Sheath blight is the most destructive disease of rice grown in Missouri. Crop losses may range from slight to heavy each year, depending on the weather, the plant growth stage when infection occurs, the extent of infection, and the rice varieties grown.

Fortunately, sheath blight can be managed to minimize yield losses. An accurate diagnosis is essential before selecting a management technique. Whenever possible, consult an expert in plant disease diagnosis; check with the University of Missouri-Delta Center, (573) 379-5431, the University of Missouri Plant Diagnostic Center in Columbia, MO, (573) 882-3019, or your local University Extension center.

Sheath blight symptoms

Sheath blight symptoms appear sometimes on seedling rice but usually at internode elongation. The first symptom is an oblong, water-soaked lesion on leaf sheaths at or near the water line (Figure 1). In two or three days the lesion will have a grayish-white center surrounded by a dark purplish- or reddish-brown margin and may be up to one inch long (Figure 2). This lesion interrupts the flow of water and nutrients to the leaf and the leaf may die. Tissue below the lesion may remain green.

As the plant grows and the leaves develop, the humidity inside the canopy increases. In this humid environment the fungus grows upward inside the sheath and on the surface, causing new lesions. The fungus can also spread to nearby plants. Severely damaged plants may lodge. These patches of lodged plants are easily seen from a combine at harvest.

Damage can range from partial infection of the lower leaves with little effect on grain development to premature plant death. On some varieties, the panicle can be attacked during hot, humid weather. Both yield and grain quality are reduced when the infection prevents the flow of water and nutrients to the grain. Grain may develop only partially or not at all. Often the grain at the base of the panicle will not fill. Poorly developed grain usually breaks up during milling thus reducing quality.

Sheath blight is more prevalent during periods of warm moist weather, and in thick, lush stands because of the high humidity which develops in the canopy.

Cause of sheath blight

The disease is caused by a fungus called Rhizoctonia solani. This fungus survives in the soil from year to year as a hard, weather-resistant structure called a sclerotium. A sclerotium will float to the surface of rice flood water, and when it contacts a rice plant, the fungus grows out from the sclerotium and moves into the leaf sheath.

Later, new sclerotia that have developed on infected stem surfaces fall from the plant to complete the life cycle. Sclerotia can remain alive in the soil for several years.

Sheath blight is usually worse in thick, lush rice stands because of high relative humidity within the canopy. Rhizoctonia solani thrives when the canopy humidity is above 95% and temperatures are hot (80- 90^oF). Little infection will occur in thin, short stands of rice because humidity within the canopy is low.

Control

The disease can be controlled by following some simple production steps:

1. Plant the least susceptible high yielding varieties. (click here for list).
2. Seed to a stand of 15 to 20 plants/per square foot.
3. Plant at the optimum time for a specific variety. Avoid extremely early planting.
4. Time nitrogen applications so 30 or less pounds are applied at internode elongation (IE).
5. Scout fields (see scouting section below) for symptoms from IE to a few days before heading. Use a labeled fungicide when the incidence of sheath blight has reached a threshold level (Table

Fungicide use

Fungicide choice depends on severity of infection, fungicide cost, and the presence of other diseases such as blast (click here for labeled fungicides). Use the most effective fungicide for sheath blight control at the internode elongation - 7 day growth stage. If blast is present or anticipated, use benomyl (Benlate) or Quadris at the second application.

Timing is essential for best fungicide effectiveness. The best time for application is during the "window" as indicated by the DD50 program, which occurs about 7 days after ½" internode elongation stage of growth. Earlier or later applications will not give effective sheath blight control. Cautions

Do not apply fungicides to non-target areas such as fish ponds. Always follow label directions fully, especially any restriction regarding endangered species.

Table 1. Sheath blight treatment thresholds

SCOUTING: The entire field should be scouted for symptoms in a zigzag pattern stopping every 50 steps (Fig. 2). Only a 3-ft long section of rice should be inspected at each stop for sheath blight symptoms. If symptoms are present the stop is positive. A minimum of 50 stops per field should be made or 1 per acre to determine the level of sheath blight for the field. If sheath blight is not widespread in the field, but concentrated in certain areas, then treating only those areas with the fungicide may be more economical. While experience may be substituted for scouting in fields with a history of sheath blight, the economic use of fungicides depend on adequate knowledge of the distribution of the disease in a field and its intensity between ½" internode elongation and early heading.

Scouting for sheath blight Damage due to sheath blight will usually be more severe when infection occurs at internode elongation. This is about the time most varieties should receive the first mid-season nitrogen application, and fields should be scouted for sheath blight at this time. The DD50 printout will provide a beginning and cut off date for scouting. The disease is likely to occur in the same fields each year, so fields with a history of the disease should be scouted. A suggested scouting technique is: Inspect for water-soaked lesions or spots on the leaf sheath just above the water line at a minimum of 40 random stops in the field. Check only plants within an arm's length (elbow to fingertip), for symptoms at each stop. Bend the rice plant over for easy observation. Apply a fungicide when the disease threshold is reached for the variety growing in the field (Table 1). Rescout every 3 to 4 days until treatment level is reached or until rice begins heading.

Sheath blight distribution will probably be erratic in the field. If distribution is not uniform, consider treating only those portions of the field that have reached the treatment threshold.

A fungicide for sheath blight is not recommended if treatment level does not occur by heading. Infection after heading will probably not cause economic injury to the plants.