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Ash Anthracnose – A Recurring Problem (05/28/15)

Cool and moist weather during the period of ash budbreak has again led to high levels of ash anthracnose, a leaf disease common to our region.

Ash Anthracnose – A Recurring Problem

Cool and moist weather during the period of ash budbreak has again led to high levels of ash anthracnose, a leaf disease common to our region. Defoliation of ash trees is the most commonly observed symptom in spring, with green leaves littering the ground under infested trees.  Additional symptoms of ash anthracnose leaf infections include brown-to-black blotches on leaf margins, causing leaf distortion, and small purple-to-brown spots in the middle of leaves (see photo). Several successive years of heavy defoliation may cause twig and branch dieback.  Trees that lose leaves in spring will often grow new leaves. Re-foliated leaves are often smaller and the crowns of affected trees may appear thin.

Treatment with fungicides is usually not warranted. Fungicides are effective only as a preventative treatment and should be applied as leaves begin expanding.  Treating trees now can prevent mid-season infections, but infection is more common in a wet, cool spring, rather than during the drier, warm summer.  For most large trees, fungicide applications aren’t very practical.  A light application of fertilizer (1-3 pounds of nitrogen per 1000 square feet of soil surface around the tree) may help reduce stress on highly susceptible ash trees.

The fungus that causes ash anthracnose overwinters in the upper parts of trees in seed samaras, on twig cankers, and on any other infected plant part that remains attached to twigs, so raking and destroying fallen leaves and twigs will 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 during budbreak. Disease severity, and therefore the extent of spring defoliation, varies from one year to the next, and among individual trees.

 forestry.zeleznik.bergdahl.ash anthracnose

 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 natural openings or wounds created by summer feeding by the ash plant bug.

Joe Zeleznik

Extension Forester

 

Aaron Bergdahl

Forest Health Specialist, ND Forest Service

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