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.
Punch holes about a half inch wide and 5 to 6 inches deep in the soil. You can almost hear the soil taking a deep breath as a sign of relief. These holes allow water penetration into the root zone which softens it. After a watering, the roots take off in the openings and you'll have happier grass in a few days. I can't stress the importance of growing the right grass varieties for our area.