midvein of the leaf, into the leaf stem and eventually into the branch.
Lesion color varies from tan to dark brown on leaves. Branch cankers are small and somewhat elliptical or football shaped.
The trees may lose their leaves. Since it takes about a week for symptoms to develop, fallen leaves may appear to be perfectly healthy.
To manage anthracnose, dispose of infected foliage and cankered branches by raking and pruning. Fungicides are not recommended unless defoliation has occurred in three of the last five years.
Anthracnose will rarely kill a tree, but it will weaken it. It is important to maintain vigor and prevent the tree from declining. To keep anthracnose from developing from a nuisance into a killer, fertilize, water and mulch trees to protect and encourage solid growth.
The bad news is that repeated loss of leaves over several successive years weakens the tree and makes it vulnerable to borer attack and winter injury.
The good news is that each anthracnose pathogen is host-specific, infecting only one tree species. The anthracnose on your ash cannot infect your maple or oak, and visa versa.
More good news is that the defoliated branches will produce new shoots in midsummer, another symptom of the disease. Other tree diseases can be mistaken for anthracnose, but if the leaves come back, it probably was anthracnose.