Issue No. 21, Article 3/August 17, 2007
Rotten Ears? Aspergillus and Others
Questions about corn ear rots have been coming in this week. Fusarium ear rot and Diplodia ear rot are our most typical ear rots, with Aspergillus being perhaps most infamous. Fusarium ear rot, the most common ear rot found in Illinois and the Midwest, is characterized by pinkish to salmon colored fungal tissue growing on the kernels. The fungus can be scattered on the cob or, often, seen toward the middle of the ear. The color is distinctive. Fusarium ear rot produces mycotoxins called fumonisins. Fusarium is favored by hot and dry conditions at pollination and high humidity, and we had a lot of both of those this season.
Diplodia ear rot is first noticeable from a bleached appearance of the husk. When you peel back the husk, you see a white, fluffy fungus. The good news is that the Diplodia fungus will not produce toxins in the grain; the bad news is that kernels will be very lightweight and shriveled.
Diplodia: White fungal growth on cob.
In areas of the state that have been droughty, Aspergillus infection could be a concern. Worries center around the aflatoxin that can be produced by Aspergillus spp. infection. Aflatoxin is just one member of the larger group of chemicals called mycotoxins, which are toxins produced by fungi. Aflatoxins are some of the most potent carcinogens on earth, so that gets everyone's attention. The allowable amount of this toxin in grain for food or feed is highly regulated by the FDA. Testing is aimed at detection of 20 ppb (parts per billion) of the toxin in grain intended for human consumption and immature animals (see Table 3).
Aflatoxin can be produced by the fungi Aspergillus flavus and A. parasiticus. These fungi are present everywhere in soils and organic matter. Stored grain can be infected by the fungi, or they can infect in the field. This typically occurs when weather is dry and hot (more than 86°F) during silking to late kernel development. Aflatoxin produced by A. flavus is most likely to be produced in the field, and particularly in storage after kernels have been damaged by insects, birds, mites, hail, early frost, heat and drought stress, windstorms, and other unfavorable weather conditions. The presence of A. flavus in a feed sample does not mean that the feed is unwholesome or that it will contain high levels of aflatoxin. Testing is necessary to determine how much toxin may have been produced.
Aspergillus fungal growth on corn kernels.
Sampling grain for aflatoxin at the elevator follows established federal protocols. A 10-pound sample is usually collected by pooling 5 or more probes collected from an auger discharge of one or more combine hopper loads. The sample is ground for analysis by one of three methods. One is a visual examination, typically a black light test. A second method is a quick qualitative laboratory immunoassay procedure; the quick tests now also have quantitative test kits for a range of toxin levels (see Table 4). Third are a number of laboratory tests for the actual quantification of the toxin. Quantification is necessary if aflatoxin is detected by one of the other methods.
If you think your corn may have been at risk, how can you minimize the chances of fungi and toxin development now or in storage? First, harvest drought-stressed and insect-infested grain at early maturity as soon as the moisture content allows minimum grain damage: for shelled corn, 23% to 25% moisture; for ear corn, 25% to 30%. Set the blower on the combine high. Adjust the combine header speed to minimize cracking and reduce the content of trash, fines, and small broken or mold-infected kernels, especially those near the tips, where mold infestation is most likely to be present. Upward of 50% reduction in existing aflatoxin levels (or below 20 ppb) can be achieved in some fields by careful monitoring of combine cylinder, screen, and air flow levels.
Drying is then essential. You need to prevent further growth. Dry all grain to at least 13% to 14% moisture as rapidly as possible, not to exceed a 24- to 48-hour period after harvest.
Safe long-term storage can be accomplished at a uniform moisture level of 13% or somewhat below (see Table 5). Moisture may be 14% if grain is to be moved or sold in a shorter period. For slow drying, the grain should contain no more than 18% to 20% moisture in full-bin drying; this may not be the best method when toxins are a concern. Another possibility is high-temperature drying until the grain reaches 18% to 20% moisture, followed by low-heat drying to 13% moisture. Avoid air-drying mold-damaged corn without heat. Research has shown aflatoxin increases of 100 to more than 2,000 ppb in three days when recently harvested field corn was stored at high moisture levels.
Delays in transit to the storage bin or buying point need to be minimal. Aflatoxins have been shown to increase in truckloads of contaminated corn by as much as 6% per hour of delay. Cool the grain after drying, and maintain dry storage conditions. When possible, continue cooling until the grain temperature reaches 35° to 40°F. Thoroughly screen and clean the grain and all bins before storage to remove dirt, dust, other foreign matter, crop debris, chaff, and cracked or broken seeds and kernels. Most of the contaminated corn is in the small and broken kernels, which will drop through a screen. Store the grain in water-, insect-, and rodent-tight structures. Continue periodic aeration and probing for hot spots at intervals of 1 to 4 weeks throughout the storage period.
Finally, you know that if you have crop insurance the crop is insured while it's in the field, but what about after? Contact your insurer prior to harvest to learn what coverage you have if aflatoxin is detected in your grain. You may be covered until December, or it may be that once you harvest you are uninsuredcheck so that you know which is true for you.
These are good online resources about mycotoxins and grain testing:
- Food and Grain Inspection Service aflatoxin handbook (Adobe PDF)
- Grain Elevator and Processing Society proceedings on mycotoxins
- University of Illinois Extension Report on Plant Disease #1105, Mycotoxins and Mycotoxicoses
- University of Illinois Agronomy Day 2003 report on aflatoxin resistance
- pp.220-228 Federal Loss Adjustment Manual (Adobe PDF)