Experts and BHG readers answer.
What should I do about lawn spots?
A number of different diseases produce that pattern in a lawn: yellow patch, necrotic ring spot, summer patch, and fusarium blight. All of these diseases are fungus-based. For immediate control, you can apply a systemic fungicide according to label directions (check with a local garden center or county extension service to see what is recommended for your area), but, as you are finding, fungicides are expensive, and you may need to use several applications.
Most healthy lawns are able to outgrow a bout with diseases; you may just have to tolerate some discol-oration while the lawn recovers. Keep your lawn as healthy as possible. Fertilize regularly to encourage the lawn to fill in. Reseed seriously affected areas. To prevent necrotic ring spot, dethatch and aerate your lawn regularly. Irrigate when necessary to avoid drought stress, and apply lawn fertilizer at regular intervals. During hot summer months, set your mower a little higher than during cool periods.