Aspergillus is a genus of molds, and there are hundreds of its species. One type in particular, Aspergillus flavus, shows up in corn and can produce aflatoxin.
Aflatoxin-producing A. flavus is sort of like influenza. More than one strain of the virus can cause it, and other strains can prevent it.
For the past 13 years, LSU AgCenter plant pathologist Dr. Ken Damann and others have been studying A. flavus, trying to figure out why some types make aflatoxin and others don’t. In the process, they’ve discovered some A. flavus isolates are not only nontoxic but that they inhibit the production of aflatoxin by toxigenic isolates.
When aflatoxin reared its ugly head in 1998 and caused significant problems for Louisiana corn growers, most observers attributed the outbreak to drought. Now, the experts aren’t so sure.
"Dry years tip the balance," Damann said. "Dry weather seems to be required, but it’s not sufficient by itself for a major outbreak. It requires something more – the presence of a high-toxin fungus.
"Our primary interest is biocontrol and the need to understand the fungus," Damann said.
Hundreds of strains of A. flavus show up in corn – some only in the soil and some in corn plants as well. The fungus lives in the soil and moves to corn plants through the air, Damann said. Winds or insects can carry the spores to the silk, which is the primary pathway to the kernels.
The window for A. flavus to enter the kernels is from about midsilking to a week or two after, the LSU AgCenter researcher said.
Aflatoxin "is a hard critter to nail down," said LSU AgCenter specialist and assistant professor Dr. Ron Levy, who’s been working with aflatoxin for the past few years.
"It’s difficult to do trials because of sporadic disease outbreaks and environmental conditions," Levy said. "But this year’s weather has been favorable for development."
Researchers have been evaluating Afla-Guard, an inoculant from Syngenta that introduces a nontoxic strain of A. flavus to outcompete the aflatoxin-producing strain.
Applied on a barley grain carrier, Afla-Guard inoculates the crop with a strain of A. flavus to overpower or win the competition with the aflatoxin-producing strain. Once it is applied to the crop and the nontoxigenic fungus has been activated by moisture, it produces spores that are carried throughout the field by wind and insects. This allows the beneficial fungus to successfully displace the dangerous aflatoxin-producing strains.
It’s important that the spores from the preventive type are present at the same time as the spores from the toxigenic strain, Damann said. "Our work indicates they have to physically touch the toxin producer during the first 24 hours for the preventative to counteract the toxic A. flavus and stop aflatoxin from being produced.
"While we’ve seen reductions in the amount of aflatoxin with the use of Afla-Guard, we need to continue to evaluate Afla-Guard to determine if we can improve its effectiveness in reducing aflatoxin," Levy added.
Corn growers have increased their use of Afla-Guard because of the potential for aflatoxin, Levy said. But it takes a lot of research and time to confirm its effectiveness. "We’ve seen that the product can be effective in reducing aflatoxin, but environmental conditions have an influence."
Last year, Damann put out trials at the Northeast Research Station in St. Joseph using Afla-Guard and three other A. flavus isolates he identified in his laboratory. None was able to inhibit aflatoxin when used alone, but each of Damann’s isolates combined with the use of Afla-Guard stopped aflatoxin from developing.
It appears aflatoxin preventatives are sort of like a flu shot – the inoculum used has to be a strain that will counter the effects of the toxigenic strain of A. flavus present that season.
"Any one of our three native strains with Afla-Guard had a significant effect," he said. "This suggests there were two strains that produced aflatoxin – one inhibited by Afla-Guard and the other inhibited by our native strains, and it takes both to get the best effect. It’s unusual to get such an even result."
Damann has the same experiments out this year to see if the results repeat. –Rick Bogren
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