Going to take more than one app but propiconazole should knock it out. You could also do the Bayer granules (longer lasting,same active ingredient) or try Daconil.

And yes, the long length is inhibiting airflow and making the problem worse.

A link on grey leaf spot

^That's a great link.

Whatever you use be sure the label says it is safe for St Augustine. Also check the temperature requirements on the label. Most limit the max air temp to 80 or 85 degrees.

I am less certain that the grass being tall is a problem. I say that because I had Floratam St Augustine at 32 inches tall for several years, through drought and flood, in Texas with absolutely no problems. I've never seen any research on really tall grass, so I experimented with it myself.

Don't expect anything but fertilizer effect from the corn meal. Corn meal depends on a fungus called Trichoderma to be living in the soil. You sprayed that out by spraying the fungicide first.

Tall grass in and of itself isn't a problem until excessive rainfall, humidity or nitrogen is introduced into the mix. Each of these, or a combination of all three, provide a hospital environment for fungus to thrive. Of course, other factors such as soil health play a role and not all fungus is bad. Compared to Houston, I would imagine San Antonio has far less fungal problems.

Houston has much denser soil than the rest of Texas, our soil is slow to accept moisture, then holds it for too long

Dchall, to clarify I only put the bayer fungus killer in the front yard. The back yard has only gotten corn meal.

We've continued to get rain everyday and the grass has stayed wet. The areas where I put cornmeal down are now growing mushrooms, probably 30 in a 10x10 area. I guess it's likely that if the mushrooms are growing that trichoderma is aswell? The back yard is getting worse and the areas with fungus are almost completely yellow

@weslsew

To clarify, are you saying that in areas treated with bayer fungus control the disease has stopped or is getting better, while in the area where only corn meal has been applied it's getting worse?

Yes, that is correct

The mushrooms all died by yesterday afternoon, but they're back this morning. I've also got several spots of black, white, and yellow slime mold this morning.

I'd hit it with fungicide asap before it's too far gone.

That's what I'm gonna have to do. It's just been way too wet and the grass just hasn't had a chance to dry out