For the last 2 years in the warmer months my lawn develops patchy dead spots. I have sprayed fungicide and also searched for any bugs/beetles grubs without success. The lawn usually recovers but takes several months to do so. The dying off occurs very quickly, usually within a matter of days. Please see attached photos. As you can see there are patches of green grass amongst the dead patches that seem unaffected.
Any ideas on what it might be would be much appreciated. Thanks!