Question
For the past five years, a 6-7 inch completely yellow (no green grass in the middle)circular spot occurs in my lawn from mid-May until August; In May the lawn is green and one morning it's there; one day in August the yellow spot is there and the next morning it's green. I use no chemicals on the zoysia and the rest of the lawn is beautiful.  My blade is up as it helps control my weeds and I like the bounce. The backyard is a healthy fescue. At first I blamed the dog, but he doesn't go near it and it's just that one small circle.  

Answer
Hi Michele;
That is really odd. If it grew, that would be uinderstandable, a fungus. But to appear in the same spot, and stay the same size, year after year???
I would suspect something else like a leak in a gas pipe or something, but it doesn't seem like that would occur at the same time of year, and in the same place, and go and come so suddenly.
That has me completely baffled.
I have never grown Zoysia, so I don't know if there is some ritual that variety of grass goes through,althought I can imagine what it would be, but ask nurserymen in the area.
I think I would dig in that area and see if there is some rotting wood etc under the soil there that causes a fungus, even though this is not acting like any fungus I have ever experienced.
If you do find out what is causing this, please come back and post and tell us what it is. There may be another question on this sometime, and you can supply us with an answer for these others.
Charlotte