The turf disease commonly known as "brown patch" is caused by a naturally occurring organism in the soil whose purpose in Mother Nature is to break down dead organic matter. The problem occurs when moisture and temperature conditions fall with in a certain range, the organism can jump on live tissue and begins to break it down.
There are a couple ways you can usually tell if this is happening. One way is by gently tugging on the individual grass blades that look affected. If they easily detach from the runners, the disease is likely present. The reason for this is the disease attacks the turf where the individual blades are attached to the runners. Another obvious sign of brown patch is a very distinct circular pattern of dead grass in the lawn that keeps getting bigger. The reason for the pattern is the disease starts in a small area and progresses outward in all directions.
Since abundant moisture is a main catalyst to turf disease, one of the first things that you should consider if the disease is present, is reducing the amount of irrigation in the lawn. After that, fungicide treatments may be appropriate.