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Scout rice for yield-robbing diseases

Knowing which diseases could be infecting your rice crop, and being able to identify their symptoms, could reap harvest-time rewards.

“Correct disease diagnosis is essential for both the economic justification for treatment and for protection against crop failure,” says Joe Street, Extension rice specialist at the Delta Research and Extension Center in Stoneville, Miss.

While situations may occur that require the use of a costly fungicide treatment, rice specialists remind growers to make sure the disease you are trying to control is a damaging one and not a minor disease. The top three potentially economically devastating diseases Delta growers should be on the lookout for are sheath blight, blast and smut.

Termed the “most important rice disease in Mississippi,” sheath blight is widespread, reducing both potential yields and milling quality of much of the area's rice crop. The reason for the rise in sheath blight cases, rice specialists say, is the increased planting of susceptible semi-dwarf, long-grain varieties, and increased nitrogen rates.

“Increased rates of nitrogen fertilizers have been shown to be correlated with increased plant susceptibility as well as making the environment within the rice canopy more favorable for disease development,” Street says.

Sheath blight is caused by the fungus Rhizoctonia solani, and is identifiable by the spots it causes to form on the leaf sheath near the water line. According to Mississippi's disease identification guide, sheath blight spots are oval and range in size from one-quarter to three-eighths of an inch wide and three-eighths of an inch to one and one-half inches long. They can have a light gray center and a dark gray border, or a white to tan center and a brown border. As the disease progresses, it moves along the plant foliage.

“The appearance of white, web-like mycelium (mold) on the surface of spots and adjacent green areas is an excellent way to distinguish sheath blight from other diseases that affect leaf sheaths,” Street says.

Because sheath blight control is difficult, Street recommends growers select resistant varieties, rotate rice with other crops, and closely follow nitrogen fertilizer recommendations.

Caused by the fungus Pyricularia grise, blast is another one of the most damaging rice diseases in the Mid-South, and is capable of causing yield losses of more than 50 percent.

Occurrence of blast is unpredictable, Street says, because of yearly changes in weather, acreage planted to susceptible varieties, and the development of new races of the fungus that are capable of attacking varieties previously resistant. This airborne disease can also travel considerable distances.

Symptoms of the disease vary depending on which area of the rice plant is infected with the disease.

On the plant's leaves, it appears as an elongated diamond-shaped spot with a center that is usually gray or white with a brown or reddish-brown margin. Similar lesions can occur at the base of the flag leaf near the sheath.

“On more susceptible varieties, spots are usually about one-half inch long, and often have no dark margin but a yellow margin or halo surrounding the spot. On more resistant varieties, the spots are very small and rounded with a thin brown margin,” Street says.

“Leaf blast is not generally considered a damaging phase of blast disease, although it is a convenient warning that blast will be a problem.”

Panicle blast occurs when the primary and/or secondary grain branches become infected with the disease. Depending on the stage of grain fill at infection, portions of the grain head, or the entire grain head, will be white in contrast to the green or tan color of healthy grain, Street says. “The blasted appearance is caused by sterile or blank grain. Seed from infested fields are often contaminated with the blast fungus.”

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