Until just a few days ago, I would have agreed with Thunderbird, especially since he always knows what he talking about. However, in a discussion under the title of "fairy rings" on this site that I can't locate right now, a subscriber described a product called Armillatox that comes well recommended. I find it hasn't been approved for use here in the U.S. except for testing, but it's basically a soap based product that one injects into the ground... you might give it a try...