Local ash trees well - for now

• Maple leaf

A silver maple leaf infested with tar spots, a fungus which only affects maples, can weaken trees over time if not prevented by cleaning up the leaf litter.

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WORTHINGTON — The emerald ash borer, found in Minnesota’s metro area in the spring, remains the largest concern to those entrusted with the health of area trees.

“It hasn’t been found here yet,” said Scott Rosenberg, Worthington City Forester.

Emerald ash borers burrow through a tree’s outer layer, just under its bark, damaging the structures that provide the tree with nutrients and water. Unlike people, whose blood vessels will heal over time if cut, a tree can’t mend a severed vessel. Nutrients can no longer move throughout the tree, and it “starves” to death.

Rosenberg hopes the beetle will never find its way to Worthington, but believes someday it will — whether it is one year away or 30 years.

The spread of emerald ash borers can be prevented by using only local firewood and by not transporting firewood from place to place. The beetles burrow into wood and move along with it.

Ash trees can be treated to prevent emerald ash borer infestations, but the cost varies by the size of the tree. A tree with a 20-inch diameter may cost around $100 to make emerald ash borer-proof. And by the time many people think to apply the treatment, the tree could be too far gone to save.

“There will be a lot of trees that will have to be removed,” Rosenberg said. “It’ll change the community forest a lot, if it happens.”

The good news is that emerald ash borers only affect ash trees, and even some ash trees aren’t affected.

Unfortunately, about a third of Worthington’s urban forest, private and municipal, is made up of ash trees, which were planted after Dutch elm disease decimated the elm population.

Dutch elm disease continues to be a problem in the area, but the city of Worthington has been proactive and requires the prompt removal of any infected tree. As a result, the fungus was not as prevalent in the area and only killed a few trees.

Dutch elm disease usually spreads from elm to elm via interconnected root systems or by elm bark beetles, which carry the disease with them from sick trees to healthy ones. There is a treatment that can prevent Dutch elm disease, but it isn’t typically used anymore because most of the large, desirable elm trees are already gone, Rosenberg said.

Because many tree illnesses affect only particular species of tree, Worthington’s Public Works Department plants a wide variety of trees each year, including maples, lindens, locusts, flowering crabs, hackberries and tree lilacs.

Worthington planted about 40 or 50 trees this year in order to save money, but normally plants around 150 trees each year. Rosenberg estimated about a third of the trees in town belong to the city.

“Every tree has its own little drawback to it,” Rosenberg said.

Local maple trees, for example, are affected by tar spots, a fungus resembling black spots on otherwise healthy-looking leaves. Tar spots won’t kill a tree, but they can stress the tree out over a period of years, causing it to become more susceptible to other problems that kill trees.

A variety of trees, including ash, can be afflicted with anthracnose in the late spring and early summer. Healthy-looking, green leaves will start falling from the tree, and the tree will look totally healthy — but the underside of the leaf will show tell-tale grayish-white spots. Anthracnose typically won’t kill a tree either, but can lead to stress on the tree over time.

Native borer worms don’t normally hurt trees, Rosenberg said, but if a tree is already stressed the worms can hurt it further. Wind storms and inadequate moisture levels can stress trees.

People can help their trees out by saturating the ground around them with water before the ground freezes up each year, because the tree will pull it in to stay hydrated over the winter, Rosenberg said.

People should also clean up their leaf litter in order to prevent any infections from recurring year after year, and also keep lawns healthy. Removing leaf litter also helps keep leaves from ending up in lakes and streams, where they can cause algae blooms.

Tags: emerald ash borer, news, worthington, trees