By Yeshi Wamishe, Extension Rice Pathologist
Automatic fungicide applications for sheath blight in rice are highly discouraged due to the potential development of fungicide resistance and non-profitability. Although sheath blight can be reduced with best cultural practices, it is one of the most prevalent diseases for which fungicides are typically applied. Prolonged periods of high humidity and temperatures favor sheath blight (Figure 1 and 2) development in rice. In some situations with susceptible and moderately susceptible rice, sheath blight starts early but mostly from ½” internode-elongation and onward. Since the pathogen is fast-growing under favorable conditions, visit your field frequently and scout.
Making a fungicide application to manage sheath blight will depend on combined factors such as: weather conditions, varietal height and susceptibility, treatment threshold, field history, and field management practices including seeding and nitrogen fertilizer rates. The optimum fungicide treatment timing if the disease is at threshold is often 7-14 days past panicle differentiation. With slow disease progress, it can be delayed until mid-boot. Fungicide application may be aligned with protective application for the smuts. If aligned make sure to use the combination fungicides. More than one fungicide application to manage sheath blight is not profitable in Arkansas.
Always make sure at least the upper three leaves including the flag leaf are unaffected.Correct diagnosis avoids unnecessary fungicide applications. Symptoms of sheath blight under dense canopy when the pathogen is active can be confused with other rice diseases such as aggregate and bordered sheath spots, black sheath spot and stem rot. Fungicides are not recommended for these diseases.
Moreover, your fungicide application decision should not be based on the disease severity at field edges or bottom of a field. These spots often have more infection due to seeding rate, inadequate fertilization or accumulation of sclerotia (the pathogen’s infective structures). To read more on how to scout and infection threshold go to productive-scouting;start-scouting-for-sheath-blight;disease-showing-scouting. To read on varietal susceptibility go to commercial-cultivar-responses.