With the harvest underway throughout Iowa, rain in some parts of the state is stopping combines from securing kernels from the field. Depending on your location, disease pressure might not have been high. However, rainfall patterns throughout the season, and even over the last several weeks, means we are not out of the woods yet. Ear rots and associated mycotoxins could be problematic as corn sits in the field awaiting harvest.
Mycotoxins are natural chemicals that are produced by certain types of ear rot fungi. These toxins are nonliving compounds and can have detrimental effects to both humans and animals if they are present in food or feed.
There is a total of five mycotoxins associated with ear rot diseases of corn:
· Aflatoxins, produced by the fungi that cause Aspergillus ear rot
· Deoxynivalenol and Zearalenone (often referred to as DON or vomitoxin), found in corn with Gibberella ear rot
· Fumonisins, found in corn with Fusarium ear rot
· Ochratoxin, found in corn infected with Penicillium spp. and some Aspergillus spp.
Allowable concentration levels of mycotoxins in food and feed varies by toxin, as well as the intended use of the final corn products. The United States Food and Drug Administration (FDA) and Health Canada have set legally allowed limits for the mycotoxin aflatoxin, and cautionary levels for fumonisins and DON. If grain or end products contain mycotoxins above the concentration limits, then law may require the grain or products to be destroyed, as these can be harmful to those consuming the grain or products.
The toxic effects of mycotoxins vary by type, dose and animal species that consumes the toxin. There are a variety of resources that illustrate effects, and some that go into great detail. Our recommendation is to begin with resources on the Crop Protection Network.
Mycotoxins occur in fields that have ear rot diseases present, and can occur wherever the crop has been stressed throughout the season, whether drought or heavy rains. The appropriate time for scouting is at kernel maturity and just before harvest.
Scout locations in which the crop was stressed, damaged or exposed to extreme environmental conditions. This include hillsides where drought stress may have been more severe (for Fusarium and Aspergillus ear rots), and low areas where moisture from fog or high-dew conditions occur (for Gibberella ear rot).
When scouting, randomly select plants and pull back husks to examine the ears. Examine 100 plants across a field, selecting 20 ears from five different areas. If a diseased ear is present, examine another 10 ears from adjacent plants. During your examination, ask three questions:
1. What ear rot disease is present?
2. How much of the ear is affected by ear rot ( severity of disease)?
3. How many plants are affected (incidence of disease)?
Testing for Mycotoxins
Mycotoxin levels may vary among diseased ears, and corn that appears to only have mild ear rot could still have very high mycotoxin levels. One should always assume that diseased kernels contain mycotoxins. With the threshold for aflatoxin being very low, if any ears with Aspergillus ear rot are found, test the grain for aflatoxin. For DON and fumonisins, the threshold is less stringent. If 30 percent of the ears examined in a field have Gibberella or Fusarium ear rots, test for DON and fumonisins. You should also test your corn if you observe severe symptoms (more than 50 percent of the ear covered with mold) of either disease on multiple ears.
Because mycotoxins are complex chemical compounds, they can be difficult to quantify. For this reason, you should never rely solely on visual methods to confirm their presence. A common visual test — the black light test — can indicate the presence of the fungus Aspergillus flavus, but it does not detect the aflatoxin it produces.
For an accurate assessment, send grain samples to a professional laboratory for analysis; the Iowa State University Veterinary Diagnostic Laboratory is able to test grain quality. Local laboratories and grain inspection services may test individual corn samples for mycotoxins; however, sample testing can be expensive. Check with your local extension personnel for a more complete list of grain testing facilities in your area. The cost and submission procedures will vary by provider.