Published May 19, 2010, 12:00 AM

Leaf blight takes toll on ash trees

For Fargo-Moorhead residents, today’s forecast calls for warm and sunny – with a strong chance of ash leaves.
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For Fargo-Moorhead residents, today’s forecast calls for warm and sunny – with a strong chance of ash leaves.

Foliage is falling from metro-area ash trees in far greater amounts than usual, as the disease ash anthracnose takes it annual toll on the hardwood.

“Basically, what you see when you drive down some of these neighborhoods is it’s raining leaves,” Fargo City Forester Scott Liudahl said.

Recent wet weather created ideal conditions for the fungus that causes the disease to flourish, he said.

City foresters in West Fargo and Moorhead also fielded a lot of calls Monday and Tuesday from residents worried about the sudden loss of leaflets, which are piling up on streets and sidewalks.

Eric Baker, owner of Baker Nursery Garden in south Fargo, said he answered about 30 calls from concerned tree owners on Monday alone.

“People are really anxious about it,” he said.

They needn’t fret in most cases, Liudahl said.

Anthracnose doesn’t seriously harm plants unless defoliation occurs repeatedly year after year. A healthy tree should grow a second set of leaves this season as conditions turn drier and warmer, he said.

Anthracnose attacks ash trees every year, but this spring it hit especially hard, said Todd Weinmann, Cass County Extension horticulturist. He said he got a call Tuesday about the disease also having a strong effect on ash trees in Grand Forks.

Leaflets may have purple or brownish-black spots as a visible symptom of the disease, he said.

Fungicide is available to treat anthracnose, but once the leaves start falling, it’s too late to spray. Owners may want to treat hard-hit trees next spring before they leaf out, Baker said.

If the tree is too large to spray all the way to the top, it probably can withstand the disease, Baker said. For younger, smaller trees already in a weakened state, “sometimes it can be one of the final straws that breaks the back,” he said.

Trees weakened by ash anthracnose are more susceptible to pests, including the emerald ash borer, a beetle whose larvae feed on the inner bark of ash trees, disrupting their ability to carry water and nutrients, Liudahl said.

Last year, Fargo removed 536 diseased ash trees from public property to replace them with other species in an attempt to thin out the city’s ash population to prevent widespread canopy loss from the borer.

The city plans to be even more aggressive with the program this year as it tries to reduce the percentage of ash boulevard trees to 10 to 15 percent, Liudahl said.

About 35 percent of Fargo’s boulevard trees are now ash, he said.

Forum reporter Tammy Swift contributed to this article.

Readers can reach Forum reporter Mike Nowatzki at (701) 241-5528