By Yeshi Wamishe, Extension Rice Pathologist
Automatic application of fungicides to manage sheath blight in rice is highly discouraged due to the potential development of fungicide resistance and non-profitability. Fungicides for sheath blight are recommended if the disease level reaches its threshold level. According to Dr. Jarrod Hardke’s projection, over 90% of Arkansas rice in 2018 will reach ½” internode by the end of June. (Arkansas-rice-update-6-22-18). Start scouting for sheath blight rice disease.
Note: Correct diagnosis of sheath blight is important to avoid unnecessary fungicide applications since symptoms of other rice diseases such as aggregate and bordered sheath spots, black sheath spot and stem rot symptoms may be confused with those of the sheath blight.
The sheath blight fungus is soil-borne and has hundreds of hosts including soybean and corn. Therefore, we can assume as no field is free from the fungus. The pathogen mostly survives as “sclerotia” (tiny masses of fungal structures called “mycelia”). Sclerotia can float on flood water and initiate infection when they come in contact with rice tissues at the waterline. Infected residues 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 favorable conditions, the disease progresses fast both vertically and horizontally. See the level of vertical progress in our experimental fields 7 days after inoculation (Figure 1).
When making a fungicide application decision you need to consider varietal susceptibility level, number of positive stops, and weather conditions. 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 and follow zigzag scouting pattern as shown in Figure 2. The lower end of a rice field will often show more sheath blight, and a fungicide application decision should not be based on information from these areas.
The best timing for a single fungicide application is at boot growth stages. While only one fungicide application is recommended, two applications may be required if the disease starts early and the environment encourages disease progress. Based on the timing for other diseases, application synchronization can be carried out. For instance, recommended timing for kernel smut and false smut fungicide protection is from early boot mid boot (swollen). If sheath blight is at threshold, combination fungicides can be used to address all the three diseases. Likewise, if threshold is at late boot or boot split, standalone fungicides can be used to address sheath blight and the first blast application. The first neck and panicle blast protection is recommended from late boot to 10% head out. For more details, refer to Page 125 mp192. Please visit the table in Rice Info Page1 for fungicide timing for selected rice diseases and on Page 2 of the same PDF for fungicides and active ingredients for rice disease management in rice.