I have a lawn that is bluegrass sod on heavy clay. I have been using Natures safe organic fertilizer and have had good visibible results. The past few weeks I have noticed discoloration in the lawn with lime green patches appearing throughout the lawn. I am able to lift the grass and roots and move them, lifting them up as if there were grubs present, but the sod does not pull back in the same way. There are not grubs in any of the root zones sampled, and the lawn was treated with Mach 2 about 6 weeks prior. I have never seen this before, and am trying to identify it if it is a fungus or not. This is not a case of POA Anna, or POA triv. Help!