Not surprisingly, the hot dry conditions cornfields have been experiencing across the Corn Belt have many farmers and the grain industry concerned about aflatoxin. Aflatoxin is a potent mycotoxin that is produced by the fungal pathogen, Aspergillus flavus, a fungus or mold which causes Aspergillus ear rot.
In the following article, Iowa State University Extension plant pathologist Alison Robertson explains the biology of Aspergillus ear rot and aflatoxin production. She also offers some management recommendations for corn farmers for the next few weeks of the 2012 growing season.
If you find the Aspergillus fungus, that still does not mean aflatoxin was produced, but the grain should be tested to see if the toxin is present, says Robertson. Aflatoxins are potent compounds that cause a variety of animal health problems if they are present at high enough levels in the feed.
* How do I recognize Aspergillus ear rot? Aspergillus ear rot is an olive-green powdery mold (see photo accompanying this article), not to be confused with Penecillium ear rot, a powdery denim-blue mold. Higher aflatoxin levels are associated with discolored, shriveled kernels of corn that are often found near the tip of the ear.
* What conditions favor infection of corn by A. flavus? Infection of corn by A. flavus and consequent disease development is favored by hot (greater than 86 degrees F) dry conditions at pollination and during grain fill. Yellow brown silks are most susceptible to infection. Spores landing on the silks germinate, rapidly grow down the silk and colonize the surface of the developing kernels. Around physiological maturity, when moisture content drops to around 32%, the fungus starts to colonize the internal tissues of the kernels, and it continues to grow until moisture content is around 15%.
* What conditions favor aflatoxin production? Aflatoxin is a secondary metabolite that is produced by A. flavus under certain conditions. Drought and high temperatures (80 to 105 degrees F) during grain fill are the most common factors associated with pre-harvest aflatoxin production. Warm nights (greater than 70 degrees F) may also increase risk of aflatoxin contamination.
Toxin production depends on kernel moisture and temperature. As kernel moisture decreases, aflatoxin production increases. Toxin production is highest at 18% to 20% kernel moisture and stops at around 15% moisture. The optimum temperature range for aflatoxin production occurs between 77 to 95 degrees F.
* Can I determine if my corn is at risk for aflatoxin? Because aflatoxins are associated with Aspergillus ear rot, farmers can scout for the ear rot. From dent through to harvest, several (five to 10) locations in a field should be assessed. Target areas of the field with plants that appear most stressed. At each location, peel back the husks of 10 ears and inspect them for olive-green powdery mold (see photo) that is characteristic of the ear rot. If greater than 10% of the ears show signs of Aspergillus ear rot, schedule the field for an early harvest.
* Harvesting fields with Aspergillus ear rot. Notify your insurance adjuster immediately if you have fields with Aspergillus ear rot. Infested fields need to be inspected before harvest or strips left in the field for a claim to be made.
Harvest grain at approximately 20% to 25% moisture. Adjust combine settings to ensure minimal damage to the grain. Damaged grain is at risk to infection by A. flavus in storage.
Cool and dry grain to 16% moisture or less immediately. One study showed that if corn grain at 21% moisture content was put in bin at 86F and cooled with air immediately, little fungal growth was detected and no aflatoxin was produced. If cooling was delayed 20 to 40 hours, the fungus grew and aflatoxin was detected.
* Sampling, testing & interpretation: ISU Extension has a newly updated publication titled Aflatoxin in Corn that explains sampling, testing of corn grain and interpretation of the results to the test regarding aflatoxin, and it explains the management steps you need to take for grain that contains aflatoxin. Go to http://www.extension.iastate.edu/sites/www.extension.iastate.edu/files/www/PM1800.pdf