St. Augustine is a warm-season grass that is highly vulnerable to common diseases, especially during the spring and summer growing seasons. We’ll introduce you to the most common diseases in St. Augustinegrass and tell you what to do about them.
In this article:
- Brown patch
- Take-all root rot (TARR)
- Fairy ring
- Nigrospora stolon rot
- Gray leaf spot
- St. Augustine decline (SAD)
Brown patch
Brown patch is a common disease in St. Augustinegrass caused by rhizoctonia solani fungus that lives in the soil. The disease attacks the new growing leaf blades during the cool season.
The disease is dominant in the spring or fall after frequent rains and hot weather. When it attacks, it leaves brown patches on the grass.
Causes of brown patch
- Cool night temperatures
- Excessive thatch
- Excess soil moisture
- Excessive application of nitrogen fertilizers.
Symptoms
- The disease leaves dark brown patches on the leaf blades that occur where the stolon sprouts.
- The infected grass blade thins out and falls loose easily.
Prevention and treatment
- Help your lawn recover using active herbicides like thiophanate methyl, Myclobutanil, and azoxystrobin.
- Aerate the soil, ensure it drains properly.
- Remove excess thatch.
- Apply fertilizer during the cool season.
Take-all root rot (TARR)
Take-all root rot occurs mostly during the peak growing months of April and May. The disease can be identified by small to large patches with weird shapes and is caused by a fungus called Gaeumannomyces graminis var. Graminis.
The fungus is common in hot and humid Southern states such as Texas and the surrounding regions. It attacks the root system of St. Augustinegrass, but is common in lawns that appear healthy, as well as disease-stressed lawns.
Causes of take-all root rot
- Poor lawn maintenance such as overwatering or under-watering
- Excessive shade
- Soil compaction
- Extreme temperatures
- Improper mowing
- Irregular irrigation schedule
- Herbicide injury
Symptoms
- Yellowing of the grass blade
- Thinning of the turf, leaving irregular brown patches on your lawn
- Rotting of the root, allowing grass to be pulled out easily
Prevention and treatment
Once take-all root rot has infected a lawn, it is very difficult to eradicate. It’s not simply a matter of tearing up the lawn and starting over because the new lawn may become infected. And fungicides have some limited effect, but only in concert with good lawn maintenance practices. Here are some tips:
- Rake up thatch and other dead material in preparation for applying fungicide. This helps the fungicide seep into the ground.
- Apply fungicides like azoxystrobin, myclobutanil, propiconazole, or thiophanate-methyl mixed in water to ensure absorption into the soil; water grass immediately after application
- Cover affected areas with a one-inch layer of sphagnum peat moss
- Improve the soil drainage of your lawn
- Proper fertilization helps. Repeat four times a year, particularly during spring and summer
- Avoid overwatering
- Mow at the best height
- Dethatch your grass where needed
Fairy ring
Fairy ring is common in St. Augustinegrass in regions with sandy soils. It is caused by a fungal pathogen, Basidiomycetes, which changes the physical and chemical properties of the soil in rings or arcs.
The disease can also result from the accumulation of ammonium to toxic levels and the introduction of hydrogen cyanide or other toxins in the root zones.
Additionally, fairy ring forms a thick fungal mat, preventing water infiltration into the ground.
Causes of fairy ring
- Sandy soil with less moisture and little nitrogen
- Excess thatch
Symptoms
- Dark green circles of grass surrounded by an extra ring of dead grass
Solutions
- Mow the grass with a vertical mower to reduce thatch
- Add a fine layer of high-quality sand to the lawn to dilute the organic matter and provide a substrate environment for St. Augustinegrass root growth
- In the regions of dark green, apply nitrogen fertilizer to mask the rings and stimulate the root growth of the turf grass
- Apply extra water and aerate the soils in regions where the lawn shows brown drought symptoms
- You can also apply fungicide
- In severe cases, dig out the soil and turf in the affected area for about 12 inches and replace it with fresh soil. Re-sod as necessary
Nigrospora stolon rot
Nigrospora stolon rot is caused by Nigrospora Sphaerica fungus, which prevents the roots from properly absorbing moisture and nutrients. The leaves turn yellow and thin, and the turf grass rots, causing the roots to decay
The disease girdles the stolons of the grass, depriving the stolon of water and nutrients. The grass suffers from moisture stress and becomes desiccated, and dies
The disease is dominant during the dry spring and summer periods. Drought conditions with high temperatures favor its development
Causes of nigrospora stolon rot
- High summer temperatures stress turf grass, preventing the roots from absorbing enough moisture and nutrients
- An increase in temperature makes the stolon turn brown and eventually rot in the grass
Symptoms
- Dark brown lesions on stolons
- New grass growth grows thin, yellow, and eventually dies
Solutions
- Water the grass adequately during the summer
- Apply a quality commercial fungicide to your lawn
Gray leaf spot
Gray leaf spot is a common disease affecting the leaves of St. Augustinegrass. It is caused by a fungal infection, Pyricularia grisea, forming oval and round spots on leaves with dark brown borders.
The gray leaf spot disease flourishes in regions with abundant moist and warm temperatures and shaded areas.
You first notice it with tiny brown to ash-colored spots with purple to brown margins that later enlarge to form diamond shapes.
Causes of gray leaf spot
- High humidity makes the leaves wet, favoring spore production and accelerating the spread of the disease
- High temperatures
- Nitrogen deficiency
- Excessive use of fertilizers
Symptoms
- Oval and round spots on the grass, with dark brown borders
Treatment
- Use fungicides containing at least two ingredients such as azoxystrobin and thiophanate-methyl
- Avoid excess watering and ensure proper soil drainage to avoid water retention
- Reduce the amount of nitrogen fertilizer you apply to your St. Augustine lawn because it holds water
St. Augustine decline (SAD)
St. Augustine decline is caused by the panicum mosaic virus. Once the grass is infected with the virus, it is likely to die because there is no cure.
However, following good lawn maintenance practices will help strengthen St. Augustinegrass and improve the grass’s immunity. It is easy to transmit the virus to your lawn using contaminated mechanical equipment or purchasing contaminated sod.
Symptoms
- Mild chloritic stippling of the leaves
- Yellow leaves resemble nutrient deficiency
- Thin, less vigorous leaves that eventually die
Treatment
- The disease has no cure
- Uproot the infected grass and replace it with a variety that is resistant to St. Augustine decline
- Thoroughly clean yard tools before use
Frequently Asked Questions (FAQs)
The diseases most commonly attack St. Augustinegrass lawns are gray leaf spot, St. Augustine decline, nigrospora stolon rot, take-all root rot (TARR), and fairy ring.
Thiophanate methyl, myclobutanil, and azoxystrobin contain active ingredients to help you control brown patch disease. Also, ensure the soil is well-drained with proper soil aeration. You can also apply water regularly and dethatch the grass.
Final Word
While St. Augustine is highly resistant to pests, it is highly susceptible to diseases under certain conditions. You may want to consider having a Lawn Love lawn care professional assess your lawn and help map out a strategy for fending off these diseases before they gain a foothold in your lawn.
Main photo credit: Phillip Pessar | Flickr | CC BY 2.0
