Gibberella Ear Rot and Mycotoxins in Corn: Sampling, Testing, and Storage

PLPATH-CER-04
Agriculture and Natural Resources
Date: 
04/07/2016
Katelyn T. Willyerd, Department of Plant Pathology
Pierce A. Paul, Department of Plant Pathology
Peter Thomison, Horticulture and Crop Science
*Corresponding author: paul.661@osu.edu (330-263-3842)

Gibberella ear rot is caused by the fungus Gibberella zeae (also known as Fusarium graminearum), the same pathogen that causes stalk rot of corn and head scab of wheat. The fungus typically infects via the silk channel, causing a pinkish-white mold to develop at the tip of the ear (Fig. 1). Cool, wet weather (rainfall or high relative humidity) during and after silking (R1 growth stage) provides optimal conditions for the development of ear rot. During infection and colonization of the ear, the fungus produces several mycotoxins, including deoxynivalenol (DON), also called vomitoxin. As a result, high levels of Gibberella ear rot severity and moldy grain are usually accompanied by high levels of vomitoxin. Mycotoxins are harmful to both humans and animals. Vomitoxin is water soluble, heat stable, and the most commonly encountered mycotoxin in food and feed.

Fig. 1An ear of corn displaying typical Gibberella ear rot symptoms.

Grain should be analyzed for toxins before being fed to animals. As a general rule do not feed any grain with >5% moldy kernels. Swine and young animals are particularly sensitive to mycotoxins. As the name suggests, vomitoxin causes vomiting, feed refusal, and suppression of the immune system. Proper sampling and testing are necessary to accuratly determine vomitoxin levels in grain.

The U.S. Food and Drug Administration (FDA) established advisory guidelines for vomitoxin to ensure the safety of the food and feed supply:

Vomitoxin Variability in Infected Grain

The number of ears infected within a field and number of infected kernels on a given ear is highly variable. Kernels with similar appearance in terms of surface moldiness may have different levels of internal fungal colonization, and consequently, variation in mycotoxin content. Healthy-looking kernels may also be contaminated. As a result, moldy grain and vomitoxin levels vary considerably within the grain lot. There are always “hot spots” and these may affect the accuracy of sampling and testing for vomitoxin. For instance, if a single sample is drawn and the location from which it is drawn happens to be a hot-spot, then the level of contamination of the lot will be overestimated. Conversely, if the sample misses the hot spots completely, vomitoxin contamination may be underestimated.

Sampling for Mycotoxins

Accurate testing depends on thorough and appropriate sampling. Poor sampling may result in considerable variation in test results, and could be the cause of some grain rejections. Guidelines for grain sampling, based on research with scabby wheat and barley, are available from the United States Dept. of Agriculture Grain Inspection, Packers and Stockyards Administration (GIPSA, www.gipsa.usda.gov).

To collect a representative grain sample, 5-10 samples should be randomly collected from multiple locations in the bin or truckload. Samples taken only from the central or outer portions of the load or from the beginning and end of the grain stream will not provide an accurate estimate of toxin contamination. For end-gate sampling, samples should be drawn from the entire width and depth of the grain stream. For sampling with hand or mechanical probes, multiple samples should be drawn from throughout the bin or truck, along an “X”-shaped pattern, for example. Once samples are obtained, bulked, and cleaned, the grain must be ground uniformly, in a clean grinder, to resemble flour. Finer particle size increases surface area of the grain and enables efficient extraction of vomitoxin.

Mycotoxin Test Options

There are three general types of mycotoxin tests: qualitative, semi-quantitative and quantitative. Qualitative tests provide a yes/no answer for the presence of vomitoxin and are useful for initial screening steps. Semi-quantitative tests estimate vomitoxin at or above certain levels (>5ppm) or within a given range, whereas, quantitative tests provide more precise vomitoxin estimates. There is a trade-off between precision, price and speed. The more quantitative tests tend to be the most precise, but are also more expensive and take longer to complete than the qualitative or semi-quantitative tests. Common vomitoxin testing methods and their pros and cons are highlighted here:

Semi-quantitative mycotoxins tests, such as ELISA, are quite accurate since they are usually engineered and calibrated against quantitative tests such as chromatography. However, ELISA-based tests are usually very specific. The kit is specifically for one particular toxin. To detect other toxins, such as zearalenone or aflatoxin, additional tests must be used. Furthermore, the test must be approved for the commodity to be analyzed (corn, dried distillers grains, wheat, barley, etc). USDA-GIPSA has approved several different types of test kits that test corn, wheat, oats and barley for vomitoxin. A list of these kits can be found in the DON (Vomitoxin) Handbook. Grain samples may also be sent to a vomitoxin testing laboratory.

 

Vomitoxin Testing Law: A Second Opinion

Poor sampling and/or testing technique may lead to incorrect estimation of vomitoxin in the grain lot. Prior to testing, producers (or their agents) may request a second sample be drawn if they feel the first sample was not representative of the entire lot. Following vomitoxin testing, producers/agents have the right to reject the commodity testers’ results and order the handler to send the sample to a federally licensed grain inspector for a re-test. Refer to Ohio Code 926.31 for details.

Minimizing the Risk of Vomitoxin in Storage

There are no commercially available treatments for reliably reducing vomitoxin in harvested grain. Planting Gibberella ear rot resistant hybrids is the best approach for reducing the disease and toxin contamination in the field. Fungicide applications at R1 have shown promising results, but further research is needed. Toxin levels can increase in storage if conditions are not dry and cool. Warm, moist pockets in the grain promote mold development, causing the grain quality to deteriorate and toxin levels to increase. Aeration is important to keep the grain dry and cool. However, it should be noted that while cool temperatures, air circulation, and low moisture levels will minimize fungal growth and toxin production, these will not decrease the level of toxin that was already present in grain going into storage. Vomitoxin is very stable and will not be reduced with drying.

Useful References

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Ohioline http://ohioline.osu.edu