Brown Patch is a fungal disease affecting the roots of lawn grasses causing small discoloured patches that grow to large patches of dead grass up to 1 metre. When looking at the grass roots they will be turning black.
Favourable conditions for its occurrence are warm humid weather patterns, particularly when soils are poorly drained, and the lawn is heavily thatched. You can minimise the chances for a brown patch with a good lawn care regimen by not over fertilising, watering early morning so the grass can dry out fully before nightfall, aerate and dethatch the lawn annually and applying fungicide where appropriate.