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Ash Decline

By Ruth Hales, Colorado State University Cooperative Extension agent, horticulture

If you are puzzled because your Green Ash trees look a little sickly, you aren't alone.

A lot of your friends and neighbors are puzzled, too.

Wilt symptoms are a key problem. Only a few branches are affected on some trees; on others entire canopies of leaves yellow, curl and drop off prematurely. Trees that are wilting now and that have done so for several years, eventually will die back completely.

Ash tree problems are wide-spread enough throughout the country that researchers from universities as far apart as Colorado State in Fort Collins, the University of Nebraska at Lincoln and Cornell University in Ithaca, New York are looking into them.

Researchers, however, find no consensus in their studies. Problems in eastern states seem to be caused by a bacteria-like organism, and symptoms include stunting and formation of witches brooms on the trees. Colorado State scientists find no evidence of this same organism on any sickly looking ash trees tested here, nor do they find the same symptoms. The best Colorado researchers have been able to do is dub the problem Ash Decline.

Ash Decline is different from damage caused earlier this year by the brown-headed ash sawfly. That insect, which can cause extensive leaf damage, is gone for the season. The wilt problem remains.

So, what seems to be the reason for Ash Decline in Colorado? Researchers say it's the "result of multiple and accumulative causes." This means:

  • Winter stresses, including fall freezes and dry winter weather with sudden, severe temperature fluctuations.

  • Excessive evapotranspiration rates.

  • Minor pest invasions (eriophyid mites, leafcurl aphids).

  • Chronic effects of weed control products (weed and feed, pre-emergents) that cause roots to function inefficiently.

  • Poor soil conditions.

What can homeowners do to protect trees before problems begin and to help affected trees recover?

  • Prune out dead branches, especially those in the center of the tree. Do the pruning this fall or in the winter.

  • Water the roots of affected trees deeply every four to six weeks this winter.

  • Apply nitrogen fertilizer to the root zone next spring at the time of leaf expansion. Base the rate on the root zone area. Use one to one-and-one-half pounds of nitrogen fertilizer per 1,000 square feet of root zone.

Photograph courtesy of Dave Roberts, MSU IPM.

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Date last revised: 01/05/2010