Corn Quality Issues in 2008 – Field Molds

October 24, 2008
ICM News

By Charles Hurburgh, Department of Agricultural and Biosystems Engineering; Alison Robertson, Department of Plant Pathology; Roger Elmore, Department of Agronomy

Fall weather has allowed late planted corn to mature, increasing grain yields expectations - the USDA October yield estimate is 172 bushels per acre in Iowa. Although high grain yields are expected, reports of quality issues are surfacing.

The cool wet fall conditions also favor the development of fusarium fungi; the white or pink ear rots that are often found in ear corn stored too wet.  Field moistures in the low 20s over a long period are favorable for these fungi, which in turn can produce several toxins harmful to people and livestock – vomitoxin, zearalenone, and fumonisin.  Grain with field mold should be tested for mycotoxins before feeding.

Scouting and harvesting fields with disease problems

High moisture conditions favor growth of many ear and stalk rot fungi. Fields should be scouted as soon as possible to determine the extent of disease problems. To minimize losses due to ear rot and increased mycotoxin levels, it is recommended that producers harvest problem fields (greater than 10-15 percent incidence of ear rot) as soon as possible.

The longer the corn remains in the field, the greater the chance of toxin production. The toxins most likely to increase in the field at this time are those associated with fusarium ear rots - namely vomitoxin, zearalenone and fumonisin.  

Gibberella Ear Rot - a consistently important mycotoxigenic fungus in the northern corn belt, producing vomitoxin, zearalenone, and other toxins.

Diplodia Ear Rot - characterized by the appearance of raised black bumps on the moldy husk or kernels.

Diplodia ear rot is more prevalent this season than in previous years. Although mycotoxins are not usually associated with Diplodia ear rot, grain quality will decrease substantially if the corn is allowed to remain in the field, thus early harvest is also recommended in these fields. 

Adjust harvest equipment to minimize damage to kernels since mold and mycotoxin levels tend to be at greater levels in damaged kernels. Dry (to less than 15 percent moisture) and cool (to less than 45 degrees F) grain as quickly as possible to reduce further mold growth and toxin production.

Elevator operators, especially in Eastern Iowa report cases of visible mold damage levels 5 percent and higher. In normal years, overall mold damage levels are generally less than 2 percent in freshly harvested corn. High damage levels in harvested grain create challenges for grain grading, particularly in the harvest rush.  Damaged corn sharply reduces the future storage life of the grain.

Storage issues

Storage and harvest management will be particularly important. Field damaged grain, regardless of reason, should not be mixed with good grain. Producers should harvest around water holes, downed grain and immature areas. Do not mix damaged grain with good grain in storage. Regardless of condition, all grain should be aerated immediately to reduce temperature and equalize moisture. 

Field damaged grain will not store beyond the winter months. Maintain 1 to 2 percentage points lower moisture than normal grain (for example, 13 percent corn instead of more typical 15 percent).  Do not try to hold field damaged corn at higher moistures to avoid drying expense. If you suspect mycotoxin problems, check with crop insurance providers to see if adjustments may be needed, and how to represent the areas to be adjusted. Your veterinarian or the local USDA grain inspection service provider (see below) can assist with obtaining mycotoxin testing. Crop adjustments for quality problems, including mycotoxins, must be done on standing corn at or before harvest.

Accurate grading of field-damaged grain is always difficult in the rush of harvest. Expect end users, such as ethanol plants, to increase their level of grading because mold and weather damage reduce processing yields/byproduct quality.

An Official USDA grade is the standard against which buyer analysis should be compared. It is important that company graders be trained to match USDA graders. Alternatively, samples can be submitted to USDA grading agencies but this process is slower and more costly. In the event of a dispute, use an Official grader. The variety of damage types will be very challenging to evaluate.  You can locate the official agency in your area from the USDA list of official inspection and/or weighing services.

Livestock Feed Issues

Livestock (swine, cattle, horses, poultry) are susceptible to certain mycotoxins.  Therefore any grain that is fed to livestock should be tested for mycotoxins.  Dairy producers should be particularly sensitive to mycotoxins. (See tables below.)

The wide variety of molds on these samples has created a range of mycotoxin possibilities – primarily vomitoxin, zearalenone and fumonisin. Prolonged cloudy, humid weather encourages production of the fusarium-based toxins. 

Veterinarians can submit samples to the Iowa State Veterinary Diagnostic Lab.  Alternatively, Official USDA grading agencies can do quick tests for these toxins.  Toxins concentrate in the distillers grains three to four times the levels in the corn. Ethanol plants are doing quick screening tests on inbound grain, especially on low test weight corn (less than 50 lb/bu). 

Please see ISU Extension publications  Aflatoxins in Corn - PM1800, and Corn Ear Rots, Storage Molds, Mycotoxins, and Animal Health - PM 1698, for guidelines on sampling and sample handling. The black light test will not respond to fusarium.


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Charles Hurburgh is a professor of Agricultural and Biosystems. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production.

The USDA – GIPSA – Grain Fungal Diseases & Mycotoxin Reference – Sept 2006 has additional information regarding Vomitoxin and Fumonisum; the FDA – Compliance Program Guidance Manual – 7371.003 – Dec 2005 contains the reference Guidance for Fumonisins in Animal Feeds .


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on October 24, 2008. The information contained within may not be the most current and accurate depending on when it is accessed.


Charles Hurburgh Professor, Agricultural and Biosystems Engineering

Dr. Charles R. Hurburgh, Charlie to most everyone, is a native Iowan from Rockwell City (Iowa, USA). He continues to operate the family farm, and is a professor of Agricultural and Biosystems Engineering at Iowa State University. He has a bachelor's degree, master's degree, and doctoral degree fr...

Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...