The popularity of St. Augustine grass is because of its hardiness and quick, reliable growth. However, it is also because of its capacity for growth that leaves it susceptible to a number of diseases. While none of these diseases are deadly, they are preventable. Well cared for and properly fertilized St. Augustine grass fairs a much better chance of fending off infection.
Brown patch frequently afflicts St. Augustine grass because they both thrive under the same conditions. Brown patch most often crops up in the spring or fall after periods of frequent rain and hot weather. St. Augustine grass grows more vigorously during these times and brown patch feeds on the abundance of new growth (consequently, brown patch can also crop up after an over-application of nitrogen fertilizer). A lawn suffering from brown patch will have circular patches of brown shriveled grass of varying sizes. As new growth continues, large brown patches may have a patch of healthy grass in their centers. Brown spot may thin and weaken a lawn but is rarely deadly. Treat it with an application of a fungicide designated for brown patch
Gray Leaf Spot
Gray leaf spot disease is another common affliction of St. Augustine grass. Like other St. Augustine grass diseases, gray leaf spot most often strikes during the summer rainy season when new growth is abundant and most often affects shady or over-watered patches of lawn. The blades of a lawn afflicted with gray leave spot will develop light brown or gray spots that have purple edges. Gray leaf spot is unsightly but not deadly and is easily controlled with the application of a fungicide prescribed for gray leaf spot.
Take-All Root Rot
Take-all root rot is a disease that most often attacks St. Augustine grass that is stressed or over watered. When take-all root rot first attacks, the roots of the grass turn black and die. The first sign that homeowners notice is withered grass blades, which is a result of their inability to receive nutrients. Take-all root rot is easily treated with a fungicide prescribed for the disease and it can be prevented thereafter with proper lawn maintenance and reduced watering.
St. Augustine Decline (SAD)
St. Augustine decline is actually a virus commonly contracted by St. Augustine grass. Once infected, St. Augustine grass will slowly turn yellow over a number of years and eventually die. There is no cure for this virus, and infected grass must be uprooted and replaced with a St. Augustine decline-resistant variety. Any tools that have been used to work the lawn should be thoroughly cleaned as they likely carry the virus.
The first sign of a downy mildew infection in St. Augustine grass often goes unnoticed. When the fungus first strikes--after a period of warm weather and rain--the leaf blades thicken slightly and become somewhat stunted. It takes a while for the fungus to produce the yellow spots and shoots that lawn owners notice. Like most other St. Augustine grass diseases, downy mildew is easily managed with the application of a fungicide.