Fungus among us. Melting out starts with patches slowly dying and looking dry. They have indefinite, irregular borders. Brown patch causes turf to turn a light brown color in circular patches from a few inches to 3 feet in diameter. Fusarium blight shows up as scattered patches of wilting turf that become light green and then fade to a straw color.
Greasy spot starts with small patches that coalesce and form a large, irregular patch of dead grass.
Let me stress to you before you reach for the chemicals that you can control most diseases by better watering and fertilizer practices. A breath of air. I also find compacted soils prevent water from penetrating the ground, which leaves the grass in a stressed condition and diseases soon follow. Anytime I see a stressed area in a lawn, I probe it with a long screwdriver or auger. Aeration is the recommendation to loosen the compacted soils.