Rice Farming

Sheath Blight

Scouting, application timing crucial to managing No. 1
Mid-South rice disease

From semi-dwarf long-grain varieties to herbicide-tolerant rice, growers have a number of innovative options when making their variety selections. As producers know, no seed variety is perfect for every situation. In fact, two of the most common semi-dwarf varieties – Clearfield and Cocodrie – are both susceptible to sheath blight and some of the other common diseases.

But should farmers be overly wary of varieties susceptible to disease? Experts say no. Other attributes make these new strains of rice much too valuable to let disease susceptibility override their benefits.

Disease management options
Sheath blight – the No. 1 rice disease in the Mid-South – is no stranger to growers, especially if they grow susceptible varieties or have a field history of infestation. Dr. Rick Cartwright, professor and Extension plant pathologist with the University of Arkansas, and Dr. Don Groth, LSU AgCenter plant pathologist at the Rice Research Station in Crowley, have done a good bit of collaborative work on rice diseases, and they describe the following symptoms of sheath blight.

When present in a field, sheath blight – Rhizoctonia solani – forms light circles in affected areas. It actually begins at the water line and progresses up the plant. When it reaches the top, very little grain will be produced. These spots often coalesce, forming large areas of the field that appear grayish. Fungus mycelium grows up the leaf sheath. The fungus can spread in the field by growing from tiller to tiller on an infected plant or across the water surface to adjacent plants.

There are a number of management options when dealing with sheath blight. Three primary strategies include selection of a resistant or moderately resistant variety, crop rotation and chemical management. Selecting a resistant variety is logical, as long as that variety fills all of the production goals of the grower. But that’s not always the case.

Rotation is another sound strategy, but rotating of rice with soybeans doesn’t break the disease cycle. And rotation to other crops is not always practical for many growers. That leaves chemical management.
Experts say the most important component in disease management is scouting.

“We would like to see all of our growers treat fields based on scouting,” says Cartwright. “Not only does it enable producers to identify the problem, it also enables them to better time the application, a must for maximum return on investment.”

Groth concurs. “It’s advisable to start scouting rice at mid-tillering or soon afterwards for sheath blight and similar diseases. Sheath blight can kill the sheath and leaf tissues. It tends to cause lodging and may affect the head, reducing both yield and quality.”

Fungicide application timing
When it comes to disease control, sheath blight has its own cycle. Syngenta recommends early applications at seven to 14 days after panicle differentiation for maximum effectiveness. Growers who also have problems with blast, which occurs after sheath blight, often make one application at heading in an attempt to control both diseases.

Cartwright and Groth warn that this can be risky, especially if the sheath blight comes on early. Their best advice for the grower who makes a single application is, “Time for the worst.” Or, time the fungicide application based on the most damaging disease present in the field. A program of split applications for sheath blight and blast is optimum, but they concede that economic constraints often limit this practice.

Quadris is the most commonly used fungicide option for sheath blight, according to Groth. “Quadris is also effective against blast. We probably had it on 70 percent of our acres last year.” Cartwright acknowledges that Quadris is also commonly used in his state, but with smut being a bigger concern than it is in Louisiana, Arkansas growers have to rely on more than one chemistry to control blight, blast and smut. Quilt, a premix formulation of the active ingredients in Quadris and Tilt, offers control of all three diseases.

The bottom line for producers is to scout fields early and often, especially if planting a susceptible variety. Choose a fungicide that has the broadest spectrum disease control available to optimize fungicide applications and to obtain potentially better return on your investment. For more information on disease rating by variety, contact your state agricultural Extension service.

Gibbs & Soell Inc., who represents Syngenta, contributed information for this article.