Native ash trees in southwest Ohio are showing tell-tale symptoms of Ash Anthracnose. The disease is produced by the fungus, Plagiostoma fraxini (syn. Gnomoniella fraxini). As the specific epithet implies, the pathogen is specific to members of the Fraxinus genus.
Host specificity is an important point relative to anthracnose diseases of hardwoods. Ash, sycamore, oak, maple, and beech anthracnose diseases are all produced by fungal pathogens specific to the hosts. Consequently, the ash anthracnose fungus does not infect sycamores, maples, etc., and the fungi that produce anthracnose on sycamore, maple, etc. do not infect ash.
Symptoms of ash anthracnose include irregularly shaped water-soaked blotches on leaflets that progress into dark green to reddish-brown leaf blotches. Infections may also occur on the leaf petioles. The necrosis often causes the leaflets to curl, and severe infections may lead to noticeable defoliation.
The publication cited at the end of this Alert notes that ash anthracnose fungus also infects twigs and samaras. However, I’ve never observed and photographed symptoms on the twigs.
There is little doubt that ash anthracnose occurs every year to some degree. The fungus overwinters in tissue that was infected the previous season, so the pathogen is available in the form of spores in the spring.
However, infections and the subsequent symptoms occur when cool, wet environmental conditions exist at the same time ash trees are in a susceptible host stage during leaf emergence. Of course, this means that infections are commonly worse in some years compared to others.
You may notice that I’ve described the three conditions illustrated by the Disease Triangle that must converge for a plant disease to develop: the pathogen must be present, the plant host must be susceptible to infection, and environmental conditions must support infection and disease development. Removing only one of these conditions will prevent disease development.
Micro-climates can also have a significant impact in terms of presenting highly localized conditions that support infections. This is why we commonly see ash trees that are heavily infected growing only a short distance from ash trees relatively free from infections.
For example, the white ash (F. americana) shown in the photo below had heavy infections meaning the tree’s leaves were at a susceptible stage at the same time cool, wet conditions supported fungal infections. Other white ash trees as well as green ash (F. pennsylvanica) trees growing within a short walking distance were relatively free of leaf infections meaning they were not at a susceptible point at the time when localized environmental conditions supported heavy infections.
Additionally, the tree shown in the photo above has been under the protection of insecticide treatments against emerald ash borer (EAB) (Agrilus planipennis). It’s a reminder that insecticide treatments do not impart protection against fungal diseases unless the treatments target an insect that vectors a plant pathogen behind a disease. Of course, EAB does not transport the fungus behind ash anthracnose.
Based on past history, anthracnose diseases that affect ornamental shade trees in Ohio are not considered tree killers. In fact, they seldom cause enough damage to seriously harm the overall health of their host trees. While trees may look bad now, there's plenty of time for healthy trees to produce new leaves when warm temperatures are less supportive of new infections.
For this reason, it's difficult to justify making fungicidal applications to suppress ash anthracnose. Sprays applied now cannot undo the damage from the infections that are already underway. Fungicide applications made later in support of re-foliation are not needed because warm, dry environmental conditions won’t support fungal infection (see Disease Triangle above).
Preemptive suppression sprays can reduce infections; however, there's no way to predict whether or not spring environmental conditions will support the levels of infection that justify the applications. History teaches us that heavy infections on specific ash trees are a rarity.
Reference
Riffle, J.W., 1986. Diseases of trees in the Great Plains (Vol. 129). US Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station.
https://www.fs.usda.gov/nac/assets/documents/research/publications/rmrs_gtr335.pdf
