How to treart red thread disease in lawn grass

First, you need to improve your drainage. Do this by spiking the lawn with a fork at approx 1 ft spacing. The soil fertility will then need to be improved, but do not use nitrogenous fertilisers after September, as this may encourage Fusarium wilt. A few fungicides are effective but avoid the ones containing mercury.
Immediately after mowing, Benomyl or thiophanate-methyl should be applied as an eradicant and/or preventative treatment, in accordance with mfrs. instructions.
Hope this helps. I seem to remember getting this question in an exam a few years ago.