There are lots of fungus types. How do you know you have them. They usually seem to rear their ugliness in the warm days of spring or hot days of summer and usually when the low temperatures at night are above 70 degrees.
Here are couple easy ones to find
One easy type has a pinkish color and is called Red Thread or Pink Patch. Another easy one to find is most of the grass is green but you have some spots about 6 inches to a couple feet in diameter. This could be dollar spot or brown spot disease.
Basically if you lawn isn’t green it is probably because it needs to be watered or has a disease issue.
What can you do?
Water it correctly, 1 inch of water a week and only in the early morning. If you have identified a fungus issue then apply a fungicide. Most of these provide protection by keeping the fungus from spreading so once it is there the grass has to grow out of the brown spots.
You can do nothing. A lot of the fungus issues will go away and the grass will grow back when the cool, rainy weather of fall arrives.
Grass seems to grow anywhere and under the right conditions. Keeping it looking its best in the summer is challenging but remember how much you have to mow in the spring? The grass always seems to grow like crazy under the right conditions.