ISSUE 4   June 4, 2009


We received two calls this week, one from Fargo and the other from Bismarck, regarding ash trees that are losing their leaves. One reason could be the disease ash anthracnose, a fungal disease that is common during cool, wet weather. Symptoms include one or more of the following: premature leaf drop, black blotches on leaves which may cause leaf distortion, and small brown leaf spots in the middle of leaves (see photos).

Is treatment necessary? In most cases, no. Ash anthracnose does not cause enough damage to stress trees in a typical year. Trees can lose up to 25% of their foliage without major consequences. However, if trees are heavily defoliated for three or more consecutive years, they may be stressed and susceptible to other pests that could kill them. Fungicides are only effective as preventative treatments as leaves are expanding, so it is too late to protect trees this year. Fertilizing trees that have lost a large amount of leaf tissue could help them refoliate. We recommend a standard lawn fertilizer application at the labeled rate, which should provide sufficient nitrogen to help the trees and reduce stress. Since the fungus overwinters in leaves and seeds, raking and destroying these tissues in the fall help to reduce disease pressure the following spring, though it will not completely eliminate the source of fungal inoculum.

Green ash and Manchurian ash - both hardy in North Dakota - are relatively susceptible to ash anthracnose. However, certain cultivars may be more resistant than others. Both 'Summit' and 'Marshall seedless' green ash (both hardy to Zone 4) are moderately disease resistant, while 'Patmore' (Zone 2) may be disease resistant, although more information is needed. In general, trees that break bud later have less infection than those that begin growth early in the spring.

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.

Joe Zeleznik
Extension Forester

Kasia Kinzer
Plant Diagnostician

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