Plant-Growth-Promoting Bacteria: A Methyl Bromide Alternative?

USDA has worked diligently to help meet the crisis that the January 1, 2001 ban on the importation and production of methyl bromide will impose on U.S. and world agriculture.

At the end of 1995, USDA's Agricultural Research Service (ARS) created five new full-time research positions dedicated to finding potential alternatives to methyl bromide. ARS filled these positions with scientists backed with years of dedicated training and research in plant pathology and physiology—all essential in finding ways to keep our agriculture productive and competitive in world markets.

With a background in soilborne pathogens that attack horticultural crops, Cynthia G. Eayre was assigned to one of these positions at the ARS Postharvest Quality and Genetics Research Unit, Fresno, California. Her mission: find biologically based alternatives to methyl bromide as a soil fumigant for strawberries and stone fruits.

"Fumigating strawberry fields with methyl bromide enhances plant vigor and greatly increases yield. And, peach and other stone fruit growers now use methyl bromide to control replant disorder when orchards are replaced.," she says. "Our research goal is to find biological control agents and alternative chemicals to control black root rot of strawberry and replant disorder in orchards. Since these root diseases are not well understood, we also need further study on the biology of these problems."

Eayre approached this daunting task by screening plant-growth-promoting rhizobacteria (PGPR) for their effect on strawberries and peaches. She also tested the chemical methyl iodide (MI) as a soil fumigant to control replant disorder of stone fruits and promote growth of strawberries.

Using Plant-Growth-Promoting Rhizobacteria

"We started with PGPR bacteria because some isolates of rhizobacteria have been found to promote growth and induce disease resistance in other crop plants," she says. Rhizobacteria are so named because they are found on, and isolated from, the surface of roots and in the soil immediately surrounding the roots.

Eayre negotiated a cooperative research and development agreement with Gustafson, Inc., a Plano, Texas, company. The purpose is to further investigate the use of plant-growth-promoting rhizobacteria to enhance root and shoot growth in strawberries and stone fruit and to reduce soilborne diseases. Gustafson supplied bacterial strains known to enhance growth in at least one crop plant.

"Initial data are promising. We have ongoing field trials, but preliminary results indicate that one bacterial strain in particular effectively promotes growth in both peaches and strawberries," Eayre reports. "And we associated several other strains with improved plant growth relative to untreated controls."

While preliminary, she says the results are very significant since this information has not been widely reported before for these crops.

"Our next step will be to see if these strains that promote growth will also induce disease resistance," she says. "These two characteristics are associated in other crops."

The fact that Gustafson already has a biocontrol product on the market puts the company in a good position to develop a PGPR product. Eayre says that growers could easily apply such a product through drip or microspray irrigation.

She is performing related trials to determine if there are interactions between soil temperature and PGPR strains and between rootstock and PGPR strains.

Applying Methyl Iodide

"I chose to test methyl iodide because it is the one compound that is chemically most like methyl bromide," Eayre explains.

Preliminary results from ongoing field tests show that methyl iodide effectively controls replant disorder in peaches. Compared to trees in control plots, tree trunks in plots treated with methyl iodide and methyl bromide were significantly thicker. Eayre is continuing these tests for another year. This ARS research is collaborative with James Sims and Howard Ohr of the University of California at Riverside and Tri-Cal in Hollister, California.

Replant disorders cause growers serious problems. Replant stunts the growth of trees and increases the amount of time before fruit can be harvested for the first time. It also causes more trees to die in their first to fifth year of growth. Peach trees planted in soil where peaches had been recently grown grow very slowly for one to several years. Since it is very expensive to let an orchard lie fallow and rotating another crop can take too much time, growers usually fumigate the soil with methyl bromide. This eliminates harmful organisms or pathogens that might be present.

"Results from our work indicate that methyl iodide could be used as a substitute if methyl bromide were not available," she says. "It would give us more time to develop biological methods to control disease and to discover more about replant disorder of peaches."

In field tests, Eayre found that preplant applications of methyl iodide controlled weeds as well as—and in some cases better than—methyl bromide.

However, a major hurdle is that methyl iodide is not registered with the U.S. Environmental Protection Agency. This is why methyl iodide is not being tested in commercial fields. Registration would require a substantial financial commitment by a private company.

Looking Ahead

Looking to the future, Eayre plans to use the methyl-iodide-treated plots to study the causes of replant disorder in peaches. She is also looking at fungi and bacteria that may be involved in causing replant disorder.

Eayre plans to test strains of plant-growth-promoting rhizobacteria found to be effective on strawberries for their ability to resist the lesion nematode, Rhizoctonia, and Phytophthora, all problems for strawberries.

"We are testing strains known to be effective on peaches for their ability to induce resistance to the ring nematode and tolerance to peach replant disorder," she says.

Last Updated: July 24, 1998