Scout for Rice Sheath Blight and Relieve its Harm

by Yeshi Wamishe, Extension Plant Pathologist - Rice - June 29, 2021

Scout before deciding on fungicide application for sheath blight and determine the threshold. It is not profitable for Arkansas rice producers to apply fungicides for more than one time to suppress sheath blight alone. If the disease progress is slowed by dry weather conditions and the upper three leaves are not threatened, it is wise to delay fungicide application further to boot stages. Sometimes fungicide application timing for sheath blight may be paired with protective fungicide for the smuts which are from early boot to mid-boot.  In such instances, fungicides may be combined or a pre-mix may be used to address more than one disease. Such an approach should help reduce the cost of application. Remember the recommended timings for a fungicide application to manage sheath blight ranges from panicle differentiation (PD) to boot split.  If the disease gets to the threshold level early in the crop stages and weather is favorable, delaying fungicide application is not an option in the interest of keeping the frequency of application to one.

The sheath blight fungus penetrates the sheath of rice plants at the water-line (Figure 1) in flooded rice.  Sheath blight in severe situations can cause significant yield loss in susceptible varieties and also weakens rice stems leading to lodging.  In addition to the sheath, the fungus attacks rice’s leaf blades eventually drying them (Figure 2). Under favorable conditions, the disease progresses vertically following the height of the crop reaching the head and sideways to neighboring plants through tissue contacts (Figure 3). Sheath blight disease is favored by warm temperatures and high humidity. The disease usually starts from a compact soil-borne fungal mass called sclerotium (Figure 4) often formed later after lesion formation and nutrients depleted. 

The Main Message: Start scouting from the green ring with greater attention between 7-14 days after jointing and continue until after heading (50% heading). For susceptible cultivars with an “S” or “VS” ratings, the recommended threshold is at 35% positive stops. For moderately susceptible cultivars rated “MS”, the threshold is at 50% positive stops. Positive stop counts assume scouting in a zig-zag pattern (Figure 5) over a bulk of a rice field not including field edges or bottoms.  Refer to MP192 pages 130 to 133 for more information. 

Note:

1. Automatic application of fungicides is not advisable due to the potential development of fungicide resistance.
2. Sheath blight can start from soil line in furrow irrigated susceptible rice.
3. Weather conditions that particularly favor the vertical progress of the disease are more damaging to rice than the micro-environment that often favors plant to plant disease transfer in lower canopy.

Fig 1. The sheath blight fungus penetrates the sheath of rice plants at water-line.

Fig 2. The sheath blight fungus attacks the leaves ultimately drying them.

Figure 3. Vertical and horizontal rice sheath blight disease progress.

Figure 4. Compact fungal masses called sclerotia are formed for lasting survival in soil.

Fig. 5. Sheath blight scouting requires a zigzag pattern monitoring technique away from field edges
or bottoms to make fungicide application decision.