Another cool, wet spring has led to a recurring problem with ash trees - ash anthracnose. This disease is most commonly seen when green leaves fall from the trees in early spring.
Additional symptoms can include black blotches on leaf margins, causing leaf distortion, and small purple-to-brown spots in the middle of leaves (see photos).
The leaf symptoms may not necessarily be visible on fallen leaves, since the infection that triggered leaf drop is likely on the stalk that joins the leaf to a stem, called a petiole, or other inconspicuous location.
Treatment with fungicides is usually not warranted. Fungicides are only effective as a preventative treatment, usually as leaves are expanding.
Treating trees now can prevent mid-season infections, but infection is more common in the wet spring, rather than the drier summer.
For most large trees, fungicide applications aren't very practical. However, there are cultural practices you can implement now, such as a light fertilization, to help reduce recurring stress on ash.
"Light" fertilization would be 1-3 pounds of nitrogen per 1,000 square feet of soil surface around the tree. The fertilizer applied with lawn treatments should be more than enough to meet the tree's nutritional needs.
The fungus that causes ash anthracnose overwinters in the upper parts of trees in the papery seed pod, called a samara, on twig cankers, and on any other plant part that remains attached to twigs, so raking and destroying fallen leaves and twigs may only help reduce inoculum rather than completely eliminate it.
As a result, ash anthracnose is a recurring problem on ash as long as we have wet, cool weather in the spring. Disease severity varies from one year to the next, and among individual trees.
In general, ash anthracnose does not cause enough damage to stress trees in a single year, but trees that are heavily defoliated for three or more consecutive years can be stressed and susceptible to other pests that could kill them.
On the upside, it may be comforting to know that trees can lose up to 25 percent of their foliage without major consequences in a single year.