When our warm season turfgrass is coming out of dormancy, it is at its weakest time of the year.
The grass is trying to overcome changes in temperature, wet conditions and a smaller root system. It is competing with winter weeds that are growing fast and are shading it out.
The turfgrass also is more susceptible to disease problems at this time, especially if it is stressed. One of the more common turfgrass diseases in the spring is large patch or brown patch.
Large patch or brown patch is caused by the fungus Rhizoctonia, which likes to grow when the daytime temperature is higher than 80 degrees and the nighttime temperature is higher than 55 degrees. The symptoms of large patch or brown patch are circular areas in the lawn that turn yellow, then brown. These areas will get larger and can grow together. Early in the morning, a grayish smoky-colored ring is visible in the area where the diseased grass and the healthy grass meet.
The best way to reduce the chance of grass getting a disease is proper turf management. There are several management practices that can reduce the chance of large patch.
The first is proper irrigation. Rhizoctonia likes water. Allowing lawns to dry out between waterings helps reduce fungus growth.
Turfgrasses need an inch of water a week to grow properly. To encourage a deep root system, I recommend putting all of this water on at one time. In our clay soils, this water will soak in 6 to 7 inches deep. In sandy soils, the turf needs one-half to three quarters of an inch of water twice in a week.
To help grass develop a deeper root system, don't water until the grass shows symptoms of drought, such as a bluish-gray color in the lawn. Drought-stressing the lawn will force the root system deeper.
In order for Rhizoctonia to grow, the fungus needs moisture for 16 hours. This is why we recommend irrigating grass early in the morning. Watering late in the afternoon means the grass will go into the night wet and won't dry until about 10 a.m. the next day. This is approaching the 16 hours that is needed for disease development. Also, watering grass every other day doesn't allow the grass to fully dry out before it is wet again.
The second practice to help reduce the chance of large patch is proper fertilization. The Rhizoctonia fungus likes and needs nitrogen to reproduce. Applying the right amount of fertilizer at the right time can reduce the incidence of large patch.
Turfgrass will lose a part of its root system every winter. This is why grasses are at their weakest when they are coming out of dormancy. If nitrogen is applied too soon, it can stress the grass and make it more susceptible to diseases.
Our warm-season grasses grow their best when soil temperatures reach 65 degrees. In our area, that usually occurs about late April to the first of May. This is the time to fertilize. This is especially true of centipede and St. Augustine grass. 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. The grass becomes stressed and then large patch can start.
Applying more than 1 pound of nitrogen per 1,000 square feet is too much fertilizer. Also, a fertilizer that has more potassium in it should be used. Potassium is the element that helps the plant build up drought tolerance, disease resistance and winter hardiness. Our soils are usually low in potassium; a soil sample will tell how much potassium is needed.
Large patch can be stopped from becoming a problem with the application of fungicides. Preventive applications of fungicides help stop diseases before they start. Most of the fungicides used by homeowners have a preventive rate listed on the label. Usually, this is half the rate of the curative rate. Read the label to determine if the fungicide allows for preventative use.
Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at firstname.lastname@example.org.
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