With corn acreage surging, Mid-South growers expected a bountiful, profitable 1998 harvest. Then, aflatoxin rode in on a pale horse and ruined the party. The fungal toxin proved devastating for a large group of growers.
When aflatoxin levels reach 20 parts per billion, affected corn can’t be shipped across state lines and elevators shy away. Aflatoxin can easily lead to food and feed safety issues. Annual losses to the disease are estimated at some $190 million.
“Many years, there are localized problems with aflatoxin,” says Paul Williams, a USDA-ARS research geneticist based on the Mississippi State University campus. “But 1998 hit the Mid-South very, very hard.”
Williams remembers the post-harvest reaction.
“Folks called when they found out we were researching aflatoxin. They were looking for answers. When researching something like aflatoxin contamination, you can labor in obscurity until the problem shows up in a serious way. At those times, everyone wants to know what you’re doing and if research has shown any solutions.”
To reduce the incidence of aflatoxin, handling corn at harvest and post-harvest is very important. Growers need to get grain in and stored quickly at 12 to 13 percent moisture. That will keep the fungus from growing and producing toxin.
Williams believes some of the aflatoxin events — “and there was already fungus and toxin” — were exacerbated by the lack of adequate drying facilities on some farms. There were too few drying units to handle the amount of corn being harvested.
“Growers were trying to dry down their corn in rice dryers and trying other things. I don’t know if the region was quite ready for all that corn.”
Lesson learned, shortly thereafter, grain dryers and storage bins were erected throughout the Mid-South.
“Interestingly, I recently read a piece saying that Kenyans were having a problem with aflatoxin contamination. That was tied — just like in the Mid-South — to having inadequate facilities for handling the crop after harvest.”
Williams began his aflatoxin research in the mid-1990s.
“Some of the scientists in the group I work with had begun researching the problem in the late 1970s. At that time — 1977, I believe — aflatoxin contamination was a big problem and it sparked interest in some of my colleagues.”
Is the research focus on breeding corn that can resist aflatoxin or suppression through other means?
“Our focus is on breeding but we realize that genetics could be just one component of the solution. We’ve been working with an MSU agronomist this year to see how our germplasm reacts in a system using the atoxigenic strains in biocontrol.
“There’s an ARS group based in Stoneville, Miss., that is working on the biocontrol aspect. They’ve used our germplasm for some experiments.”
Germplasm part of solution
Unfortunately, no one has yet completely solved the aflatoxin problem.
“I think germplasm will be a part of the (eventual) system to deal with aflatoxin, not the whole thing.”
One aspect of Williams’ project is to screen new sources of germplasm to locate resistance to aflatoxin. The researchers secure germplasm from various places, including the GEM (Germplasm Enhancement of Maize) Project — which involves both public institutions and private companies — and the International Maize and Wheat Improvement Center in Mexico (CIMMYT).
“Also, exotic germplasm from other countries has been crossed with proprietary germplasm that companies have provided. We screened that looking for new sources of resistance.
“We’ve also screened old, Southern germplasm lines we could find. Old in-breds could be another source of resistance.
“So, the first thing we do is screen sources. If we find some resistance, we begin the breeding to get that into a line that could eventually be released to the public.”
In some cases, the researchers find resistance in plants that are very late or poor-yielding. Those situations may require crosses, breeding and reselection so the resistance can be moved into a more user-friendly line.
The team is also expending significant effort in identifying molecular markers — genes, or groups of genes — that control resistance.
“When we have a source of germplasm, obviously it contains something responsible for resistance. If we can identify quantitative trait loci (QTL) — groups of genes or portions of chromosomes associated with resistance — it allows a breeding program to move those into elite germplasm lines that would be useful to the corn industry. We’ve been successful with some of that.
“After that, whoever is doing the breeding can select for the molecular markers rather than have to evaluate all the germplasm in field and phenotype it for how much aflatoxin it has after infection.”
To identify the molecular markers, the researchers do field tests to assess what level of resistance different lines are showing. Then, they make crosses and assess resistance in the progeny.
One advantage of working on aflatoxin researcher in Mississippi: it’s easy to get high levels of contamination.
“That’s particularly true when you inoculate the ears with the fungus. Most years, we can rely on having hot, humid weather in the state with dry periods. That’s a given. While that’s a problem for farmers it works well for those of us researching and doing disease screenings.”
Williams’ team saw success with two lines — Mp715 and Mp717 — in 2008 trials. Another line, Mp04:97, did well in 2009 trials. Mp715 and Mp717 have been released to numerous university and commercial breeding programs.
“We will likely release Mp04:97 after we review this year’s aflatoxin contamination data.
“When we release lines, they’ll be used by both commercial and public research institutions. Unfortunately, with corn we can’t really keep track with the germplasm after it’s publicly released.
“I do know that commercial companies have used it in their breeding programs. But as far as which lines end up in which hybrids on the market, we don’t know. Some of the company people have assured me that the better job we can do and get them good, molecular markers for resistance, the more likely they are to put resistance into commercial hybrids.”
When releasing a new variety, getting the balance right is difficult. “When someone buys hybrid seed for the farm, they don’t want aflatoxin contamination. But there are a lot of things they do want: high yields, lodging resistance, grain quality, and other things. Resistance to aflatoxin contamination is only one component of a desirable seed.”
Williams is also researching lines resistant to fall armyworms and corn borers.
“Of course, with the advent of Bt there’s been less interest in the native resistance in developing commercial hybrids. Bt did such a great job of controlling corn borers and some did quite well with fall armyworms.
“Even so, we’ve continued work on insect pests. There are still times when you’d like to have native resistance instead of the transgenics.”
And there’s a link between insect damage and aflatoxin.
“We feel that some work on insect resistance could contribute to lessening the aflatoxin problem. Experiments show that in a year when aflatoxin can be a problem, insect damage can make it even worse.”
The solution to aflatoxin is coming, says Williams.
“I hope the readers will think what we’re doing is worthwhile and know we’re tackling the problem. We think the genetics approach will be an important component to reducing the likelihood of an aflatoxin problem. We feel using both conventional breeding methods and molecular markers will yield fruit and that’s the right way to go.”