necrotic ring spot:
Necrotic Ring Spot is another serious lawn disease that can destroy a lawn very suddenly. Its patches or rings of dead grass are brought on by dry, followed by wet, weather. Lawns 2 to 8 years old are favorite victims. The disease is most active in spring and fall, but it also shows up in summer during times of water stress. If your lawn has Necrotic Ring Spot, you'll first notice scattered light yellow patches, usually 2 to 6â€ wide. The disease leaves a ring of dead turf that becomes brown or straw color with a healthy-looking green area in the center. This has prompted the nickname frog-eye disease.
https://www.spring-green.com/lawn-patch- ... eases.aspx
Here's what they suggest:
â€¢Regular core aeration helps nutrients and water reach roots and breaks up thatch, which is one place diseases like to breed.
â€¢Deep, infrequent watering in the early morning, keeps moisture from remaining on the grass surface too long, which attracts disease.
â€¢Mowing frequently, at a high height and with a sharp mower blade, further helps to keep disease out.
â€¢Overseeding with disease-resistant grasses is another option you can consider.
Here's another article:
Necrotic ringspot (NRS) is a perennial disease of Kentucky bluegrass.
NRS results in circular or doughnut-shaped patches of dead grass.
Symptoms often develop in late summer.
NRS can be controlled by the use of resistant varieties, good turf management practices and fungicide applications.
Necrotic ringspot (NRS) is the most destructive disease of Kentucky bluegrass in Colorado. The disease also damages red fescue and annual bluegrass
Overseeding diseased patches. Perennial ryegrass seed germinates rapidly, competes favorably with Kentucky bluegrass and is immune to NRS. Thus perennial ryegrass can be used to quickly fill patches created by NRS. However, the texture and color differences between perennial ryegrass and surrounding bluegrass may be objectionable to some. In theory, overseeding with one of the resistant bluegrass varieties listed above should also be possible, although attempts to do this in University field trials were unsuccessful. Overseeding will not prevent the development or intensification of NRS in other parts of the lawn.
Do not overwater. This is perhaps the most important management practice for NRS. It is tempting to irrigate lawns with a history of NRS more frequently. However, this will enhance the disease. Water the lawn to a depth of 6 to 8 inches as infrequently as possible, usually no more than twice a week, without creating water stress. Also check to make sure irrigation heads are working properly and limit overlapping sprays that may create puddles in the yard.
Follow good management practices on established lawns. Maintain the turf at a height of 2 Â½ to 3 inches. Remove no more than 1/3rd of the blade at any one mowing. Since the NRS pathogen attacks the roots and not the leaves, it is not spread by mowers. Returning clippings with a mulching mower may actually help turf recovery by recycling nitrogen during the leaf decomposition process. Core aerate established lawns at least once a year (spring or fall) to help reduce thatch buildup and improve soil drainage
Avoid applying excessive amounts of nitrogen fertilizer
Sulfur amendments. Sulfur amendments can reduce NRS severity. The exact mechanism by which this occurs is still unclear, although it is thought that sulfur acidifies the soil surrounding the roots and inhibits, either directly or indirectly, the NRS pathogen