To reduce aflatoxin contamination of crops, farmers may use biocontrol strains of A. flavus that do not produce aflatoxins. New research suggests native strains may work better.

Tim Lundeen, Editor

April 5, 2019

3 Min Read
Agronomy 196957_web.jpg
This corn cob is infected with Aspergillus spp. in a field located at the Upper Coastal Plain Research Station in Rocky Mount, N.C.

The principle of competitive exclusion in the gut microbiome centers on loading the gut with beneficial microbes to prevent potentially pathogenic bacteria from having adequate space and nutrients to thrive and cause disease.

In grain crops, contamination with the wrong strains of fungus can lead to mycotoxins and other negative effects, but safe native fungi may show promise in the fight against toxic fungal contamination, according to an announcement from the American Society of Agronomy (ASA).

The fungus Aspergillus flavus can infect several crops, including corn, and some varieties or strains of A. flavus produce aflatoxins. Aflatoxin contamination costs U.S. farmers billions of dollars every year and are harmful for humans and animals, the announcement said.

To reduce aflatoxin contamination of crops, farmers use safe commercial strains of A. flavus that do not produce aflatoxins. When applied to crops, the biocontrol strains outcompete the harmful aflatoxin-producing fungi (i.e., competitive exclusion). That reliably reduces levels of aflatoxins in the harvest, transport and storage stages, according to the announcement.

However, commercial strains may not be the only answer. A new study shows that using safe, native strains of A. flavus can be as effective, or even more effective, than commercial strains, ASA said, pointing to a new study led by researchers at North Carolina State University.

"Using native A. flavus strains could have many advantages," said Ignazio Carbone with North Carolina State. "Native strains may be better adapted to the soil type and weather conditions. Therefore, they may perform better in the field compared to non-native strains."

Moreover, using commercial strains can have some disadvantages, ASA said, suggesting that these strains usually need to be reapplied each year, at a cost of $20 per acre. Also, the application has to be done aerially or manually. "That can deter farmers from using commercial strains," Carbone said.

Native strains, on the other hand, occur naturally in growing areas. They may be more persistent in the soil and not need to be reapplied every year.

Carbone and colleagues tested native strains of A. flavus that produce no or low levels of aflatoxin. They also tested commercial strains. Both reduced crop aflatoxin levels, ASA said.

Corn for human consumption can have maximum aflatoxin levels of 20 parts per billion, per Food & Drug Administration regulation. FDA also sets regulatory limits for aflatoxins in livestock feeds, which vary depending on class of livestock and type of feed.

In the North Carolina study, untreated crops had aflatoxin levels above 35 ppb, while native and commercial strains reduced aflatoxin levels to lower than 10 ppb, ASA said.

Unexpectedly, the study also showed that certain combinations of native strains were more effective than commercial strains in reducing aflatoxin levels, ASA said, explaining that the combinations may take advantage of fungal biology: their mating types are compatible, allowing them to reproduce and sustain their population.

When the researchers applied native strains of compatible mating types to the test plots, aflatoxin levels were reduced to less than 2 ppb in some cases. This was a better outcome than any commercial strain, the researchers said.

"Our results suggest that using native strains could lead to sustained reductions of aflatoxin levels. Using native strains could be very cost-effective for farmers over the long term," Carbone said.

Although this study was conducted in North Carolina, Carbone anticipates the approach can work in other areas. A preliminary experiment in Texas also showed that paired native strains reduced aflatoxin levels more efficiently compared to a single commercial strain, ASA noted.

"We need to continue testing this approach in cornfields across different states," Carbone added. "We also need to monitor aflatoxin levels over several growing seasons."

Future field trials may include testing current commercial strains plus a compatible mating partner strain. Different combinations of native strains may also be tested, Carbone said.

Can this approach go beyond aflatoxin? Carbone is optimistic. "Fungal toxins pose a continual threat to food safety. Our approach can potentially be applied to other toxin-producing fungi as well."

Read more about Carbone's research in Agronomy Journal.

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