A student research team led by Fresno State biology professor Alice Wright is looking for ways to help California’s agricultural industry transition from the use of methyl bromide to less toxic chemicals that will still help control unwanted soil organisms.

The team has spent the last three years studying the performance of four alternative chemicals that may be useful to agriculture but less of a threat to the environment.

Twelve Fresno State students worked on various aspects of the research over its duration. Several conducted their work while in master’s degree programs, and four have continued on to doctoral programs at top national universities based on their work on microbial degradation.

“Methyl bromide is a fumigant heavily used in California during pre-planting and soil preparation,” Wright said. It is especially effective in killing fungi, microorganisms and nematodes, but it also is highly toxic. Methyl bromide is targeted for elimination under the U.S. Clean Air Act because it causes depletion of the earth’s ozone layer, she noted.

In agriculture, methyl bromide is commonly applied into the ground where certain types of crops are to be planted. It kills soil organisms that could damage plant seedlings or compete for plant nutrients such as nitrogen, which are added to stimulate plant growth.

“Methyl bromide is very effective. That’s why it’s so popular,” Wright said. “However, with its elimination on the horizon, our goal in this research was to develop pest management practices that are environmentally friendly and still allow sustained or improved crop production.”

Wright developed a protocol for testing the effects of four chemicals that have been developed or are under consideration for use as soil fumigants. They are methyl iodine, chlropicrin, propargyl bromide, and dichloropropene.

Petri dishThe protocol involved applying different formulations of the fumigants to soil samples with typical bacterial and fungal populations, then monitoring the samples for up to 36 weeks to determine microorganism mortality rates.

The researchers used molecular study techniques to characterize changes in the soil microbial communities after each treatment application.

“Based on the analysis of DNA levels in the soil, we found that the number of microbes in the soil was severely decreased after the application of the pesticides, suggesting that most, but not all, of the microorganisms were killed by the fumigants,” Wright said. One fungal pathogen, however, was still found to be thriving one week after fumigation, she noted.

Long-term soil monitoring indicated no significant increase of microorganisms after three weeks, suggesting the impacts of the fumigants is lasting. Examinations at 36 weeks indicated that the soils had returned to normal condition in terms of microorganism repopulation.

Another phase of the study was to isolate and monitor soil microbes that showed resistance to the fumigants and actually helped to degrade them by metabolizing fumigant molecules into water and carbon dioxide. Several bacteria species were found capable of aerobic degradation of the fumigants, Wright reported.

One of the fumigants, propargyl bromide, showed promise in initial testing. However, because of the chemical’s unstable nature, it is unlikely that product companies would pursue its manufacture and sale, Wright said.

Wright and several of her students presented study results at the Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions.

“It was the first report of microbial degradation of some of the alternative pesticides and the

first to show that the biodegradation process could be significant in soils,” Wright said. This raised concerns about soils becoming “immune” or resistant to the pesticides, she noted.

“However, our further testing shows that the initial treatment kills most microbes, even the degrading ones. Furthermore, soils return to a normal microbial community structure after 36 weeks.”

Details of this work are available on the website of the California State University Agricultural Research Initiative (ARI), which funded a portion of the research along with several other public agencies and private agricultural industry groups. The ARI web address is http://ari.calstate.edu. The project title is “Methyl Bromide Alternatives,” ARI Project No. 03-1-012.

(Copy by Steve Olson of the California Agricultural Technology Institute.)