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Seeds battleground for disease war

FAYETTEVILLE, Ark. — Seeds planted in Arkansas' rich agricultural soils often face an assortment of lurking organisms that can severely damage crops.

Soilborne pathogens like Pythium species and Rhizoctonia solani wait for environmental stresses to weaken resistance in soybean, rice and other plants, said Craig Rothrock, plant pathologist for the Arkansas Agricultural Experiment Station.

Without protection, these organisms can cause seed rot or pre-emergence damping-off so that the plants never get out of the ground, Rothrock said. Soilborne pathogens can also cause postemergence damping-off, in which the plants fall over and die after emerging.

"Pythium and other pathogens have strategies to survive," Rothrock said. "They can exist in all soils used in crop production and can infect a wide range of host plants."

He said soilborne pathogens remain dormant until they detect exudates — nutrients that leak from seeds or roots. Detection happens at close range, usually within about a millimeter. Pythium species are similar to algae. These "water molds" produce motile spores that can actually swim toward a host plant.

"Certain conditions put plants under stress," Rothrock said. These conditions, depending on the crop, can include flooding or cold or any number of other adverse conditions. "When the plant is unable to defend itself, the organisms can attack and we see stand losses."

University of Arkansas Division of Agriculture scientists are studying strategies to protect soybeans and rice from these pathogens.

"It's best to plant a crop when conditions are favorable for germination and growth," Rothrock said. "Many farmers are planting earlier in order to conserve water. An earlier planting date means putting seed in cooler, damper soil. This can put stress on the plants that makes them vulnerable to pathogens."

Rothrock and UA plant pathologist John Rupe, and their graduate students Gary Bates and Luciana Rosso, are looking at individual fungicides and combinations of fungicides for use as seed treatments.

"Seed treatments protect seeds from those soilborne pathogens that are close enough to attack," Rothrock said. "This won't kill all the pathogens in the field, but it protects germinating seed and uses much less fungicide than is needed to treat a whole field."

Rothrock said resistant varieties are always the best strategy for controlling diseases. "You don't pay extra for it, and it's always out there to defend your crop during times of plant stress."

Although it is unusual to find resistance to these soilborne pathogens, research revealed that Archer, an ultra-short season soybean grown in northern states has good resistance to Pythium species, Rothrock said.

"We're looking at the nature of resistance for clues that will help us develop improved crop varieties," he said.

Studies indicate pathogens are stimulated more by the exudates from susceptible varieties than they are from resistant varieties, he said. Archer also has two genes that give it resistance to Phytophora sojae, another soilborne disease that causes stem and root rot in soybeans. Rothrock said one of these genes also gives almost complete resistance to Pythium.

"Finding that gene allows us to move from learning about the nature of resistance to breeding the resistance into cultivars adapted to Arkansas," he said.

Fred Miller is science editor for the Arkansas Agricultural Experiment Station. e-mail: [email protected]

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