Writer: Rick Bogren (firstname.lastname@example.org)
BATON ROUGE, La. – Louisiana corn farmers learned a tough lesson in 1998 when aflatoxin showed up in amounts that caused significant problems.
Over the years, researchers have learned much about aflatoxin and Aspergillus flavus, the fungus that produces it. They also have learned that not all strains produce the toxin, and those can actually inhibit the strains that produce it.
“We’ve been looking for specificity and finding which strains are effective inhibitors and their characteristics that differentiate these strains,” said LSU AgCenter plant pathologist Ken Damann.
This has set the stage for the use of biocontrols – nontoxic strains of A. flavus that can get into the corn plant and prevent the toxin from being produced.
The window for A. flavus to enter the kernels is from about midsilking to a week or two after, Damann said.
“It’s important that the spores from the nontoxic strain are present and infect first,” Damann said. “Our work indicates they have to physically touch the infecting toxin producer during its first 24 hours of growth to prevent toxin production by the toxigenic A. flavus.”
This mechanism can be called “competitive inclusion,” which refers to the requirement that both strains are growing together when they come into contact in the infected kernel, he said.
Damann is cooperating with plant pathologists Burt Bluhm of the University of Arkansas and Tom Allen of Mississippi State University to study these biocontrols. Their work is partially funded by a grant from the Aflatoxin Mitigation Center of Excellence administered by the National Corn Growers Association from funds contributed by state corn grower boards throughout the Southeast.
“Last year all three of us tested a spray formulation of Afla-Guard, Syngenta's biocontrol, applied a week before and at the same time as the toxigenic strain. Both treatments were highly effective in reducing aflatoxin contamination,” he said.
Afla-Guard contains a strain of A. flavus that does not produce aflatoxin. It is normally applied to denatured barley seed that acts as a carrier to introduce the material to a cornfield. Once the barley has been spread, the nontoxigenic fungus can be activated by moisture to produce spores that are carried throughout the field by wind and insects and infect the ear through the silks.
One problem with A. flavus biocontrol is that more than one strain can be toxigenic and cause an infection in a corn kernel. And because any one biocontrol strain doesn’t control all toxic strains, biocontrols aren’t 100 percent effective by themselves.
For the past several years, Damann put out trials at the LSU AgCenter Northeast Research Station in St. Joseph using Afla-Guard and three other A. flavus isolates called 17, 19 and 51 that were identified in his laboratory. Each biocontrol isolate in combination with Afla-Guard stopped aflatoxin from developing better than any of the isolates did when applied alone.
“This year we are applying the conventionally treated barley formulation of Afla-Guard and our 17, 19 and 51 strains alone and each of the three with Afla-Guard in Louisiana, Arkansas and Mississippi,” Damann said. “It will be late August or early September before we will have results.”
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 strains of A. flavus present that season.
“In laboratory tests, we found that two biocontrols mixed together were better than any individually,” Damann said. “Each inhibits different parts of the toxigenic spectrum.”