Rhizoctonia Brown Patch of Tall Fescue

Ned Tisserat 
  Extension Specialist, Plant Pathology

Symptoms

Brown patch is a summer disease of several cool-season turfgrasses grown in Kansas, but it is particularly severe on tall fescue. With increased planting of tall fescue in urban and commercial landscapes, brown patch has become a significant management problem.

Brown patch symptoms develop rapidly (24 to 48 hours) during warm, humid weather and can result in blighting of large areas of the turf. Symptoms on tall fescue vary depending on weather and management practices.  In some cases, the disease appears as distinct circular patches of blighted turf that range in size from a few inches to several feet in diameter (Figure 1).  Patches initially are dark purple-green, but then quickly fade to light tan or brown as the diseased leaves dry out.  In most cases however, blighting tends to occur in a more irregular or diffuse pattern without formation of circular patches (Figure 2).  Diseased turf may appear droughty even though sufficient soil moisture is present.

Figure 1. Circular patches associated with brown patch on tall fescue (courtesy P. Sell)

Figure 2. Irregular pattern of blighting on tall fescue caused by the brown patch fungus.

Symptoms on individual plants are also helpful for diagnosing brown patch. The brown patch fungus initially attacks the leaves causing the formation of irregular, water-soaked spots (Figure 3).  The spots are bordered by a dark brown band.   Thin strands (mycelium) of fungus can sometimes be seen in early morning growing between diseased leaves (Figure 4).  This mycelium has a distinctive right-angled branching pattern when viewed under a compound microscope (Figure 5). Leaf symptoms of brown patch are sometimes confused with another disease called dollar spot. However, dollar spot is rare on tall fescue.   

Figure 3. Straw-colored leaf spot on tall fescue leaf caused by the brown patch fungus. Note the brown border surrounding the irregularly-shaped lesion.

Figure 4. Fungal strands (mycelium) of brown patch fungus may be visible on leaves soon after sunrise.

Figure 5. The brown patch fungus has characteristic right angle branching of the mycelium.

Conditions for disease development

Brown patch is caused by a fungus called Rhizoctonia solani. There are several subgroups (called anastomosing groups) within the Rhizoctonia solani species, with subgroup AG1 being the most common on tall fescue. Other AG groups are found on bentgrasses, zoysiagrass, and ryegrass.

The brown patch fungus survives in the thatch and soil as resting structures called bulbils. During favorable weather, fungal mycelium from bulbils begins active growth and infects grass leaves. Disease development is favored by nighttime temperatures above 70 degrees F and leaf wetness periods exceeding 10 hours. This may occur anytime from early May through mid-September.

Tall fescue lawns under high management, especially high nitrogen fertilization, are more susceptible to damage from brown patch. In most cases, the fungus attacks only the leaves, but during severe disease pressure, the crowns may also be killed.

Disease management

Several cultural practices will suppress brown patch development. Avoid seeding rates greater than 6 to 8 lb per 1000 square feet when establishing new lawns. A high seeding rate results in an excess number of turfgrass plants. This makes it physically easier for the fungus to move from leaf to leaf and also increases humidity and leaf wetness periods that are favorable for infection.

Most tall fescue varieties are susceptible to brown patch. Some newer varieties reportedly have a higher level of resistance but these also may be severely bighted under high disease pressure.

Brown patch occurs less frequently when the available nitrogen supply is adequate to low and phosphorus and potassium levels are adequate. Apply the majority of the nitrogen fertilizer in the fall and do not overfertilize. Applications of more than 4 lb of actual nitrogen per 1000 square feet per year will predispose the turfgrass to increased brown patch activity. This is true even if the majority of the fertilizer is applied in spring and fall when the disease is not active. Don’t apply nitrogen fertilizer when brown patch is active. However, a light fertilization (less than ¼ lb nitrogen per 1000 square feet) after a brown patch epidemic may speed turfgrass recovery.

Turfgrass managers have been concerned about the potential for increasing brown patch severity by returning grass clippings to the lawn with a mulching mower. Recent research has demonstrated that returning clippings does not enhance brown patch severity.

Some research has indicated that the use of organic composts (turkey litter, cow manure, brewery waste, etc.) reduces brown patch severity. Other studies show no significant control of the disease with these types of organic amendments.

There is still debate on the effects of mowing height on development of brown patch severity. Low mowing (1to1 ½ inches) can initially decrease brown patch severity, but eventually may increase blighting as a result of a denser turf canopy. Since there is no apparent advantage to low mowing, continue to mow tall fescue at 2 1/2 to 3 inches. Mowing at shorter heights may predispose the turf to other management problems.

Do not irrigate lawns in late afternoon or evenings if possible. This extends the number of hours the leaves remain wet and increases the likelihood of brown patch development. Irrigation after midnight to mid-morning is preferable. These are the hours the turf would normally be wet from dew, and irrigation at this time does not extend leaf wetness periods.

Several fungicides are labeled for control of brown patch on tall fescue. Before applying a fungicide, consider whether the application is necessary. In most cases, brown patch causes temporary injury to the lawn. Our research indicates that the turf often fully recovers from blighting after two to four weeks without fungicide treatments. Nevertheless, initial damage from the brown patch disease is unsightly and often unacceptable to the homeowner.

Preventive fungicide spray programs are effective in suppressing brown patch. Fungicides such as azoxystrobin, flutolanil and triadimefon can provide three to five weeks suppression of brown patch. Thus, only two to three applications may be required during the summer to provide season-long control of brown patch. Begin preventive sprays in early to mid-June. Other fungicides (listed in table) are also effective for shorter time intervals.

Another option is to apply a fungicide as soon as symptoms appear. This is called a post-infection or curative spray program. In general, these curative sprays are less effective because significant turf injury has occurred by the time the product is applied. Nevertheless, post-infection applications of chlorothalonil or other fungicides will suppress further disease development. Unfortunately, the best post-infection fungicide chlorothalonil is no longer labeled for use on home lawns.

excellent

 14-28

good

7-14

excellent

21-28

good

14

fair

7

good

14

good

7-10

good

14-21

good

7-14

good

7

excellent

14-21

excellent

14-28

good

7-10

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Web updated 9/01/06
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