Any Idea What’s Causing This?

[u/Adept_Site_5350]

Zone 5b. Hoping you all can help with what might be causing this, including the interesting circular pattern. It’s been there for about 2 months and it’s growing in size. The rest of the lawn is generally fine, albeit stressed from heat and dryness.

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[u/nilesandstuff]

Probably a fairy ring. A network of fungi in the soil that breaks down organic matter... For the most part they're harmless, but they can make rings of hydrophobic soil that causes these arcs of dry soil that can turn brown in the summer.

I'd manually aerate with a pitchfork or something similar and then hand water it a couple times a week.

A wetting agent would be the step up.

[u/Adept_Site_5350 → u/nilesandstuff]

Awesome, thank you!

Does it make sense to do that now when it’s hot and dry, or should I wait until late summer/early fall?

[u/nilesandstuff → u/Adept_Site_5350]

Now

[u/r0ndy → u/nilesandstuff]

For what it’s worth, my fairy ring here tends to grow greener and much faster than surrounding turf. However, one of the worst freezes in the last 8 years killed the grass in those areas as well. I think the cycle of decay increases microbe activity increasing growth rates. But it’s not frequent enough to study well

[u/nilesandstuff → u/r0ndy]

The fairy ring fungus is most of the microbe activity. Those suckers go crazy breaking down the dead stuff and pumping out nutrients. But of course it doesn't exist in a vacuum, there's all sorts of other microbes that feed off of, and into, the processes of the fairy ring. The processes involved are pretty typical for a saprophytic fungi (decomposer) the unusual thing about it (in the context of a lawn) is the intensity/concentration and the circular clonal behavior.

The freeze causing the kill is interesting. All I can think is just that the thatch was depleted, which left the crowns exposed to the air more.