A very common fungus you will see this time of the year is one called Take All Root Rot.  It has several other name such as Dead Patch, Die Off or (grass name) Decline but they are all the same thing.  Sometimes it will show up in the fall but is most common in the Spring and early summer.  The first signs you will typically see are a lime green yellowish color on the grass blades.  There will also be some dead or dying patches of grass in the area.  The fungus affects the roots and is in the thatch layer so this discoloration and dead grass is usually a delayed sign of an already present and active fungus.  When these signs show up it is important to act fast because with Take All there are deadly consequences for the area in question.  We take about several things that cause this issue, ways to handle it and ways to treat it.  Let us know if you have any questions or need any help identifying a possible fungus in your lawn.