By the hackberry, near the street, in the St Augustine, some purplish black stripes about 6" wide appeared. The stripes are a total of about 20' long, sometimes curving and sometimes straight. When they first appeared, i wiped some off a St. Augustine blade and it was waxy. Now, at least a week later, the color is more grayish-black, and the substance is ashy not waxy. The grass appears slightly scorched where the trail is.
In 2 years, i have been only organic. The last thing i applied to my lawn was corn gluten meal, months ago when the gurus advised us to. i have not watered of course because of the rain. Any idea what this is, and whether it is temporary or harmful? Thank you!!!!!

For what it's worth, is it possible it's something from a neighbor's exhaust or something like that? The thought occurred because you specifically stated that it's near the street and that the place where the stripes are looks scorched.

It may be some weird fungus or something -- I don't know -- but my first thought was something like the above.

Or, does a landscape service, or neighborhood mowing service, work on your yard? Might it have been from something they did? Wheeled vehicle up in your yard? Or exhaust from blowers or edgers?

I'll be interestedly watching your post to learn the solution to the mystery...

In the Lake Highlands neighborhood I live in, many yards are experiencing this same fungal problem! Especially the ones who are watering at the same time it is raining . I might recommend an application of vermicompost tea or compost tea with living microbes to eat the fungus, but a stop to watering too much (which you are not) will help during this gray and rainy season we are having. Just too much water and gray skies.

I agree with the fungus idea. Usually the more obvious the fungus, the less damage it does. There is one fungus that is really foamy looking. It's called dog barf fungus, and it is harmless.

so what do you guys think about sprinkling horticultural cornmeal on the areas? Would that help at all?

might help, as would a very thin layer of finished compost or compost/vermicompost tea.