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This common foliage disease might be the reason behind your tree's leaf loss

"Fielding Questions" columnist Don Kinzler also answers questions about the disadvantages of using stone around tree trunks and the recommended spacing for carrots in the garden.

ash anthracnose June 18, 2022.jpg
A reader wonders what might be causing the leaves on their ash tree to develop dark spots and edges and fall off.
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Q: I have a beautiful green ash tree in my front yard and hope to keep it protected from the ash borer as long as possible, but today I noticed a lot of leaves have fallen off. There are little dark spots and dark edges on most of the leaves, as shown in the photo. Can you help diagnose and advise me as to whether there is anything I should do? — Rachel M.

A: Your ash tree has a common foliage disease called ash anthracnose, which is showing up in the area because of the cool, wet spring. Leaves become distorted and some drop. Caused by a fungus, the disease has been present from time to time in the area for many, many years.

North Dakota State University Extension Forester Joe Zeleznik says, “Ash anthracnose occurs during wet springs, and the leaf loss may continue for a couple of weeks until drier conditions prevail. Leaves that don’t fall off may grow into curled shapes as the leaf margins turn black and die while the remainder of the leaf tissue grows normally.

“At this point, fungicide treatments are likely not warranted,” Zeleznik says. “Fungicides for ash anthracnose should be applied only before the disease hits, and we’ve missed that window of opportunity.”

The amount of leaf loss can be worrisome to some people, but it is not considered major until 25% or more of the leaves are gone, according to Zeleznik. At that point, a tree can begin to get stressed. Trees that lose this quantity of leaves for three or more consecutive years are the most susceptible to decline.

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About the only action you can take now is to help defoliated trees grow new leaves by giving the trees a light application of nitrogen fertilizer, and Zeleznik recommends the rate of 1 to 2 pounds per 1,000 square feet.

In the fall, raking and destroying fallen leaves also will help minimize next year’s disease pressure because the fungus overwinters in dead leaf tissue, among other substances. “Mostly, though, it’s simply a matter of waiting out the disease and allowing nature to take its course,” Zeleznik says.

Keep up with "Fielding Questions" and "Growing Together" columnist Don Kinzler and see his past columns by clicking here.

Q: Someone told us that it’s not good to put rocks and brick around our boulevard trees. What are your thoughts, and if it’s not recommended what is your suggestion to put around trees to make them look nice and be able to continue to grow well? — Linette K.

A: Putting a circle of mulch around trees has definite advantages. It’s easier to mow without damaging the tree trunk, and mower and trimmer damage is a common killer of trees. A grass-free circle around young trees reduces competition from the turf for water and nutrients.

So mulched circles around trees are good, but the preferred mulch is wood chips or shredded bark instead of rocks. Rock mulch around a tree won't automatically kill the tree, but there are definite disadvantages which can be detrimental to the tree’s health.

Rocks around a tree heat up on hot summer days, which many trees don't like. Rocks tend to eventually mesh tightly together and in many cases the tree trunk can't expand because the rocks won't push away, and rocks become embedded in the trunk. Rock mulch is heavy and causes soil compaction.

Wood chips or shredded bark keep the soil cooler, don’t compact the soil and are more naturally compatible with tree growth than a layer of rock.

Q: I always hate to thin out my carrot row, because it seems like a waste, but then they grow too thick and don’t develop. What’s the recommended spacing? — Jim L.

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A: To develop full-sized carrots, plants need to be spaced 1 to 2 inches within the row, so excess seedlings should be carefully pulled or cut away. Thinned seedlings are delicious in salads and soups.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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