A. The symptoms you describe are most likely caused by a fungus called Rhizoctonia solani. The disease is called Brown Patch or Rhizoctonia Disease. Brown patch can grow in all warm season turfgrasses but St. Augustine is more frequently damaged than the others.
The fungus develops in high humidity, when day time temperatures are between 75 to 90 degrees F. and nighttime temperatures drop to 70 degrees F. or below. These conditions occur in the spring and fall but because the fungii stop growing when the air temperature is 90 degrees F. and above, the symptoms are more obvious in the fall.
The first symptom appears as a small, circular, water soaked dark grass that soon wilts and turns yellow or brown. As the fungus grows outward from the center, there is a grayish-black colored ring of wilted grass around the perimeter that is called a smokey ring. The patch begins small, a few inches in diameter and can grow to over fifty feet in diameter. As they expand and merge with other patches the shapes become irregular.
Another symptom is the leaves of the grass are easily pulled from the stolons. This is because the fungus has destroyed the tissue at the base of the leaf. The stolons are not usually killed unless the infection is severe and the warm, humid weather continues. If this is the case the roots and crowns will rot and the grass dies. If the stolons are not killed, the grass in the center of the patch may recover in a few weeks and put out new growth.
Brown Patch can be controlled by applying a fungicide. There are many on the market to chose from. Generally, the fungicide should be applied at the first sign of infection in the fall. Subsequent applications can be made according to the manufacturer while the day time temperatures continue to be warm and humid. The grass should be kept as dry as possible to slow the spread of the fungus. It should be watered as little as possible and only in the early morning.
Homeowners can reduce their chances of the fungus returning by following a balanced program of watering and fertilizing. Lawns with high nitrogen and high moisture are more likely to develop Brown Patch than a less cared for lawn. Watering should be done only when the grass needs it, in the early morning and to a soil depth of six inches, if possible. Avoid leaving grass clippings on the lawn when the fungus is active. Avoid over fertilizing with high nitrogen fertilizers. Preventative fungicides can be applied during the period the fungus is active.
All lawns are susceptible to Brown Patch, but a strong healthy lawn is more likely to recover quickly.
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