Rice producers should start scouting fields for sheath blight and blast once joints begin to move. These two diseases pose the most threat for economic losses to Arkansas growers.
Most rice in Mississippi County, Ark., does not have severe pressure from sheath blight which is seen more frequently on prairie soils west of Crowley's Ridge. However, isolated fields may have high enough levels to require a fungicide application.
According to Rick Cartwright, Arkansas Extension plant pathologist, sheath blight is the most common and economically devastating rice disease in Arkansas. Fields with severe pressure typically suffer loss in yield of 10 to 25 percent and reduced milling quality. Weakened stems may also result in more lodging, causing additional yield loss and harvest difficulties.
Properly timed fungicide applications are effective in reducing sheath blight. Cartwright said thresholds should be used to determine the need for a fungicide application.
Sheath blight is more likely to be a problem if a field has a history of the disease or is in a continuous rice rotation or if high nitrogen rates are used to promote lush growth. Rhizoctonia solani is the pathogen responsible for the disease. This fungus overwinters in the soil and crop residue.
Once rice is flooded, sclerotia float to the water surface and begin infecting plants at and just above the waterline. Infection occurs more rapidly under moist, humid conditions, with daytime temperatures in the 80-to 92-degree range and generally develops more rapidly on a producers' best fields which have dense canopies. In those fields, the morning dew persists longer, allowing more time for the disease to develop.
The first symptoms of sheath blight are oblong water soaked lesions near the waterline. In two to three days, the lesions turn light grayish-white, surrounded by reddish-brown margins. As the rice canopy closes and plants come in closer contact, the fungus can spread from plant to plant and up each plant.
A producer's objective is to protect leaves in the upper canopy from infection because they manufacture a large majority of the nutrients that go into the developing kernels.
Scouting should start at internode movement and continue through heading. When scouting, several random points should be checked in a field. Three row feet should be observed at each stop. The stop is considered positive if any signs of the disease are present. One stop per acre or 50 stops per field are recommended.
Thresholds for treating sheath blight vary by variety. For semidwarf varieties like Cocodrie, which is rated very susceptible to the disease, the threshold is when 35 percent of the stops scouted in a field are positive for the disease from seven to 14 days after half-inch internode elongation. For taller varieties like Wells, which is rated only moderately susceptible, the threshold is when 50 percent of your stops are positive.
The threshold levels indicate the need for fungicide applications only if the disease is approaching the upper canopy. If lesions are found low, near the waterline, hold off fungicide application, and continue to monitor the field twice a week.
Cartwright said research shows Quadris 2.08 SC at 12.8 ounces per acre to be the most effective fungicide for control of sheath blight on fields with severe infections. However, for the majority of fields, which have only moderate levels of infection, the 8.5-ounce rate of Quadris has been adequate.
In addition, for moderate infections, other products labeled for sheath blight control may provide adequate control with less expense. These include Moncut, Tilt, Propimax, Stratego and GEM.
If a field with sheath blight is planted next to Cocodrie and the producer is planning a fungicide application to control kernel smut (recommended timing is late boot), he should consider using Tilt, Propimax or Stratego. All three products contain propiconazole, the active ingredient which has performed well against kernel smut.
Make sure to not get sheath blight mixed up with a couple of other diseases — such as stem rot and black sheath rot — that have similar symptoms. They are not as widespread as sheath blight and do not pose nearly the same threat for reducing yields. In addition, there is very limited research on whether an economic response is possible using fungicides on these diseases.
Blast is the other major disease for the producer to keep an eye out for in his rice. Blast is a windborne disease just like rust in wheat. Cartwright advises that many rice fields in the southern part of Arkansas are showing leaf blast, which has been favored by the wet weather the last few weeks.
Cartwright stresses the need to watch fields closely. Most rice grown in the state is planted to varieties that are susceptible (Wells, Francis) to moderately susceptible (Cocodrie) to blast. If favorable weather persists for disease development, head protection with fungicides may be warranted.
Blast leaf lesions should be checked for from joint movement through heading. Leaf lesions from blast are generally diamond or linearly shaped and from 0.25 inch to 1.5 inches long. The lesions have white/tan centers surrounded by dark reddish-brown margins.
If a producer finds suspect lesions, he should take samples to the local county Extension office. The samples can be examined under the microscope to confirm the presence or absence of blast spores.
Blast is more likely to be a problem on fields in which the flood is not properly maintained. Leaf blast lesions are often first found on drought-stressed plants (out of the flood) on the southwest (major prevailing wind) side of the field. Research has shown one way a producer can help reduce losses associated with blast is to maintain a slightly deeper flood (3 to 6 inches).
If a farmer has seen leaf blast lesions in a field and there is a damp, cool forecast, he should consider the use of fungicides to protect the head. Quadris and GEM are the two most effective products labeled for control in Arkansas. Research has shown both to be comparatively effective against blast.
Generally two fungicide applications will be needed for effective blast control. The first shot should be put out at 10 percent headed, followed by the second application when 70 to 90 percent of the heads have about emerged.
Dave Freeze is Extension agent for Mississippi County, Ark.