Corn ear rots, mycotoxins, and grain quality in 1993

Last fall, ISU conducted two separate surveys of the corn crops condition. One survey, conducted by the Plant Pathology Department and Veterinary Diagnostic Lab, focused on the incidence of ear rots in the field and the associated mycotoxins. The other survey, conducted by Agriculture and Biosystems Engineering and the College of Veterinary Medicine, focused on grain quality, including mycotoxins, in random samples of corn stored by 240 pork producers in Iowa. It was conducted by Dr. James McKean, Dr. Charlie Hurburgh, and others. Mycotoxin analyses for both surveys were conducted by Dr. H.M. Stahr of the ISU Veterinary Diagnostic Lab and Dr. P.F. Ross of the National Veterinary Services Lab in Ames. Mycotoxi analyzed were aflatoxins, fumonisins, ochratoxin, T-2 toxin, vomitoxin, zearalenol, and zearalenone. Together, these two surveys provide a good picture of the condition of the crop in terms of diseases and grain quality.

This article presents a summary of the results of the two surveys. For the field survey, sampling was conducted by the Iowa Agricultural Statistics Service (IASS) in Des Moines. Samples were taken from the same fields used in the IASS objective yield survey. Fields were selected using a random sampling procedure designed so that each acre of corn in the state has an equal chance of being selected. Just before each field was harvested, a 10-ear sample was taken and sent to Ames for analyses. A total of 230 samples were received. The number of samples from each crop reporting district was roughly proportional to the corn acreage in the district, except for the EC district, which was under-represented.

The occurrence of ear rots was evaluated by visual examination and culturing of the pathogenic fungi. Table 1 shows the percentage of samples on which each ear rot was detected. The numbers in parentheses are the numbers of samples from each district. This table is a good indication of how common each disease was, and the distribution of the diseases, but it does not indicate the level of damage. Table 2 (next page) indicates the amount of damage caused by each ear rot in the infected samples. This table includes only the samples that had each ear rot. For example, the average amount of damaged kernels by Penicillium was 2.8 percent, but this was true of only 3.91 percent of the samples. The rest of the samples (96.09 percent) had no Penicillium. While any ear rot fungus can reduce grain quality, Fusarium, Gibberella, Aspergillus (and rarely, Penicillium) ear rots can be toxigenic. Fusarium ear rot is caused by Fusarium moniliforme and other Fusarium species that can produce fumonisins. Gibberella ear rot is caused by Gibberella zeae, also known as Fusarium graminearum. This fungus can produce DON (vomitoxin). No Aspergillus ear rot was detected.

Aside from the visually rotted kernels, about 46 percent of all kernels had symptomless infections by Fusarium species, including F. moniliforme and F. graminearum. These fungi can potentially produce mycotoxins in symptomless kernels.

Table 3 shows the percentage of samples that had various mycotoxin levels. Not quite all the samples have been analyzed, so the final results may vary slightly. No aflatoxins, ochratoxin, or zearalenol were detected. Two samples had a trace of T-2 toxin. Vomitoxin was the most commonly detected toxin. The recommended safe level for swine is less than 1 ppm in feed, and 91 percent ofthe samples were in this range. Vomitoxin levels were highest in the NC district, which also had the highest levels of Gibberella ear rot. The recommended safe fumonisin level in feed is less than 5 ppm for horses, less than 10 ppm for swine. All the samples were below 5 ppm. For zearalenone, the recommended safe level is less than 1 ppm for swine. All the samples were below 1 ppm.

Fusarium ear rot was the most common, but usually caused little damage to kernels. Fusarium moniliforme can produce fumonisins, but all the samples analyzed have been within safe limits. Gibberella ear rot was fairly common, and caused more kernel damage than Fusarium. The NC district in particular had significant Gibberella ear rot. It can produce vomitoxin and zearalenone, and some samples, particularly in the NC district, were over 1 ppm. Many of the samples in the <0.5 ppm range actually had traces of vomitoxin, especially in the NC district. The wet summer resulted in a relatively high incidence of Gibberella ear rot. This was potentially a high risk situation for vomitoxin, as indicated by the presence of trace amounts of the toxin in many samples. However, since the fall turned dry and the harvest progressed quickly, these Gibberella infections apparently did not have time to develop significant vomitoxin levels.

Table 4 summarizes the results of the stored corn survey. In general, mycotoxin results were similar between the two surveys, with slightly lower levels in the stored corn. In this survey, the NC district again had the highest levels of vomitoxin. Test weights were below normal almost throughout the state, with the NC district having the lowest value. Protein levels were below normal, but slightly higher than in 1992. Lysine levels were also slightly below normal. The overall quality of the stored corn was better than expected for such a poor growing season.

Because of the presence of ear rot fungi, low test weight, and mechanical damage, the 1993 crop will not store as well as a normal crop. This should be a consideration in managing 1993 grain. See the December 1993 issue of Integrated Crop Management for more information on storing and handling the 1993 crop.

Updated 03/24/1994 - 1:00pm