"Is there anything we can do to treat the fungus?"

If you would like to avoid chemicals then set about aeration using a garden fork for starters. This will let air into the root zone.

One of the major reasons you will have this fungal disease is lack of air circulation in the garden, probably overhanging branches which are dripping water onto one spot constantly and low light levels.

Improve the soil condition by top dressing and overseeding and a regular feed and you will start to restore this area to grass..