It sounds like a fungus to me. You can spray with a
baking soda fungicide which in my experience helps
prevent further spreading. I've only had so-so luck
with corn meal.

You didn't mention what kind of grass you've planted
but 4"+ sounds awfully high to me. I wonder if you're
getting good air movement at that height?

Rust fungus?

The grass is a mix, kgb/fescue. It's actually the thinner stalks that appear to be affected, which I believe is the fescue.

In terms of the length, I've wondered myself if it is too long, but I thought longer was the way to go. Maybe I'll do the second highest notch on the mower instead of the highest.

Assuming it is a fungus, how can I treat it naturally? Do I make a baking soda fungicide myself?

Red thread or rust? Both kind of surprise me as the season's been kind of dry lately for MA and PA, although I've seen both be perfectly happy in hot/dry weather.

I think the ratio is 4 tsp of baking soda per gallon of water, but check me on that. I'm not sure if sodium bicarbonate is salty enough to do damage in hot weather, but if I were applying I would assume it is--and not apply on a 90 degree day.

I'm not a big organic guy, but have read lots about it on here. I don't know if corn meal will work for red thread or rust, but it won't hurt to give it a try as it also acts as a fertilizer. 20lbs/1000 sq ft is good for a treatment. 10lbs/1000 sq ft about every 45-60 days is good as a preventative measure.

When was the last soybean meal application? If it was 3 weeks or longer, then why not give the entire lawn a 1/2 dose of corn meal, and add a full dose on the affected area.

Your lawn will love the extra nitrogen, the microherd will thank you too. And this would help prevent any fungus from getting started anywhere else.

While you're at it, over the next two cuttings, bring the cutting height down to 4" then 3.5". The appearance will be better, the weed seeds will still be shaded, and the grass roots will still be plenty long because yolu'll probably let the lawn grow to 4.5" to 5" before you cut back to 3.5"

Can I do both cornmeal and the baking soda fungicide? I sprayed 1 gallon of my homemade fungicide on the area today (recipe from the internet--1 1/3T baking soda, 1 t. soap, 1 gallon of water). I don't know if that is enough, but it seemed kind of light. How often should I repeat the application of this? We'll have some thunderstorms daily for the next few days so it might wash away. By the way, I think last year I had this same fungus in the same spot--what's up with that? Only last year I treated it with chemicals since it was before I turned organic. And last year it (the fungus) killed half my lawn.

Today I also mowed at 3 7/8", though I discovered after I finished that one of the notches had slipped so it might have been a little shorter.

My regular grain place is out of corn meal. Could I just buy it from the grocery store and apply it to the affected area (which is seeming bigger...)? Can I do both corn meal and baking soda fungicide?

Sorry for all the questions, but I appreciate everyone's help! I thought I was off to such a good start this spring, so I'm really disappointed. I'm too emotionally invested in my little lawn.

"My regular grain place is out of corn meal. Could I just buy it from the grocery store and apply it to the affected area (which is seeming bigger...)?"

I have exactly the same question... is this the same as the stuff you can buy in the grocery store? I'm planning on using cornmeal on the lawn this fall for recurring snow mould over the winter. If someone could please answer it would be appreciated. Thanks.

Could I just buy it from the grocery store

Yes, but it's usually more expensive from there. They grind it finer, and it's listed as fit for human consumption.

If that's your only option, then that's fine. If you only have small areas, then the gas to go to the grain mill probably more than makes up the difference!

newlyorganic, I cannot add to the constructive advice you have already obtained but I certainly wish to compliment you on monitoring your landscape so regularly and carefully. Whatever tne condition and whatever the cure, you stand a better chance of success for having jumped on it so early.