From Aflatoxin to Zearalenone: Mycotoxins You Should Know – Deoxynivalenol (DON)

Mycotoxins are toxic substances produced by fungi on the substrate where they grow. Mycotoxin-producing fungi are commonly found in corn and small grains, causing ear and kernel rots and potentially contaminating crops with mycotoxins. Exposure to mycotoxins can lead to chronic or acute toxicity in humans and animals. In addition, mycotoxins can lead to market losses, discounts, rejection of grain lots at elevators, and reduced livestock efficiency and productivity.

The most economically important mycotoxins include aflatoxins (AF), deoxynivalenol (DON, also known as vomitoxin), fumonisins (FUM), zearalenone (ZEA), ochratoxin A (OTA), T2, HT-2, ergot alkaloids, and patulin (PAT). The fungal species that produce mycotoxins have worldwide distribution; therefore, mycotoxin contamination occurs everywhere grain crops are grown. Accordingly, mycotoxins have been detected in feed, silage, food, and beverages derived from cereal grains and animal products exposed to contaminated feed.

This is part two of an ongoing series covering the important mycotoxins in our agronomic systems.

Deoxynivalenol (DON, Vomitoxin) 

Deoxynivalenol, also known as DON or vomitoxin, is the most common mycotoxin in corn and small grains. In animals, the acute response to DON ingestion is vomiting, while chronic exposure to low doses leads to anorexia, immunotoxicity, reduced weight gain, growth impairment, and reproduction issues (1). The toxic effects of DON vary according to the levels of exposure and the animal species, with swine being the most susceptible. Based on toxicological data, the International Agency for Research on Cancer (IARC) does not classify DON as a carcinogenic hazard to humans. 

Deoxynivalenol is produced by the fungal pathogens that cause Gibberella ear rot of corn (Figure 1) and Fusarium head blight in small grains (also known as head scab) (Figure 2).  Therefore, the negative impacts of these diseases on grain production come in two forms: yield reductions due to small, damaged, lightweight kernels and mycotoxin contamination of infected grains that lead to discounts, grain lot rejections, and loss of markets. DON is a heat-stable mycotoxin that can withstand temperatures commonly used for food and feed processing and ethanol production.

Figure 1. Reddish mold typical of Gibberella ear rot. Photo credit: Adriana Murillo-Williams. Penn State.

Figure 2. Wheat spikelets with orange spore masses typical of Fusarium Head Blight. Photo credit: Adriana Murillo-Williams. Penn State.

How Does the Contamination Occur?

The fungal pathogen Fusarium graminearum is the primary causal agent of Gibberella ear rot and Fusarium head blight of small grains; however, other Fusarium species can be involved depending on the location and climatic conditions.  When the fungus infects different plant parts, it produces mycotoxins that will remain in the affected tissues, even when the fungus moves to a different tissue or dies. DON production helps the fungus spread throughout the plant. Therefore, contamination with DON occurs while crops are in the field and may continue to increase if storage conditions are suboptimal and favor fungal growth. 

The main source of inoculum in the field is wheat and corn residues, where the fungus survives in the winter. The risk of Gibberella corn ear rot increases under wet and cool weather conditions during silking.  Under these conditions, spores produced in specialized structures on corn and wheat residues will be released and land on the ear, infecting the ear through silks. When rainy conditions delay corn harvest, the severity of the disease increases (2). Kernel damage associated with insects, birds, and hail can increase the risk of all ear rots.

Wheat infection and head blight development occur when high humidity and warm temperatures are prevalent during flowering and early grain development (3). Under these conditions, airborne fungal spores will enter the wheat head through flowers, spikelets, stomata, and natural openings.  The resulting disease is Fusarium head blight, or sometimes called head scab.

Disease Management

Management of Gibberella Ear rot and Fusarium head blight cannot be achieved by relying on a single practice. Best management occurs when the focus is on reducing the effect of the disease since dealing with the contaminated grain is more challenging than preventing contamination in the first place. Continuous corn and a corn-wheat rotation increase the risk of DON contamination, since the same fungus produces both Gibberella ear rot and Fusarium head blight. For both crops, rotating to non-host crops for at least one year is a good management practice.

Farmers should consult with their seed dealers about varieties with some resistance to Gibberella Ear Rot that are also well adapted to local conditions. Scouting before harvest is critical. Fields with high levels of Gibberella ear rot must be harvested as soon as possible, handled separately, and grain should be dried to 15% moisture content or less (4).  

For Fusarium head blight management, farmers should select moderately resistant varieties as the first line of defense and determine the risk of Fusarium head blight in their areas with the Fusarium Risk Tool for fungicide application decision-making. Triazole fungicides are used for FHB management, since strobilurins can increase the risk of DON contamination. 

Even when all these practices are implemented, DON can exceed the advisory levels set by FDA when conditions are highly favorable for disease development. If contamination has occurred, affected fields must be harvested first and handled separately, and the combine should be adjusted to remove as many fines and broken kernels as possible.

Occurrence of DON 

Gibberella ear rot and DON contamination of corn are more common in the northern United States, while other ear rots, like Aspergillus Ear Rot, are more frequent in the southern US. In Pennsylvania, the most common ear rots include Gibberella ear rot and Diplodia ear rot. Fusarium head blight occurs wherever small grains are grown, however, the pathogen species and the type of mycotoxins they produce may vary according to the geographical location.   

DON Regulations

The Food and Drug Administration (FDA) has established advisory levels for DON in finished wheat products for human consumption and grain and grain by-products used for animal feed. The advisory level of DON for finished wheat products intended for human consumption (i.e., flour) is 1 ppm (parts per million, or mg/kg). There are specific advisory levels for grain and grain byproducts for beef and dairy cattle, chickens, and swine. For all other animals, the advisory level is 5 ppm, with the recommendation that the ingredients do not exceed 40% of their diet.  Brewers and maltsters also use the level of 1 ppm of DON as the maximum level of DON allowed in malting barley. 

It is important to note that Fusarium graminearum can produce other toxins besides DON, including zearalenone, 3-acetyl-DON, 15-acetyl-DON, nivalenol, and the novel mycotoxin NX2 (5).  Therefore, single contamination with DON is not the norm; it is the exception. When DON is present in agricultural commodities, it is likely to be accompanied by other toxins that can also threaten human and animal health. 

Take-home Messages:

• Contamination of crops with DON starts in the field when the mycotoxigenic fungal pathogens infect the crops.  
• Avoid planting corn-corn and wheat-corn rotations. 
• Scout your field before harvest to determine the incidence and severity of Gibberella Ear rot and Fusarium Head Blight
• Fields with high incidence and severity of Gibberella Ear rot and Fusarium Head blight must be sampled and tested for DON before being fed to animals or sold to other markets.

References

1. Pestka JJ. (2010). Deoxynivalenol: mechanisms of action, human exposure, and toxicological relevance. Arch Toxicol. 84(9):663-79. doi: 10.1007/s00204-010-0579-8. PMID: 20798930.
2. Crop Protection Network (2016). Corn Disease Management. Ear Rots. CPN-2001.
3. Crop Protection Network (2022). Fusarium Head Blight of Wheat.
4. Woloshuk, C, and K. Wise. Diseases of Corn: Gibberella Ear Rot. Purdue Extension BP-77-W.
5. Varga E, Wiesenberger G, Hametner C, Ward TJ, Dong YH, et al. (2015). New tricks of an old enemy: isolates of Fusarium graminearum produce a type A trichothecene mycotoxin. Environ. Microbiol 17:2588–600.

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