An Overview of Ear Rots (Archived)
Published: 07/01/2016
DOI: doi.org/10.31274/cpn-20190620-001
CPN-2001
***The newest version of this publication can be accessed at https://cropprotectionnetwork.org/publications/an-overview-of-ear-rots***
Ear rots are some of the most important corn diseases throughout the United States and Canada. Ear rots decrease yield and can greatly reduce grain quality (Figure 1).
It is critical to identify ear rots in the field because many of the fungi responsible for ear rots produce toxic chemicals (known as mycotoxins), which can harm livestock and humans. Grain that has been contaminated with mycotoxins can be difficult to market and may be docked in price.
Therefore, it is important that farmers and other agricultural personnel are able to diagnose corn ear rots and manage affected grain according to the specific ear rot present. This publication:
Describes how to identify the most common corn ear rots observed in the United States and Canada
Discusses the mycotoxins associated with each ear rot
Describes diseases and disorders easily confused with corn ear rots
Briefly addresses how to manage ear rots and affected grain
Figure 1. Corn ears infected with a fungus that causes an ear rot disease.
Aspergillus Ear Rot (disease) & Aflatoxin (mycotoxin)
Aspergillus ear rot is one of the most important diseases of corn. It is caused primarily by the fungus Aspergillus flavus, but a few other Aspergillus species may be involved. Typically, this disease is more common in the southern United States than in other areas.
Aspergillus species produce a mycotoxin called aflatoxin. Aflatoxin affects grain quality and marketability and is primarily a threat to livestock health. Aflatoxin is extremely carcinogenic and most countries (including the United States and Canada) have regulations in place to prevent aflatoxin from entering the human food and livestock feed supply.
The Aspergillus fungus survives in soil or crop residue and infects ears during late silking (Figure 2). Hot, dry conditions favor infection. Stressed plants (from nutrient deficiencies, drought, or feeding damage from ear-invading insects) are often more susceptible to this disease.
The fungus produces aflatoxin, which will accumulate as the fungus spreads in subsequent hot and dry weather. The fungus can infect the ear and produce more aflatoxin after physiological maturity, particularly during periods when rainfall delays harvest.
It’s important to note that kernels with no visible injury or mold may still contain aflatoxin.
Signs and Symptoms
Aspergillus ear rot appears as an olive-green mold on the kernels (Figure 3). The fungal spores appear powdery and may disperse like dust when you pull back the husk. These signs are most commonly observed at the tip of the ear, but can be scattered throughout the ear and all the way to the base of the ear (Figure 4).
Figure 2. The Aspergillus ear rot life cylce. A. Inoculum of A. flavus survives in corn residue and soil. B. Insects and wind spread conidia to the ears, infecting through silks and insect wounds. C. Olive-green powdery mold forms on kernels.
Figure 3. The tip of this corn cob shows signs of Aspergillus ear rot, which is caused by the fungus Aspergillus flavus.
Figure 4. The fungus that causes Aspergillus ear rot produces olive-green spores that are scattered throughout the ear.
Fusarium Ear Rot (disease) & Fumonisins (mycotoxins)
Fusarium verticillioides is the primary fungus that causes Fusarium ear rot. The fungus survives in crop residue, and moves readily by spores on the wind (Figure 5). The fungus can infect seedlings and developing kernels, and it can grow for a time in the stalk and ear without producing symptoms or signs.
Plants can develop Fusarium ear rot under a wide range of environmental conditions. The most severe disease outbreaks occur in warm regions and in fields with extensive insect, hail, or other damage to the ears.
While Fusarium ear rot often has only a minimal effect on yield, farmers should be concerned about the risk of fumonisin contamination. Fumonisins are a group of mycotoxins that affect both human and livestock health.
Concentrations of these mycotoxins in affected fields often increase when wet, warm weather conditions persist just before harvest. Visible signs and symptoms of Fusarium ear rot can indicate fumonisin contamination, but the fumonisin concentration in the grain can be very high even when the disease does not appear to be severe.
Figure 5. The Fusarium ear rot life cycle. A. Inoculum of F. verticillioides survives in corn residue. B. Airborne conidia infect ears via silks or insect injury; soilborne conidia infect plant roots. C. White to purple, cottony mold can appear anywhere on the ear. Affected kernels are scattered and are discolored or have white streaks.
Figure 6. Ears with Fusarium ear rot have white to purple mold visible on kernels.
Signs and Symptoms
The signs and symptoms of Fusarium ear rot vary from other ear rots. Diseased kernels are scattered or in patches on the ear, especially on kernels damaged by European corn borer, earworm, or bird feeding. Fusarium-affected kernels appear purple, tan, or brown (Figure 6). When fungal growth is visible on the ear, infected kernels will appear white to pink or salmon. In some cases, kernels have white streaks (called a “starburst” symptom), which is caused by the pathogen growing under the kernel pericarp (seed coat) (Figure 7).
Figure 7. The starburst symptom on kernels is often seen on ears affected by Fusarium ear rot.
Gibberella Ear Rot (disease) & Deoxynivalenol and Zearalenone (mycotoxins)
Gibberella zeae (Fusarium graminearum) is the fungus that causes Gibberella ear rot. This fungus produces two different mycotoxins: deoxynivalenol (DON, sometimes called vomitoxin) and zearalenone. These mycotoxins can affect livestock, especially swine. Gibberella ear rot is most prevalent in the northern United States and Canada and is rare in warmer regions of the southern United States.
The fungus overwinters on corn and small grain residue and can infect soybean roots (Figure 8). Spores produced on the residue lead to infection during silking (Figure 9). The prevalence of Gibberella ear rot tends to increase when cool, wet weather occurs during early silking. Extended periods of rain in the fall that delay harvest increase disease severity.
Gibberella ear rot will be most severe in continuous corn fields or in areas where there is wheat affected by Fusarium head blight (scab), which is caused by the same pathogen.
Signs and Symptoms
Gibberella ear rot produces a pinkish mold that often begins at the ear tip. On severely affected ears, the husks and silks may adhere tightly to the ear because of mold growth — such ears are called “mummified ears” (Figure 10).
Except in highly susceptible hybrids or under severe conditions, the disease usually affects only part of the ears. It is relatively easy to identify Gibberella ear rot in the field on intact ears, but it is much more difficult to identify after the grain has been shelled. The concentration of the mycotoxins (DON or zearalenone) can be high even when the disease does not appear to be severe.
Figure 8. The fungus that causes Gibberella ear rot (Gibberella zeae) can survive on corn stubble from one season to the next.
Figure 9. The Gibberella ear rot life cycle. A. Inoculum of G. zeae survives in infected corn and wheat residue. B. Splashing water and spores ejecting from specialized fungal structures spread inoculum to the ear where they infect through silks or the base of the ear. C. Red or pink mold forms, typically beginning at the ear tip.
Figure 10. Corn ears with Gibberella ear rot.
Diplodia Ear Rot (disease)
No Mycotoxins in North America
Diplodia ear rot is caused by the fungi Stenocarpella maydis and S. macrospora and has become a common (and troublesome) disease on corn. These fungi produce mycotoxins in South America and Africa, but no mycotoxins have been associated with Diplodia ear rot in the United States and Canada. Pycnidia (the small, black, spore-producing structures of the fungus) overwinter on corn residue and are the source of infection for the subsequent corn crop (Figure 11). Pycnidia appear as black specks that may be scattered on the husks, cobs, and sides of kernels.
Dry weather before silking, immediately followed by wet conditions, favor Diplodia ear rot. Fields under conservation tillage also favor Diplodia ear rot, as do fields of continuous corn. Hybrid susceptibility also contributes to disease development. Earworm damage at the ear shank is often associated with the disease.
Delayed harvest and wet weather before harvest can allow fungal growth to continue, further reducing grain quality and yield.
Signs and Symptoms
On infected ears, the ear leaf generally dies prematurely when kernels are at the milk or dough stages. The Diplodia ear rot fungi produce a dense white to gray mold that appears on and between the kernels at the base of the ear and progresses toward the tip (Figure 12). Rarely, the white mold will occur only at the tip or middle part of the ear.
Infected ears weigh noticeably less than healthy ears. Occasionally, the white mold will not be prevalent, and kernels will have a brown discoloration. This appearance is called “hidden Diplodia,” and you can observe the symptoms only by breaking the ear in half and observing the fungal structures (pycnidia) in the cob (Figure 13). Often, the entire husk of affected ears will have a bleached appearance.
Figure 11. The Diplodia ear rot life cycle. A. Inoculum of S. maydis and S. macrospora survive in infected corn residue on the soil surface. B. Splashing water spreads conidia to the ear, infecting it through silks or ear shank. C. Husks and ear leaves can die prematurely. D. Dense white mold begins at the ear’s base, and becomes grayish-brown, eventually rotting the entire ear. E. Small, black pycnidia can form on kernels later in the season.
Figure 12. White mold growth on corn, indicative of Diplodia ear rot.
Figure 13. A cross-section of a corn cob with small, black pycnidia produced by the fungus that causes Diplodia ear rot.
Dangers to Livestock
Mycotoxins can severely threaten the livestock that consume them. Different mycotoxins affect livestock differently, which means it is very important to correctly identify the responsible ear rot and its associated mycotoxin (Figure 14).
The U.S. Food and Drug Administration (FDA) and Health Canada have set action levels or advisory levels on several mycotoxins. If a mycotoxin has an action level it means there are legal restrictions on the grain when mycotoxin concentrations reach that level. If a mycotoxin has an advisory level it means there are strong cautions regarding the grain’s use when mycotoxin concentrations reach that level.
Figure 14. Moldy corn has the potential to contain dangerous mycotoxins.
Aflatoxin
Aflatoxin (associated with Aspergillus ear rot) is a liver toxin and potent carcinogen. When livestock consume aflatoxin they can experience a variety of health issues including suppressed immune systems, reduced weight gain, cancer, and death. The toxicity of aflatoxin varies among animal species, but young animals are most sensitive to the toxin. Furthermore, when lactating animals consume contaminated grain, the aflatoxin is present in the animal’s milk (Figure 15).
Figure 15. This illustration shows the structure of the aflatoxin compound. Lactating animals that consume feed contaminated with aflatoxin can pass the toxin to their young through their milk.
Fumonisins
Fumonisins (associated with Fusarium ear rot) can cause fumonisin poisoning, which is associated with a number of toxic effects, including equine leukoencephalomalacia (ELEM, also called blind staggers) and porcine pulmonary edema.
Equine and swine are the most sensitive to fumonisins.
Deoxynivalenol and Zearalenone
Deoxynivalenol and zearalenone (associated with Gibberella ear rot) can be dangerous to livestock.
Deoxynivalenol (also called DON and vomitoxin), can cause swine and other animals to vomit and refuse to eat.
Zearalenone has estrogenic properties, which means it can cause infertility, abortion, and other breeding problems. Swine are the most sensitive livestock to zearalenone. A feed ration with as little as 1 to 5 parts per million (ppm) of zearalenone may produce an estrogenic effect in swine.
Diplodia Ear Rot
There are no reports in the United States or Canada that the fungi that cause Diplodia ear rot have produced mycotoxins. If you observe adverse effects on livestock after feeding them Diplodia-affected grain, promptly contact your local extension service.
Scouting for Ear Rots
To manage and minimize the effects of these ear rot diseases, it is critical to assess fields before harvest. You should assess fields each year, because these pre-harvest assessments can alert you to potential problems and provide time for livestock producers to segregate, obtain alternative grain, or hold onto stored corn from the previous year.
Scouting practices are similar for all corn ear rots. Begin scouting fields at late dent stage to determine the presence and severity of ear rots. When scouting, randomly select plants and pull back the husk to examine the entire ear (Figure 16). A quick method is to select 100 plants across the field (20 ears each from five different areas). For each ear, be sure to peel back the husks and examine the entire ear.
If a field contains a significant level of ear mold, collect a representative sample at harvest and have it tested for mycotoxins before storing the grain or feeding it to livestock. A lab test is often the only reliable way to definitively diagnose an ear rot or mycotoxin.
More information about grain sampling and mycotoxin testing is available in Corn Disease Management: Grain Sampling and Mycotoxin Testing (CPN-2003). If you suspect a field is contaminated with a mycotoxin, contact your crop insurance agent. If you need to file a claim, your agent may require an adjuster to visit the field before harvest.
Figure 16. When scouting for corn ear rots, pull back the husk to examine the entire ear.
Managing Ear Rots
Corn infected by ear rots will often result in significant discounts on the grain. Kernels with an ear rot disease can be lighter than healthy kernels (which will lower the test weight of a sample), and elevators can dock grain that contains mold. Mycotoxin contamination can lead to further discounts.
General management practices apply to most ear rots.
Choose what you plant carefully. In fields with a history of ear rots, choose a corn hybrid that is less susceptible to the specific ear rot. You may also want to select hybrids with insect resistance traits, which can help reduce the occurrence of ear rots.
Promote conditions that favor healthy plant growth and reduce plant stresses. Make sure plants receive adequate water and nutrients, and minimize insect-related and other damage.
Don’t rely on fungicides. It is important to note that the foliar fungicides currently available are notgenerally recommended to manage ear rots and mycotoxins. There may be some fungicides available for Gibberella and Fusarium ear rots in the United States, but these products currently require a FIFRA Section 2(ee). Check with your state extension service (or in Canada, with the Pest Management Regulatory Agency) before using fungicides to control corn ear rots.
In areas with high levels of Aspergillus ear rot and a history of frequent aflatoxin contamination, consider using an atoxigenic fungal strain to reduce aflatoxin accumulation. More information about using atoxigenic strains to manage aflatoxin can be found in Corn Disease Management: Using Atoxigenics to Manage Aflatoxin (CPN-2005).
Harvest infected fields early and segregate the grain. Leaving diseased grain in the field allows the ear rot fungi to keep growing, which will increase the risk of moldy grain and mycotoxin contamination. Most ear rot fungi continue to grow (and, if applicable, produce mycotoxins) until the grain has less than 15 percent moisture. In severely infected fields, it may be worthwhile to harvest grain at a higher moisture and then dry it to less than 15 percent to minimize the further mycotoxin accumulation (Figure 17).
Never mix grain from a field affected by ear rots with grain from a field that has not been affected.
During harvest, adjust the combine to discard lightweight or damaged kernels. These kernels may contain mold and mycotoxins. Segregate poor-quality grain from good-quality grain, and clean moldy grain out of your equipment before using it on clean grain to prevent cross-contamination.
Figure 17. Dry infected grain to avoid further mycotoxin accumulation.
Storing Affected Grain
It is crucial to properly store corn affected by ear rots. You must quickly dry and cool grain after harvest to limit fungal growth and the further mycotoxin accumulation in storage.
The standard recommendations for long-term storage are to dry contaminated grain to less than 13 percent moisture and to cool it to 30°F (-1°C). Whenever possible, only store affected grain during the cold weather season.
More information about storing grain is available in Corn Disease Management: Storing Mycotoxin-affected Grain (CPN-2004).
Diseases and Disorders with Similar Symptoms
There are several conditions that have symptoms similar to the ear rots that can produce mycotoxins. Corn is also susceptible to several other ear rots that are less harmful than the others described above. These other ear rots are usually minor in incidence and severity, but can be confused with the more important ear rots, namely Aspergillus ear rot.
Figure 18. Corn showing symptoms of Cladosporium ear rot.
Figure 19. Corn with signs of infection by Nigrospora oryzae, the fungus that causes Nigrospora ear rot. The inside of the broken cob (bottom) shows the small, black spores the fungus produces.
Figure 20. Corn with signs of Penicillium ear rot.
Figure 21. (Top). An ear with blue-green mold, indicative of the fungi that cause Trichoderma ear rot. (Bottom) Spores of the Trichoderma fungus growing in between the kernels on a corn ear.
Figure 22. The gray, tumor-like gall on this cob was caused by Ustilago maydis, the fungus that causes corn smut.
Figure 23. Black corn typically produces black or dark-colored fungal growth, primarily on senescing tissue (leaves, husks, etc.).
Acknowledgments
Authors
Kiersten Wise, Purdue University; Tom Allen, Mississippi State University; Martin Chilvers, Michigan State University; Travis Faske, University of Arkansas; Anna Freije, Purdue University; Tom Isakeit, Texas A&M University; Daren Mueller, Iowa State University; Trey Price, LSU AgCenter; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin; Albert Tenuta, OMAFRA; and Charles Woloshuk, Purdue University.
Reviewers
Gary Bergstrom, Cornell University; Alyssa Collins, Pennsylvania State University; Andrew Friskop, North Dakota State University; Doug Jardine, Kansas State University; Heather Kelly, University of Tennessee; Dean Malvick, University of Minnesota; Hillary Mehl, Virginia Tech University; and Alison Robertson, Iowa State University.
All photos were provided by and are the property of the authors and contributors except the cover photo and Figures 1 and 8 by John Obermeyer, Purdue University; Figures 6 and 7 by Burt Bluhm, University of Arkansas; Figure 13 by Martha Romero, Purdue University; and Figures 18, 19, and 23 by Gary Munkvold, Iowa State University.
Sponsors
Funding for this project was provided by the United States Department of Agriculture-National Institute for Food and Agriculture (USDA-NIFA) project: Integrated Management Strategies for Aspergillus and Fusarium Ear Rots of Corn (NIFA Award Number: 2013-68004-20359). We also thank the Grain Farmers of Ontario for support.
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