By Yeshi Wamishe, Extension Rice Pathologist
Scouting for sheath blight rice disease is recommended starting at the green ring developmental stage. Automatic application of fungicides is highly discouraged due to the potential of fungicide resistance development and nonprofitabiliy if applied in the absence or when sheath blight is below the threshold level. Additionally, correct diagnosis of the disease is important to avoid unnecessary fungicide applications since symptoms of other rice diseases such as aggregate and bordered sheath spot may be confused with those of sheath blight.
The sheath blight fungus mostly survives as “sclerotia” (tiny masses of fungal structure called “mycelia”) (Figure.1). Sclerotia can float on flood water and initiate infection when they come in contact with rice tissues at the waterline. Infected residue can also serve as a source of inoculum. Therefore, sheath blight disease of rice can be more severe in flooded rice than row rice.
The sheath blight disease is favored by warm temperatures and wet/humid conditions. Under the right conditions, the disease progresses fast both vertically and horizontally. Application of fungicides twice just to manage sheath blight may not be profitable. The best timing for a single fungicide application is at boot growth stage. While one fungicide application is recommended, two applications may be required if the disease starts early and the environment encourages disease progress. Therefore, continuous scouting starting from green ring until after heading is advised. Susceptible or very susceptible (“S” or “VS”) cultivars are recommended to be treated at 35% positive stops; and moderately susceptible (“MS”) cultivars at 50% positive stops as shown in Table 1.
Varietal susceptibility level, number of positive stops and weather conditions all must be considered when making fungicide application decisions. For more details, refer to MP 192 Page 125 https://www.uaex.edu/publications/pdf/mp192/mp192.pdf. Number of positive stops at field edges should not be used to make application decision across the whole field. However, field edges which may have been double drilled, can be scouted for spot fungicides treatment.
Strobilurin fungicides (Quardris, Quilt Xcel, Stratego) are still effective at managing sheath blight disease of rice. These fungicides can also be rotated with alternative mode of action fungicides such as fluxapyroxad (Sercadis) or flutonil (Elegia). Refer to MP 154 page 68 https://www.uaex.edu/publications/pdf/mp154/rice.pdf for more info on rice fungicides. Follow the most recent label instructions. Labels are the rule.