A. After nearly 20 years I would think that your lawn is a mixture of several types of grasses. Unless you are extremely diligent in taking care of your lawn, I would guess that you have a mixture of grasses including older tall fescue and common Bermudagrass. 90% of the problems on lawns in the hot desert are related to irrigation. The next most common reason for brown spots to occur in lawns is from disease. Insects like white grubs is not all that common here on tall fescue and bermudagrass. But it can happen.
Make sure this is not an irrigation problemTurn on your irrigation system and watch the sprinklers. They should pop up out of the ground perfectly straight and not tilted in any direction. The water should spray evenly from the nozzle in all directions. After you run a normal irrigation cycle, use a long screwdriver to judge whether the soil is wet in the brown areas.
Brown spots caused by grubsInformation you will find on the Internet about damage caused by grubs comes from cooler parts of the country where Kentucky bluegrass is the common lawn. We seldom grow that here. Our grasses are primarily tall fescue and Bermudagrass. When tall fescue gets damaged by grubs, it doesn't lift up like a carpet.
That information is wrong for us. If grubs are causing damage to a tall fescue lawn in our hot, desert climate, individual grass plants will pull up easily at the edge of the damaged area. Pull on green grass In several places right on the edge between the damaged area and a healthy lawn. If green grass pulls up easily from these spots then your lawn has a good chance of having grubs. Any of the grub control chemicals should stop it if you follow the directions exactly. Scott's Grub-ex granular is a good product as well is the Bayer product you mention. If you go in this direction, use just one of them.
DiseaseIf irrigation and grubs are not the problem, then it is likely a disease problem by default. Disease problems run their course and then they are over when the weather changes. If this problem repeats every year, consider applying a fungicide as soon as you see a problem beginning to develop. Fungicides are preventive.
You should consider aerating the lawn if it hasn't been done for a while. This helps to open up the soil for better water movement to the roots. It can help to prevent disease problems. You can aerate the lawn anytime but the best time is in the spring months before it starts to get hot.
Dethatching or Vertical Mowing
Consider removing dead grass from the lawn if the lawn feels "spongy" when you walk across it. Removing this dead grass requires a power rake and should be done in mid fall or very early spring to give the lawn a chance to recover and grow a little bit before winter or hot weather.
Importing Disease Problems
Some lawn diseases can be carried into your lawn from other lawns. Improve the general health of your lawn by using a good quality lawn fertilizer or applying a thin layer of compost to the lawn during the spring or fall.