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St. Augustine Grass Disease

St. Augustine grass (Stenotaphrum secundatum) offers a lush green to bluish-green color in hot, humid areas. The grass tolerates both salt spray and shade well, which makes it suited to coastal regions. Unfortunately, the grass is not drought tolerant and will quickly turn brown without adequate irrigation or rainfall. The grass also suffers from numerous diseases which can be difficult to treat.

Types of Diseases

St. Augustine grass is often afflicted by brown patch fungus (Rhizoctonia spp.) and gray leaf spot fungus (Pyricularia grisea). Both diseases cause the grass to appear unsightly. The growth of the grass blades will become stunted and deformed. Gray leaf spot (Pyricularia grisea) will kill large sections of the lawn if left untreated. Irregular circles of complete lawn death occurs over time.


St. Augustine lawns affected by brown patch will develop spotting on the grass blades that appear brown with a darker circle around the margin. The fungus is seen on grass growing in the shade, predominately. The blades often rot and discolor near the soil's surface. Gray leaf spot produces gray spotting with a dark or black margin on the grass blades. The circles become larger as the fungus grows.

Time Frame

During the summer, gray leaf spot fungus often kills large patches of St. Augustine. The fungus enjoys a humidity range that is greater then 95 percent, according to the University of Florida. The high humidity allows the fungus to flourish because the grass remains wet for an extended time. In the spring and fall, brown patch fungus grows well in temperatures that range from 70 to 90 degrees F, according to the Texas A&M University.

Treatment and Prevention

Fungicides such as propiconazole, triadimefon, myclobutanil and PCNB help to control brown patch and gray leaf spot. They also work as a preventative when applied on a regular schedule.

Over fertilizing St. Augustine grass with nitrogen-rich fertilizers makes the grass susceptible to both gray leaf spot and brown patch. The nitrogen encourages the grass to produce young, tender shoots The lush foliage is the perfect breeding ground for the fungus. Both brown patch and gray leaf spot fungus quickly establish themselves on the young growth, notes Texas A&M University


St. Augustine grass that is allowed to become infected with fungal diseases will gradually lose its vigor and perish. Large dead areas will appear in the lawn. Left to progress with no treatment, the grass will often need to be replanted because there will be no hope of saving the existing lawn. Prompt prevention and treatment will significantly help the lawn maintain its overall health and green looks.

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