It sounds like Take-all-Root-Rot from what you explained about the short black roots. Cut out a 1x2 foot section of the affected area and take it to your local extension office to have them examine it/determine what it is. This disease is primarily observed during the summer and early fall months. By the time the leaf symptoms appear, the pathogen has been active on the roots for at least 2-3 weeks, probably longer. Any stress placed on the grass will encourage or worsen the disease. This disease is very difficult to control once the above ground symptoms are observed. Therefore, measures that prevent or alleviate stress are the best methods for completely controlling the disease or at least decreasing the potential damage. Because the roots are affected, they will not be able to efficiently obtain water or nutrients from the soil, nor will they be able to store the products from photosynthesis. Here are a few things to do NOW. 1. Frequent foliar (leaf) feeding of all nutrients (N, P, K and micronutrients) in small amounts will be necessary. The roots are not functioning properly, and so will not be able to efficiently obtain nutrients from the soil. Spray Soluable Seaweed powder weekly with a pump sprayer/fine mist until the leafs are wet. Spray in the early morning or late evening. Use molasses as a wetting agent. 2. Extra potassium is useful in late summer and early fall. Use Sulfate of Potash if staying organic. Apply immediately @ 2-lbs. per 1000sqft. Do not apply lime or herbicides. Raise the mowing height to the highest recommended height/slightly higher until conditions improve. Mow as frequently as necessary so that only one third (1/3) of the leaf tissue is removed.

I'm guessing that you are watering too frequently, possibly daily, which has led to the lawn developing a fungal disease. Notice that there is a distinct arc to the brown blades from the bottom right of the picture to the middle left...that is another clue that it's a fungal disease and not caterpillars (worms). Quite often a fungal disease will form a near perfect circle. Can you post another picture? Get about 3 inches away from some of the grass at the edge of the brown area. The best pictures are taken on a cloudy day or in the shade to reduce the contrast. Fungal diseases show up on the blades of grass. If it is a fungal disease, my recommendation would be to apply ordinary corn meal at a rate of 20 pounds per 1,000 square feet. Corn meal attracts a predatory fungus which feeds on the disease fungus. It goes to work immediately but it takes about 3 weeks to see the improvement as the green grass returns. I have used corn meal successfully every year since 2002 to eradicate disease in my St Aug. Although, now that I have Floratam in the back yard, the grass fights off the disease faster than it can spread. I still see signs of disease...and then it's gone. If you don't have Floratam now, you might consider putting down one flat of it and letting it spread into the lawn. If you had Floratam in the lawn currently, it would be spreading into the thin (brown) areas and taking over even while the disease is present. Cool stuff. Oh and as to the watering, this time of year you should be watering no more frequently than once every 2 weeks in your area. If you get high temps in the 70s, then you could back off to once every 3 weeks. If you are not doing that, please write back and tell us what you are doing so we can get you backed off to watering less often.

Okay then good news about the top dressing. It could be so old that it was never graded properly the first time. Or it could be optical illusions. My inlaws in El Monte had a perfectly flat lawn where the soil met the concrete at the driveway and sidewalk with no bulges, so in that respect it was perfect. They didn't water, so it was a mess, but... Yes, dig for grubs just for your own edification. You might find as much as a few per square feet. That's fine up to a dozen per square feet. If you find it squishy with hundreds of grubs, then you should have treated it in July. Treating now is like closing the door after the horses are out. They have stopped feeding for this year. Hot? Okay here's the watering schedule broken down by temperatures. Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall. If once every 5 days works in Phoenix at 115 degrees, then 2x per week would never be right for the San Gabriel Valley. It is likely, too, that 1/2 hour is not enough to get an inch. Here's another tip: spray the yard with any clear shampoo at a rate of 3 ounces per 1,000 square feet followed by 1/2 into to 1 inch of irrigation or rain. That will help the soil absorb the water much deeper. j4 is a non-believer in corn meal. I don't know if he's tried it, but it has worked for me every year for the past 14. I have never let my lawn get as bad as your mother's is. It does look dead. I have recovered a dead lawn, but it was from the outside in and required serious patience. Mine was in the Texas (almost) desert and caused by a broken sprinkler system before I moved in. St Aug only spreads when temps are in the 70s, so fixing it in the heat took a little longer than I had hoped. I fixed it with deep and infrequent watering. You might be surprised to see new grass popping up in the middle of the dead. Or not. Just because everyone is mowing short in the area, that won't make mowing short the right thing to do. St Aug will absolutely thrive at 32 inches tall. Here's a picture of my dog walking through my semidesert St Aug. You can see her tail and ears. That grass was never fertilized (or mowed) in the 2.5 years I lived there. When the grass grows up tall it grows deeper roots which can get moisture and nutrients from much deeper in the soil. These roots were in uncharted territory picking up nutrients from way deep down. As it turns out you don't have to water nearly as frequently when the grass is up tall like that. I had one spot that grew up to 32 inches quickly. I never watered that area in 30 months through a full year of drought and then some rains. This hyperinfrequent watering was the point of growing the grass that tall.

Those caterpillars might be part of the problem, but you definitely have a fungal disease. The corn meal can go down while it's raining or during a drought. Doesn't matter. As to whether it flows through a spreader, you probably already know the answer to that. No, not very well. Corn sucks up all the humidity in the air and starts to clump in the spreader. If you have an uneven soil surface that causes the spreader to bounce, that helps. I spread it by hand. I have a 2-pound coffee can that I fill. Then I "eyeball" about a 10x10 foot area and try to put the 2 pounds of corn meal evenly on that area. As to the caterpillars, the BT Worm Killer should kill them all in 24 hours. Nothing works faster. If there are more eggs waiting to come out, then you will have to repeat. I believe it would be impossible for you to spread the eggs and certainly impossible to do it as efficiently as the moths do it. They lay the eggs in the soil. I would not use another insecticide. That would be overkill, a waste of money, and an unnecessary contribution to pollution. But while you are at this, you should dig up 1 square foot of soil looking for grubs. If you see fewer than a dozen grubs, then cover it back up and leave it alone. If you see hundreds of grubs, then go back and do the granules for grubs. Have you sprayed shampoo on your lawn? Shampoo helps with water penetration in all types of soil. The application rate is 3 ounces per 1,000 square feet. Follow that up with 1/2 to 1 inch of rain or irrigation. In your case just spray it and let the rain take it down into the soil. You defs don't need more water for awhile. Deep and infrequent watering only works on established turf and only after the roots have grown deeper into the soil. New sod needs frequent watering to get the roots into the soil at first. This lawn will recover from all the above. Stick with it.