ISSUE 2   May 20, 2010


Based on the amount of ash leaves falling ‘like snow’, we expected many calls this week with questions about what is happening to ash trees. We weren’t disappointed. A disease known as ash anthracnose is once again occurring on ash this year, as it does every year when the spring is cool and wet. Symptoms may include one or more of the following: premature leaf drop, black blotches on leaves causing leaf distortion, and small brown leaf 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 may be on a petiole or other inconspicuous location.

The pathogen (a fungus) that causes ash anthracnose overwinters in the upper parts of trees in seed 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 spring. It seems to vary in severity from one year to the next, and among individual trees. This is not surprising since the amount of inoculum can vary from year to year and from tree to tree.

Treatment with fungicides is usually not warranted. Fungicides are only effective as preventative treatments as leaves are expanding, so it is too late to protect trees this year. Although it is too late for fungicides this year (and for most trees, fungicide applications aren’t practical anyway), there are cultural practices you can implement now, such as applying a light fertilization, to help reduce recurring stress on ash.

In general, ash anthracnose does not cause enough damage to stress trees in a single year, but trees that are heavily defoliated for 3 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% of their foliage without major consequences in a single year. Lightly fertilizing trees that have lost a large amount of leaf tissue should help them refoliate and reduce stress. Heavy fertilization should be avoided.

Ash leaves showing symptoms of ash anthracnose. Note the dead leaf margins and distorted growth in the first photo. The second photo shows a dead leaf margin plus small dots where the fungus has entered the leaves through wounds created by feeding of the ash plant bug.


Kasia Kinzer, Plant Diagnostician

Joe Zeleznik, Extension Forester

Ron Smith, Extension Horticulturist

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