It definitely looks like that area isn't get enough water. Also, once a week deep watering wouldn't work in Fresno. We need 3 days a week because of the heat and dry air 24/7. I have a feeling that your area might be the same.

I have pop up spray sprinklers.

Will measure with tuna cans again. Admittedly, I don’t think I measured in the corners. Probably placed them more in the middle of the lawn.

Upon closer inspection, looks like there are some yellowish spots on the blades of grass that are still alive. Hope this isn’t some disease or fungus (rust?).

Yeah, that is rust.

Pop-up sprinklers doesn't tell much. What brand? Who designed the irrigation system? Were heads ever broken and possibly replaced with the wrong output?

They are Rain Bird 1800 sprinklers. We
Recently renovated the house and my contractors gardener did the irrigation system. Pretty sure all the heads were brand new.

CA Kate said, Also, once a week deep watering wouldn't work in Fresno. We need 3 days a week because of the heat and dry air 24/7.

That's weird because in Phoenix, where the temps run in the low 100s during the day and sometimes in the night, they deep water once every 5 days. If your grass needs water more frequently than that, you might have the wrong grass. The only turf type grasses appropriate for Fresno are bermuda, St Augustine, and buffalo grass (UC Verde, in particular). If you have Marathon fescue, you most defs have the wrong grass. There are some pasture grasses you could grow which look okay if you mow every week.

After burning the front lawn with Scotts Weed and Feed, but not experiencing burning with my backyard after adding Milorganite, I think I’m definitely sold on organic fertilizer. That’s a pretty striking photo showing the effectiveness of alfalfa.

Regarding the rust, do I continue to mow high and water deep/infrequent? Read online that proper soil conditions (I.e. nitrogen), watering and frequent mowing is about all you can do and it will eventually go away. But I’m worried about it spreading through my whole lawn. Just noticed it is in my front lawn now too. Should I re-seed the brown patches corners of my lawn?

Rust looks like this.

Although my case may not be as severe as those pictures from the google search, I think I definitely have a case of rust.

Here are a couple pics of my lawn mower with orange dust after my last mow:

Also checked sprinklers and it’s likely the brown patches were around sprinklers where they were shooting so far there could be areas where the area under the arc is not getting enough water. Tried to dial down the spray distance to see if that will help.

Durwin you have lawn rust. The orange dust on the mower is the smoking gun and if you take your new fancy I phone and magnify a picture you'll see the fungus on the glades.

It's in the corner by the sprinkler head because it's where the grass gets the most water and as the water evaporates it makes humidity and what Rust fungus love. Water in the morning.

@kevin9408, any other tips to get rid of it. I’ve always run my sprinklers at 5:30am.

Feed a bit extra in that location to overgrow the rust; rust is not a strong fungus and the best way to combat it is to simply overgrow it. Fungicides are generally strictly third or fourth on the list of things to try, after cultural solutions.

5:30 is fine for running the sprinklers. The sun rises, the grass dries off, that's perfect.

I endorse morpheuspa's recommendation and is also the same approach if you had red thread fungus. A Fertilizer with a NPK ratio of 3:1:2 or 4:1:2 is needed like 18-6-12 to get rid of the fungus and not 6-4-0 like milorganite has.

I'm not a fan of Milorganite, it has no potassium in it and potassium is essential to life. To name just one of many reasons potassium in needed is its ability to help protect plants from fungus attack. Milorganite's name to fame is the high iron content it has, 2.5% and is what makes the grass lush and green. The low nitrogen content and slow release will make fungus laugh at it.

In this particular instance, I agree. Don't use Milorganite and, when trying to overgrow rust in June, don't use a slower organic. Go with a synthetic, even this late, just use it at half the normal rate.

Without knowing what the soil balance of Durwin Tsay's soil is, I'd normally hesitate to add other resources. However, potassium is harmless enough that a little extra doesn't bother me and I'm not troubled by kicking a little extra in.

In this case, a fertilizer around 25-0-5 would be just fine, at half the normal rate, to overgrow the rust. Feed that small area every 3 weeks or so--a total break of the rules but, in this case, called for.

If that fails, then by all means, use any general fungicide like DiseaseEx. Rust is not a strong or durable fungus and should go down at the first blow. We avoid using fungicides because we don't want it to become resistant.

(Milo actually does contain potassium, but it comes in at just under one percent and has to be listed as zero on the bag for that reason. Corn, sold as fertilizer, has to be listed as 1-0-0 even though most of us report it more accurately as 1.6 - 0.6 - 0.4 due to the poundage used. Horse manure is technically 0-0-0, but in reality, 0.44 - 0.17 - 0.35).

Yes the red dust on the mower or on your shoes is the main way to ID rust.

Thanks to all for the suggestions. Really appreciate the help.

I have leftover Scotts Turf Builder Lawn Food which is 32-0-4 that I could use at half rate. I noticed rust in other parts of the lawn, albeit not nearly as severe. Should I just apply to the whole lawn to ensure total overgrowth of the rust and prevent further spread?

I'd only bother on the areas where you see rust. Through summer, rust usually becomes less prominent in the heat.

I get rust and red thread in October, but I tend to simply ignore it. During fall, my lawn (as others should be) is very well-fed and outgrows both very quickly. Conditions are such that rust and red thread simply happen.

Great, thanks again!

Update: Have applied two doses of Scotts Turf Builder Lawn Food (32-0-4) at half rate twice (3 weeks in between). I also re-measured my sprinkler output and as a result, increased my weekly watering. Still not out of the woods with the rust. Below is what the main affected areas look like now. There are other areas where I can see the rust but hasn't been browning (yet).