Answer: Could be a fungal disease or feeding by grubs - I can't tell without examining the turf. Sampling for grubs is a good first step in determining the cause of your lawn problem. Random holes throughout the lawn will give you a general idea of how large the grub population might be. You can cut a square foot of turf out, pull it up and look on the soil surface and on the roots for fat, white grubs; less than six grubs per square foot can usually be masked by water and fertilizers. Populations between 10 and 15 per square foot can cause significant turf damage in September and October. Of course, populations occasionally reach 40 to 60 grubs per square foot and these levels can cause significant damage. If you find grubs you can treat your lawn with a product like Grubex or Bayer Advanced Season-Long Grub Control granules, applied according to label directions. If you don't find grubs, you might want to take a sample of the yellowing turf to the cooperative extension office for diagnosis. Helpful folks there can positively identify the problem and suggest control measures. Contact Maricopa County Cooperative Extension, 4341 E. Broadway Road Phoenix, AZ 85040-8807, Phone:602-827-8200. Good luck with your lawn.
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