Q.lawn
I had a new lawn laid last year, and although it was lovely and green and growing well, I cut it on a high setting on the mower and DID NOT fertilize it. After about 2 months, it started dying off in huge patches….now I’m left with a dreadful mess with what looks like lots of worm casts, which I brush off, but then they seem to reappear overnight. Someone has said it may be contaminated topsoil which was used, as it was raised about a foot in height on top of my old lawn which I never had any problems with. Is there anything I can do without having to dig the whole lot out? Many thanks.

Certified GKH Gardening Expert
It is possible that a toxin or disease agent in the topsoil is responsible- maybe you could contact the supplier and find out the source.
Sometimes the death of new lawn grass is caused by timing (planting in summer when it's too hot for young grass to thrive) or by drought conditions in your area. On the other hand, excessively wet conditions can kill new lawns by promoting disease, as discussed here:
https://ask.extension.org/questions/125039
Grub damage is another possibility:
https://extension.illinois.edu/lawnfaqs/grubs.cfm