On Feb 26, 6:17 pm, "Neil" wrote:
The grass has gone really thin in a triangular area in each corner, and I
can see a lot of little mud spirals on the surface instead. The areas
affected are visibly growing each week.

I'm getting dim memories of worm casts here, but I can't see how that
could harm the grass. Perhaps the thinning of the grass is making the
casts more visible, but the problem lies elsewhere.
--
Rob