Sounds like maybe "fairy ring fungus".
<http://en.allexperts.com/q/Lawns-725/Ring-dead-grass.htm>
That link gives the treatment as iron sulfate. But this link
<http://turfgrass.com/ubb/Forum13/HTML/000041.html>
gives an interesting treatment using ordinary corn meal. It says
-----------------------------------------------------------------------
In case anyone missed the answer to this on another message, I'll repeat
it here.
Of course fairy rings are caused by fungus, hence the mushroom circles.
Sometimes the mushrooms don't grow but the grass will die in a perfect
circle. Sometimes you will see lots of circles ranging from 6 inches in
diameter to 50 feet in diameter.
The cure for all (yes, all!) fungus disease in turf is whole ground corn
meal. This is the same corn meal found at the grocery store. I get mine
at the feed store in a brown bag labeled, "feed." For $5-$6 you can get
50 pounds at the feed store. The application rate for prevention and
fertilization (it is also a great organic fertilizer) is 10 pounds per
1,000 square feet ($0.0011 per square foot). The application rate for
existing fungal disease is 20 pounds per 1,000 square feet ($0.0022 per
square foot). Don't let the feed store folks get you confused with corn
gluten meal. They are two different products. You want corn meal, chops,
or any whole corn kernel that is ground up. If you try whole corn seeds
like some animal feed, you will end up with a field of corn. It must be
ground.
The Texas A&M University at Stephenville has found that ground corn meal
works like a fungicide because it attracts a member of the Trichoderma
(try-ko-DER-mah) fungus family. Trichoderma acts like a disease to the
disease-causing fungus and kills it in about 3 weeks. If I remember
right, the Trichoderma attacks the cell walls of the other fungus.
In peanut fields, corn meal works as well as crop rotation for
eradication of many different fungi diseases. This means that peanut
farmers no longer need to spray fungicides nor do they need to rotate
their peanut fields out of the prime cash crop.
The same fungi that attack peanuts also attack turf
------------------------------------------------------------------
Chuck Norcutt
Jim Nichols wrote:
> I know there are many knowledgable people on these two lists. I am looking
> for a hint on the cause of a circle of dead grass that is very well defined
> on my lawn and extends, faintly, into the adjoining lawn.
>
> I recall that a cherry tree once existed in this general area in the
> neighboring lawn. I had a maple in my yard that was damaged by a wind storm
> and removed. However, neither of these was near the center of the circle. I
> have heard that maple roots can cause problems in grass, but no
> previously-existing root circle matches what I now observe.
>
> Anyone got a suggestion?
>
> http://gallery.leica-users.org/v/OldNick/Lawn+Circle.jpg.html
>
> Jim Nichols
> Tullahoma, TN USA
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