By Yeshi Wamishe, Extension Rice Plant Pathologist
Time is approaching for April planted rice reaching the need to scout for sheath blight. The earliest rice planted on April 17 at RREC in my research plots is now reaching ½ inch IE (internode elongation). The hot weather in the last couple of weeks helped it to grow like normal and the drier weather in the last couple of weeks did not support any disease development. Based on the prediction provided here nearly 93 percent of Arkansas rice will get to ½ inch IE and beyond by July 12. Currently, there is rain hither and thither across the state. With the warm temperatures, it should be ideal for sheath blight to take off particularly in fields planted with susceptible or very susceptible varieties in dense canopy and excessive pre-flood nitrogen fertilization.
We encourage you to plan ahead to start scouting for sheath blight. Determine the threshold before you make decision to apply fungicides as shown in Table 1. Automatic application of fungicides to manage sheath blight in rice is not recommended due to the potential development of fungicide resistance and note that we do not have several options of fungicides for rice. Moreover, it is not profitable to apply fungicides for sheath blight alone more than one time. Making a decision on fungicide application to manage sheath blight should depend on combined factors–treatment threshold, weather conditions, varietal height, varietal susceptibility level, field history and field management practices such as seeding and nitrogen fertilizer rates. If sheath blight is below the threshold, it is wise to delay until boot stage for two important reasons: (1). To pair your fungicide application for sheath blight with fungicides to suppress false and kernel smut. (2). Boot application may keep the disease suppressed until the crop stage advances and there is no threat to the upper three leaves. The optimum fungicide treatment timing if the disease at threshold is often 7-14 days past panicle differentiation. Our research indicated boot application as an acceptable timing as long as the disease progress is slow enough. In situations where pairing fungicides to target more than one disease is possible, combination fungicides can be used at one go and application cost can be reduced by half.
Correct diagnosis of sheath blight in rice is always important. Because when the pathogen is active under dense canopy, symptoms (Figure 1) of sheath blight can be confused with other rice diseases such as stem rot or foot rot. Sometimes sheath spots are also confused with sheath blight later in the crop development. Since fungicides are not recommended for these diseases, incorrect diagnosis can incur unnecessary cost on producers.
You can scout field edges and bottoms. However, your decision for fungicide application should not depend on what you see at field edges or bottoms. You need to walk in a zigzag pattern as shown in Figure.2 further from the edge or the bottom of your field to determine the need for fungicide application across the field. Yet, spot application on the edges and field bottoms can be carried oud if the disease is bad and disease progress is fast. You do not want your rice to look like Figure 3 with sheath and leaves consumed by the pathogen.