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Rice blast, sheath blight active now
Author: Yeshi Wamishe, Extension Rice Plant Pathologist
Long, medium, tight shots of leaf blast.

Leaf blast in rice.  (Click image for larger, clearer view)

Leaf blast was reported in the second week of June on CL151 in some areas in Louisiana (Groth, Rice Newsletter, No. 12). There came another report of blast on CL152, Jupiter, and Mermentau in the third week of June from Louisiana (Groth, Rice Newsletter, No. 13) followed by a report from Texas in experimental plots of “Rex” and several other rice varieties in Jefferson County, Texas, (Shane Zhou, Texas A & M). The infected plants were in late tillering stage.  Reportedly, Rex was one of the rice varieties devastated by blast in LA during 2012 (Walker, Mississippi State University).  Based on blast marker genes, Rex has been reported to have the Pi-k gene that provides resistance to one of the races of blast (IB54) that occurs in the southern United States (McClung, USDA-ARS DBNRRC). However, because of changes in the blast fungus over time our resistant varieties may fail, based on the prevailing races of blast present during the growing season. We always need to keep alert on the need for timely scouting and managing the disease with this in mind. Remember, blast can cause up to 100 percent loss under near-perfect environmental conditions for the disease.

Leaf blast reported 

Leaf Blast was reported on Jupiter rice near Hunter, in Woodruff County, Ark., this week (Photos A, B,C). The crop was at green-ring and the field was surrounded by trees, which tends to extend dew period on the leaves.  The field was located close to a bayou on the west side, and leaf blast appeared to be spreading across the field from south to north. Flood depth was lower than recommended to suppress blast disease development so overall, conditions in this field were ideal for leaf blast (Photos A,B,C).

Fields with any history of blast disease in past years or fields planted to susceptible varieties in fields with favorable conditions like those mentioned above should be scouted at this time for leaf blast.  This is especially true for fields with intermittent flood management, shallow flood, row-watered or other potentially water-stressed rice fields.  Blast can be managed if you can catch it early, without sacrificing much yield if everything works out. We recommend an integrated management package – increase and maintain flood to 4 inches or deeper if possible; do not use too much nitrogen at midseason; and be prepared to use a labeled fungicide at late boot and early heading.  For leaf blast epidemics where rice plants are “burning down” in large spots or areas, it is a field judgment call as to whether to slow it down with a fungicide application earlier than late boot.  Usually, if you can pump the field up and hold the water deep, then leaf blast development slows or stops and you can wait until late boot to use fungicides.  However, there are instances where an application for severe leaf blast may be warranted.  Contact your county agent and field consultant for advice because someone is going to have to assess the situation on the ground – there is no substitute for experience in these situations.

Close up of sheath blight on rice.

Sheath blight.

Sheath blight

Active sheath blight disease was also reported on Jupiter near Hunter, Ark., this week. The disease (Photo D) was more severe where rice was more lush and had a dense canopy. The crop was at green ring and while symptoms were still low, lesions were very active and warm, humid conditions in the field indicated the disease would be developing rapidly over the next few days.  Typically, Jupiter and other medium grains are not considered very susceptible to sheath blight, but may benefit from fungicide treatment under conditions where the field is lush, has lots of nitrogen, where soil potassium is historically low or planted thickly.  Since sheath blight is a “patchy” disease and does not spread rapidly like blast or other airborne fungi that infect rice, then many early patches of active disease across the field are needed before potential yield loss and a fungicide is likely.  In this case, sheath blight was widespread in the field and moving rapidly, so we decided to treat soon and prevent the disease from developing to the upper canopy too soon.  Effective scouting and treating with the correct fungicide to keep the upper leaves of the canopy healthy during booting and grain fill is the most economical way to use sheath blight fungicides on most varieties in Arkansas.

Remember that careful use of nitrogen fertilizer and keeping soil potassium levels at adequate levels really help in the management of this disease, and likely will minimize the need for fungicides.  While still being validated, the N-STAR nitrogen soil test appears to have potential to help us manage this disease by providing a better estimate of field nitrogen need than previous methods.

Please contact your county Extension Office for more information on rice disease management.


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