Why Does St. Augustinegrass Get Brown Patches?

large, circular brown patch on grass

If it looks like the aliens have landed in your St. Augustine lawn, you might have large patch. Large patch manifests as large, brown patches and is common in St. Augustinegrass lawns. With a little prevention and the proper treatment, you can get rid of this fungal invasion and restore order in your lawn.

What is large patch?

Large patch is a fungal disease caused by a strain of the fungus Rhizoctonia solani. This fungus lives in the soil and affects warm-season turfgrass during the spring and fall transitional periods. If you’ve heard about brown patch in warm-season lawns, it’s the same disease. Large patch used to be called brown patch until experts renamed it recently.

Signs you have large patch

  • Circular patches that are at least 1 foot in diameter but can be over 3 feet wide
  • Patches are yellow, tan/brown, red, or orange
  • Pull test: If you pull on the grass blades, they will sever easily from the plant. This fungus causes the base of the leaves to rot. (Note: Rhizoctonia solani does not affect the roots.)

Before you decide to treat these spots on your lawn, it’s best to get an official diagnosis. Take a sample of the affected area to your Cooperative Extension office to get a positive ID before you move forward with treatment.

Conditions that lead to large patch

Nearly all fungi thrive in damp, wet conditions. The conditions below encourage large patch in St. Augustinegrass lawns.

  • Continuously wet grass
  • High humidity
  • High nitrogen levels
  • Poorly drained soils
  • Mowing too low
  • High thatch levels
  • Excessive shade

The good news is that some of these conditions are under your control.

How to prevent large patch

The best way to prevent large patch, or at least lessen the severity of it, is proper lawn maintenance. In other words, continue to do the things you already do, but do them correctly and at the right time. 

Water: Water before 8 a.m. and only if your lawn is showing signs that it needs water. When one-third to one-half of the lawn starts to look stressed, turn on the sprinklers.

Signs it’s time to water:

  • Grass turns from green to blue-gray
  • Footprints remain on the lawn long after you do
  • The oldest leaf blades start to curl up into a V shape or wilt

Fertilize: Avoid fertilizing the lawn during fall and spring. Wait until early summer (or late spring) to apply fertilizer only if your soil test recommends it. According to the University of Florida, over-fertilizing the lawn with nitrogen can cause repeated fungal outbreaks (along with overwatering). 

Mow: Mowing won’t prevent this fungus from attacking your lawn, but it can help prevent it from getting worse

Mowing tips:

  • Mow at the proper height for your grass: 3.5-4 inches for standard cultivars and 2-2.5 inches for dwarf cultivars.
  • Mow only when the grass is dry and mow diseased areas last (or don’t mow them at all). Collect the grass clippings from diseased areas, and wash off your mower blade between mows. This helps prevent the disease from spreading.

Dethatch: If your thatch layer is greater than 1 inch, consider dethatching. Dethatching removes excess material from the surface of the soil so that air, water, and fertilizer can reach the soil surface. Do this in late spring or early summer. 

You can rent a machine or buy a liquid biological dethatcher to help break down the excess thatch. If you have a very small area, a manual dethatching rake works, too.

Aerate: If your soil is compacted, aeration is a means to open up the soil and allow more air and water to circulate. Aeration machines pull plugs from the soil, allowing air, water, and nutrients to reach the grass. The result is stronger, healthier roots and a more beautiful lawn.

How to get rid of large patch

Once you’ve properly identified the disease and corrected your cultural practices, you can consider whether additional treatments are necessary. If so, here are a few natural and chemical solutions to consider.

Natural solutions: The active ingredient bacillus amyloliquefaciens strain D747 is marketed as effective against brown patch disease. Since large patch is caused by the same bacteria (albeit a different strain), this may be worth a try on large patch, as well. You can find these products online or at a local store. Most will market them as suitable for organic gardening.

Chemical solutions: If you’ve had problems with large patch in the last transitional season, it’s best to use preventative treatment since you’ll likely have another outbreak next spring or fall. Ask your local Extension agent (or search their fact bulletins online) to find the best time to apply preventive chemicals in your state. With preventive applications, timing is key, so be sure to get the timing right for the best results.

As with most chemicals, alternate between two, or use a product with more than one active ingredient, to keep your lawn from building a resistance. Azoxystrobin with propiconazole, pyraclostrobin with triticonazole, and fluoxastrobin are highly effective against large patch.

A word of caution: Since warm-season grasses grow more slowly in the shoulder seasons, don’t expect the fungicides magically to turn the lawn green again. The chemicals simply prevent the fungus from expanding. The only way to turn the lawn green again is to wait until it grows new leaves (or paint it in the meantime).


1. Are there other causes of brown patches on St. Augustine lawns?

There are many other reasons why your St. Augustine lawn may have brown spots.

Maintenance or cultural issues: 
–Lack of water
–Herbicide burn
–Scalping the lawn
–Freeze damage
–Too much shade (it will thin out and you’ll see the soil below)
–Excessive wear or foot traffic

–Chinch bugs
–Mole crickets
–Tropical sod webworms
Armyworms/grass looper

Other lawn diseases:
–Take-all root rot
–Grey leaf spot

As with any mysterious disease on your lawn, contact your local Cooperative Extension office to rule out alien invasion or other unexplainable causes.

2. Are certain varieties of St. Augustine more susceptible to large patch?

According to the University of Florida’s IFAS Extension service, there are a few St. Augustine cultivars that are more susceptible to large patch than others. If you’re starting from scratch and have had issues in the past, avoid choosing a cultivar that is susceptible to this disease.

St. Augustine cultivars that are susceptible to large patch:
“Captiva” — a dwarf cultivar that is “somewhat susceptible” if it gets too much fertilizer or water
“Delmar” — a dwarf cultivar

3. Are other grasses affected by large patch?

Yes, all warm-season grasses are affected, including bermudagrass. However, with bermudagrass, the damage may not be as noticeable because of its quick growth habit.

If this latest fungal invasion has taken you to the outer limits, contact a local lawn care professional to bring your lawn back into focus.

Main Photo Credit: Scot Nelson | Flickr | public domain

Sarah Bahr

Sarah is a writer who has previously worked in the lawn care industry. In her spare time, she likes to garden, raise chickens, and mow the grass with her battery-powered lawn mower.