Rice Sheath Blight Advisory
By Brian Ottis

As we approach the later reproductive stages in rice development, it is important to be on the lookout for sheath blight disease. Sheath blight is caused by Rhizoctonia solani, the same pathogen responsible for aerial blight in soybean. Although we don’t have the disease pressure in Missouri similar to states further south, there are always problem fields that need to be scouted frequently. Several factors affect sheath blight development in the field. Some of these factors we can control, but Mother Nature most often determines where disease will show up.

When considering sheath blight, some of the factors we can control are variety selection, seeding rate, and fertility. If you discover fields on your farm with a history of sheath blight problems, consider switching to a more resistant variety. For example, most of the semi-dwarf varieties grown today are more susceptible to sheath blight than the conventional-height and hybrid types. A few examples of susceptible varieties are CL131, CL161, Cybonnet, and Trenasse. If you have a red rice problem and utilize the CLEARFIELD* production system, a new variety, ‘CL171’ should be available in limited quantities next year with better sheath blight resistance. CLEARFIELD* hybrids should also be considered in fields with a history of sheath blight as they are rarely affected by sheath blight.

Seeding rate is another factor we can control in the fight against sheath blight. Rice has an uncanny ability to tiller to compensate for voids in the canopy. If you have had a history of sheath blight in your fields, you may have noticed that it tended to be worse in areas where rice was double-planted on the ends of the field. When the canopy is thick, air movement through the canopy is impeded, which allows moisture to persist longer providing perfect conditions for disease development. Lower seeding rates produce a more ‘open’ canopy, which allows more efficient air movement through the canopy.

Sheath blight development is also related to nitrogen fertility. In cases where too much nitrogen has been applied, or overlapped on the edges, sheath blight development can be severe. Excessive nitrogen leads to lush vegetative growth, which creates a similar situation to high seeding rates.

In the event that sheath blight develops on your farm, there are a couple of management options that must be considered. First, determine if a fungicide application will be warranted by considering the variety, yield history, and stage of growth. If the rice is heading, a fungicide for sheath blight is usually not warranted at this stage as the panicle has ‘escaped’ the disease, or is too late. If the variety is CL161 or another susceptible variety, scout these fields twice a week if possible.

Once the disease is found at a third of the stops in the field, an application of a strobilurin fungicide is usually warranted between ˝” internode and late boot. On less susceptible varieties, such as Wells and Francis, scouting does not need to be as frequent, but a fungicide application may still be warranted if disease begins moving up the culms toward the flag leaf. With these varieties, wait until late boot before applying fungicide as sheath blight may not become severe enough to warrant an application. Research has shown that yield loss associated with sheath blight is most likely due to infection of the flag leaf, which supplies most of the energy to the panicle during grain fill. Therefore, even if disease thresholds have been reached, it may be better to wait until the disease is two to three leaves below the flag leaf until the application is made in order to get more ‘bang for your buck.’ In the case of hybrid rice, research has shown that more often than not, a fungicide application did not pay for itself in protected yield. Similar results were found for medium-grain varieties such as Bengal, Medark, and Jupiter.

Brian Ottis
Rice Agronomist
Delta Research Center
573-379-5431