Texas-based research uncovers biological weapons against sheath blight of rice

Researchers in Texas and the Delta region have some good news for rice growers concerning sheath blight. They have discovered a combination of pest management tactics that will effectively control sheath blight and require only half of the amount of fungicides currently needed.

Caused by the pathogen Rhizoctonia solani, sheath blight is the most devastating disease of rice. Although the disease may not kill the crop, it severely affects the yield and the quality of the grain, sometimes making it unmarketable. R. solani is a fungus that lives in the soil undetected, and once it attaches to the sheaths, it quickly spreads to other plants, facilitated by their close proximity, until the disease has infected the entire crop. Since the fungus infects soybeans as well, rice-soybean rotations—typical in the Delta region—do not help farmers rid the soil of the pathogen.

“If a farmer had sheath blight the first year, he would grow soybeans the second year,” said Xin-Gen “Shane” Zhou, Texas A&M AgriLife Research scientist and primary project director. “But they wouldn’t reduce the incidence of the disease. The sclerotia can survive in the soil for two years.”

Traditional control is aerial spray of strobularin fungicides, but because the pathogen lives in the soil and initially infects the lower sheaths and leaves, fungicides often can leave plants partially untreated. In addition, fungicides are labeled for the first (main) crop only, so in double-cropping situations, rice farmers have no fungicide control option for a second (ratoon) crop.

Growers use nearly one million pounds of fungicides every year in rice-producing states to control the disease. Because only one class of fungicides is labeled for sheath blight in rice, researchers were concerned that the fungus might develop resistance if the full dose of fungicides were used every year.

In 2011, their worst fears materialized, as the fungus began showing the signs of fungicide resistance in localized areas. So Zhou and others began looking into biologically-based options that would control the fungus and reduce the need for fungicides at the full strength. Reducing the amount of fungicides would delay the onset of widespread fungicide resistance and keep already resistant populations from multiplying too quickly.

Zhou’s research examined two biologically-based control options: rhizobacteria that would attack the fungus itself, and a brassica cover crop that would “fumigate” the soil with allelopathic chemicals toxic to the fungus. A USDA Southern Regional IPM grant  funded the research.

Out of 70 initial strains of rhizobacteria, Zhou’s research team discovered one that was very effective at suppressing sheath blight: rhizobacteria strain MBI600.

When sprayed on rice leaves, MBI600 colonizes the hyphae, or main fibers, of the fungus, causing them to shrink and shrivel. Eventually the bacteria engulf the entire fungal structure, and the fungus coils and shrinks, finally breaking up completely.

Although field trials proved that MBI600 controls the disease, when used without fungicides it does not increase yield. However, when used with half of the recommended amount of azoxystrobin, it not only controls the disease at the same rate as would a full dose of fungicides, but it also increases grain yield.

“Based on controlled environment studies, seed treatments with MBI600 had an increased germination in addition to disease control,” Zhou says.

Rice seeds can be treated with MBI600, so by combining the treated seed with a fungicide treatment at 50 percent, growers spend less on sheath blight control than when using only fungicides. Because the treatment is seed-based, the entire plant is protected rather than only the parts reached by the aerial fungicide sprays. The bio-based nature of the treatment also gives double-crop rice growers a weapon to use on their second crop.

In addition to the MBI600, Zhou discovered a cover crop that would add to the arsenal by reducing the amount of fungus present in the soil. In separate trials, Zhou tested various brassica crops as potential allelopathic cover crops. Brassica species are often used as cover crops, since many of them exude chemicals into the soil that are toxic to fungi and some weeds.

Out of 37 initial brassica species, researchers found one that was particularly effective at reducing the sheath blight pathogen: mustard ‘Caliente 199.’

Repeated trials over a two-year period resulted in an optimal planting rate for ‘Caliente 199’: about 5 pounds of seed per acre. The plants grow to maturity and then are plowed into the soil before winterkill. According to soil tests, ‘Caliente 199’ reduced fungus populations significantly more than did fallow fields. The allelopathic properties of the cover crop reduced other pathogens in the soil as well.

Researchers are still experimenting with the timing of planting the cover crop, since planting dates differ from state to state, and growers using field crop rotations would have to time their plantings carefully. Because the crop fumigates phytotoxic chemicals into the soil, growers need to wait about a month between the removal of the cover crop and the planting of the next field crop.

“Brassica needs a long planting time, and farmers need information on how to plant it,” says Zhou. “The MBI600 and fungicides at the 50% rate are tools they can use now.”

Strain MBI600 is the active ingredient in the biopesticide Integral®.

Contact: Xin-Gen (Shane) Zhou, Research Plant Pathologist & Assistant Professor, Texas A&M AgriLife Research, 409 752 2741 ext 2210 (Office), 409 223 0178 (Cell), xzhou@aesrg.tamu.edu

Writer: Rosemary Hallberg, Communication Director, Southern IPM Center, 919-513-8182 or rhallberg@sripmc.org

2 Responses

  1. Great news! A growing number of people are beginning to express concern for the amount of pesticides and fungicides used to grow the food we eat. This discovery will surely ease the minds of the health conscious.

  2. Hello! This is my first visit to your blog! We are a group of volunteers and starting a new project in a community
    in the same niche. Your blog provided us beneficial information to work on. You have done
    a marvellous job!

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