What do you recommend to treat grass fungus?

Call or visit your local nursery to get more information for your area and project. I never heard of grass fungus. You learn something new everyday.

cut back watering, let grass dry out well between watering,rake up brown areas to promote aeration/air circulation & throw dead grass in trash, area will regrow, don't overfertilize lawn use slow release fertilers only

If the weather prohibits the things that Lynn recommends, maybe grass is not a good solution for you.

Applying anti-fungals on large areas like grass is a lot of work and expense, for something you can’t eat.

Consider regrading the yard to accommodate better drainage, put in bordered gravel paths, rock gardens and other alternatives.

Definitely contact a company that specializes in grass care