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Weather Pattern Conducive for Anthracnose Fungus on Shade Trees

This disfiguring disease can become severe when cool, wet spring weather persists just as the leaves are unfolding

This year, the wet and cold spring weather is conducive to the development of anthracnose on shade trees. The anthracnose fungus can appear on just about any woody plant if conditions are right, which, for most North Dakota homeowners, they are.

In North Dakota, the most commonly planted shade tree is the green ash. The green ash also seems to get the most attention from fungus-caused plant diseases, such as anthracnose.

“This disfiguring disease can become severe when cool, wet spring weather persists just as the leaves are unfolding,” says Ron Smith, North Dakota State University Extension Service horticulturist. “In addition to ash being the highly visible tree species affected, other species, such as maple, elm, walnut and birch, also may be affected."

The typical symptoms show up on the lower limbs. The branches on the lower limbs are defoliated almost completely. However, about the upper 25 percent of the canopy is untouched. In examining the foliage, tissue death will be apparent on or adjacent to the veins of the leaf.

Because green ash trees get so much attention from this disease, homeowners tend to think they are the only vulnerable species. That’s because ash trees are the first to show disease symptoms.

“However, the point also should be made that the fungi that cause anthracnose are very host- specific,” Smith says. “This means that the anthracnose that infects an ash tree will not attack a maple or elm tree.”

Ash and other tree species that have had anthracnose attacks in previous spring seasons likely will have it show up again this spring.

“While it is common to feel a wave of panic when one goes out and sees most of the leaves of the tree lying on the ground in the morning, it is not a death knell for the tree,” Smith says. “Most healthy trees will push out a new canopy of foliage sometime in early to mid-June.”

Here are some steps for homeowners to take:

  • Infected trees need to be fertilized as the renewal of foliage is taking place.
  • The tree should be watered deeply during periods of summer drought or heat stress.
  • Picking up all the fallen leaves and twigs is an important step in preventing the reinfection of the tree next spring.

When the symptoms of anthracnose are apparent, the typical question is what to spray on the tree to alleviate the problem.

“My usual response is nothing for now,” Smith says. “However, if the tree has a history of this fungal disease showing up every spring, then a spray with a fungicide known as Daconil 2787 (chlorothalonil) will help control an outbreak. This needs to be done just as the buds are about to open and again about 10 days later. Be sure to follow label directions carefully.”

For a positive identification of this disease, contact the NDSU Extension Service agent in your county. Homeowners also can contact the NDSU Plant Diagnostic Lab at ndsu.pdl@ndsu.edu to make arrangements for submitting a sample.


NDSU Agriculture Communication – May 3, 2011

Source:Ron Smith, (701) 231-7123, ronald.smith@ndsu.edu
Editor:Rich Mattern, (701) 231-6136, richard.mattern@ndsu.edu
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