As spring warm-season lawns continue to green up, diseases rear their ugly heads. The main culprit this time of year is a fungus that causes large patch. Large patch can infect all warm-season grasses, but centipede and St. Augustine are particularly susceptible.
Large patch appears in roughly circular patches that are yellow, tan or straw-brown with orange- brown borders. The patches are initially 2 to 3 feet in diameter, but can expand up to 10 feet or more, as the name “large patch” indicates. Early in the morning, a grayish ring can be seen in the area where the diseased grass and the healthy grass meet.
Large patch occurs in the spring and fall when environmental factors are favorable. Favorable conditions include humid days with temperatures ranging from 75 to 90 degrees and nighttime temperatures above 60 degrees. The higher temperatures and humidity lead to an extended period of leaf wetness.
Turfgrasses are also more susceptible when coming out of – or going into – dormancy. Therefore, spring and fall are the times of year the grass is most venerable because it is not growing as actively and is more stressed.
The best way to protect your grass from disease is to properly manage the turf.
The best way to prevent large patch in your grass is by following good lawn-care practices. This is much easier and less expensive than the use of fungicides and can be very effective. Here are a few practices to keep in mind:
• Avoid high nitrogen rates on warm-season grasses in mid- to late fall or in early spring. The disease-causing fungus readily attacks the lush growth of grass that nitrogen promotes. Avoid fast-release forms of nitrogen fertilizer.
• Irrigate grass only when needed and to a depth of 4 to 6 inches (1 inch of irrigation water per week). Water early in the morning to reduce extended leaf wetness. This disease can spread fast when moisture is present.
• Avoid spreading the disease to other areas. Remove clippings to prevent spread to other areas during mowing.
• Keep lawns mowed on a regular basis to the proper height for the grass species. Lower than optimum mowing height can increase disease severity.
• Provide good drainage for both surface and subsurface areas. Correct soil compaction by core aeration. Prevent excessive thatch buildup. A pitchfork can be an excellent aeration tool.
• Test the soil, and apply lime according to test recommendations. Disease may be more severe if the soil pH is less than 6.0
Centipede and St. Augustine are at peak growth when soil temperatures reach 65 degrees. In our area, the soil reaches this temperature in late April or early May. This is the optimum time to fertilize. This is especially true of Centipede and St. Augustine. When these grasses are fertilized too early, they turn yellow from stress. The nitrogen in the fertilizer causes more top growth than the root system can support, and large patch can start.
If prevention is not an option and treatment is warranted, fungicide can control this disease. There are many fungicides on the market labeled for use on lawns, and most will control large patch. In order for the fungicide to work properly, follow the directions on the product labeling.
After treatment, the size of the patch should stop increasing. If the size of the patch continues to increase, another treatment is needed. The grass then will fill in the areas affected by large patch. A good rule of thumb to follow on warm-season grasses is to initiate fungicide sprays when nighttime temperatures reach 60 degrees, and stop applications when nighttime lows are forecast to be below 60 degrees for five consecutive days. Typically, applications are made at 14- to 28-day intervals, depending upon the fungicide.
Also remember to alternate fungicides to prevent a buildup of resistance to a fungicide.