WOOSTER, Ohio - In the struggle to protect soybean plants against root diseases, one Ohio State University researcher is taking the fight below the soil surface.
Brian McSpadden Gardener, a plant pathologist with the Ohio Agricultural Research and Development Center, is studying biological control of plant pathogens using beneficial bacteria that colonize plant roots. The goal of the research is to identify the distribution of soil bacteria across Ohio to determine which soybean fields would be ideal candidates for biological control applications.
"We know that some bacteria can promote plant growth but what we don't know is how much of those beneficial bacteria are in individual fields or how widely distributed they are across any defined geographical area," said McSpadden Gardener. "The ultimate goal is to find out if there is a way to assay a field and say okay, there are very low populations of beneficial bacteria here, so this would be an ideal spot to apply biological control."
In his work, in collaboration with Ohio State University Extension soybean agronomist Jim Beuerlein, McSpadden Gardener has identified two bacterial genotypes specific to Ohio soils that produce a natural fungicide known as DAPG (2,4-diacetylphloroglucinol). DAPG-producing bacteria that colonize soybean roots protect the plant from a wide range of root rot diseases, including Phytophthora, Pythium, Rhizoctonia and Fusarium.
"What we are doing is isolating different representatives of these DAPG-producing bacteria and treating seeds with them to see how well they colonize soybean roots and how well they protect roots against diseases," said McSpadden Gardener. "Our goal is to improve stand and increase plant yields by applying these DAPG-producing bacteria to seed as natural fungicides."
The DAPG compound works much like a chemical seed treatment fungicide in that it diffuses away from the plant roots, protecting the plant from fungal pathogens. But, unlike chemicals that lose their effectiveness once the plant grows out of its juvenile stage, DAPG-producing bacteria provide the plant protection from germination up to harvest.
"With synthetic chemicals, once that plant grows out of its sphere of influence, the fungi can sniff out the roots and attack," said McSpadden Gardener. "The idea behind biological control is that if you put the protective bacteria on the seed, as the plant germinates, the roots get colonized by the beneficial bacteria and they keep reproducing as the roots grow so more of the roots are protected more of the time. There's no other way to protect plants like that with a seed treatment."
During the two-year study, funded by an OARDC Seed Grant and the U.S. Department of Agriculture National Research Initiative, the researchers found that at 10 soybean sites throughout Ohio only about half of the plants harbored DAPG-producing bacteria. McSpadden Gardener said soil type, bacterium genotype, soybean variety, weather and production management are just some of the factors that determine the level of beneficial bacteria that are present to colonize plant roots.
"That's why it's so important to identify genotypes that are indigenous to Ohio," said McSpadden Gardener, who is also studying bacillus species that may prove effective in Ohio against soybean diseases. "A non-indigenous bacterium is not familiar with the Ohio soils or the environment, so it may not be as effective."
Seedling diseases cause a loss of about 3/4 of a million tons of seed annually, accounting for over $150 million in U.S. losses. Beneficial bacteria may be used as natural, effective and economical disease controls because of the unique relationship plants have with soil microbes. Between 5 and 10 percent of the carbon a plant fixes is released into the soil providing food for bacteria that colonize plant roots. Managing those plant-microbe associations can improve plant health.