Q: My lawn looked beautiful in spring, but I started seeing yellowing spots, then brown circles, now bare patches in my St. Augustine grass. Is this fungus? What is the best spray?

A: Kudos for a good description! Half the battle is identification. The other half is reaching down deep into your gardening well to find the will to work outside in this August weather!

Three things cause lawns to suffer, especially in August heat: lawn culture, insects and disease, usually fungal. Diagnosing a sick lawn is a process of elimination. Don’t think “instant fix.” Think of building a healthy lawn for next season. A good dose of delayed gratification builds character.

Lawn culture matters — if you want a good lawn, that is. I hate to tell you, but there are no miracle cures. Your lawn needs: water — to six inches deep once weekly while growing (which is equal to about one inch of water per week); nutrition — including enough sun; proper mowing (for St. Augustine, this means every 5-7 days to a height of three inches); good soil texture; and structure. Late spring lawn aeration allows oxygen, water and nutrients to penetrate to roots, encouraging deep growth.

Insect damage can look like advanced disease, but don’t bomb the whole lawn because you see one grub worm. Identify and target. The common damaging insects you can successfully treat at this time of year include chinch bugs (prevalent in hot, dry, sunny areas), mole crickets (at their vulnerable stage July to August) and grub worms (near the soil surface in August).

And never forget, when you kill indiscriminately, you kill the beneficial and the beautiful. Monarchs, bees and dragonflies are insects, too.

As a Master Gardener intern, I was determined to find out how to have one of those lush, deep green, cool and soft St. Augustine carpets. So while the other interns did interesting presentations about exotics or butterfly perennials, I slogged through fungus. I know a thing or two about fungi, and I agree that is your problem.

The appearance of circular yellowing to brown patches in early spring, especially given the rainy spring we had, shouts common brown patch, a fungus prevalent in warm spring or fall days with cooler nights. Both moisture and shade contribute. But brown patch typically improves in hot weather (though it can return in rains) and yours remained and worsened.

Another St. Augustine disease is take all root rot. The soil-inhabiting fungus Gaeumannomyces graminis var. graminis is the culprit. Pull up a weakening runner at the edge of a dead area. Does it pull up as though the roots are not attached? That is a good sign of take all, because it is a disease of the roots. Look at the roots. Are they white, fleshy and healthy, or have they darkened and hardened, some even turning black? If so, you may have take all, a tenacious fungus that worsens through the hot summer, overwinters in the soil then reblooms the next year.

While difficult, you can give your lawn a fighting chance against graminis. Get a soil test, maintain soil at pH 6.0 to 6.5 and avoid heavy liming or heavy nitrogen fertilization. Use a fungicide that targets take all, use it exactly according to the label directions, and alternate fungicide active ingredients to avoid resistance.

There are fungicides that treat both brown patch and take all, so those would probably give you your best result. Follow the label directions carefully. Keep in mind that brown patch often clears up without chemicals when the weather heats up.

And if you should tire of fighting the fungi, consider the current sustainability trend of small, easily maintained patches of lawn for accent (or no lawn at all), conserving soil and water for home gardens. Plant food, not grass! Remember, next summer’s beautiful lawn begins now.

To learn more about pesticides or any other topic, visit www.aces.edu/pubs/docs and type your question into the search box.
Email us your questions at [email protected] or call (toll free) 877-252-4769, the Master Gardener Helpline answered by Mobile and Baldwin County Master Gardener Volunteers.