An Overview of Ear Rots

***Updated in 2024, this version replaces the previous 2016 Overview of Ear Rots publication.***

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, feed, and food quality. From 2012 to 2023, ear rot fungi were estimated to have reduced grain yield by 1.9 billion bushels, worth approximately $8.6 billion USD. Depending on the price of grain and disease severity in a given year, estimated average per acre losses range from $2-21 USD across corn production regions. 

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 others that work in agriculture can 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

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.

Some Aspergillus species can 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 aflatoxins from entering the human food and livestock feed supply.

Aspergillus fungi survive in soil or crop residue and generally infect ears during late silking (Figure 1). Hot, dry conditions favor infection. Stressed plants (from nutrient deficiencies, drought, hail or feeding damage such as birds, or ear-invading insects) are often more susceptible to this disease.

Aflatoxin accumulates in corn kernels as the fungus spreads in subsequent hot and dry weather. Aspergillus fungi 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 2). 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 3).

Fusarium Ear Rot (disease) & Fumonisins (mycotoxins)

Fusarium verticillioides and closely related species are the primary fungi that causes Fusarium ear rot. The fungus survives in crop residue and moves readily by spores on the wind (Figure 4). 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  high even when the disease does not appear to be severe.

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 the European corn borer, earworm, or bird feeding. Fusarium-affected kernels appear purple, tan, or brown (Figure 5). When fungal growth is visible on the ear, infected kernels will appear white to pink or purple. 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 6).

Gibberella Ear Rot (disease) & Deoxynivalenol and Zearalenone (mycotoxins)

Fusarium graminearum (also known as Gibberella zeae) 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. Spores produced on the residue lead to infection during silking (Figure 7). 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 can increase Gibberella ear rot severity and potentially mycotoxin concentrations thus, impacting grain quality.

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 fungal pathogen.

Signs and Symptoms

Gibberella ear rot results in 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 8).

Except in highly susceptible hybrids or under severe conditions, Gibberella ear rot  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.

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 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 9). 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 Diplodia ear rot.

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. 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 10). 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 signs only by breaking the ear in half and observing the fungal structures (pycnidia) in the cob (Figure 11). Often, the entire husk of affected ears will have a bleached appearance.

It is critically important to identify ear rots correctly. If you are not sure what disease is present, send samples for laboratory diagnosis.

Dangers to Livestock

Mycotoxins can severely threaten the livestock that consume corn that contain them. Different mycotoxins affect livestock differently, which means it is important to correctly identify the responsible ear rot and its associated mycotoxin.

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.

Table 1. A general guide to corn ear rots.

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.

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 and result in decreased nutritional efficiency.

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 ear rots, 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 (R5) to determine the presence and severity of ear rots. When scouting, randomly select plants and pull back the husk to examine the entire ear. 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 before or 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 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.

Managing Ear Rots

Corn affected by ear rots will often result in significant discounts on the grain. Kernels affected by ear rots 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.

Read fungicide labels carefully and consult extension before spraying for ear rots. Only certain foliar fungicides are labeled for use against specific ear rot diseases and mycotoxins, and efficacy varies significantly, depending on the targeted pathogen. 

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 Using Atoxigenics to Manage Aflatoxin (CPN-2005).

Harvest infected fields early and segregate the grain. Leaving diseased grain in the field allows 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 further mycotoxin accumulation, or harvest stressed areas of the field last to minimize mixing of high-quality grain with mycoxotin-contaminated grain. 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 equipment before using it on clean grain to prevent cross-contamination.

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 Storing Mycotoxin-affected Grain (CPN-2004).

Diseases and Disorders with Similar Symptoms

There are several conditions that produce symptoms like 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.

Cladosporium Ear Rot (Cladosporium spp.)

Cladosporium ear rot is caused by Cladosporium fungal species that do not produce any known mycotoxins. Look for scattered dark, green-black kernels throughout an ear (Figure 12). The kernels can also appear slightly shriveled and may split open, revealing tufts of fungal growth. The fungus can cause kernels to develop black, irregular streaks. The dark green, fuzzy fungus can also grow on and between kernels. Wet weather near harvest favors Cladosporium ear rot. Plants with damage from frost, hail, or insects are more susceptible to infection.

How to distinguish Cladosporium ear rot from major ear rots:

The major ear rots do not form irregular black streaks on corn. Infected kernels also tend to be more scattered across a corn ear than the other major ear rots and do not tend to form a mat of fungal growth.

Nigrospora Ear Rot (Nigrospora oryzae)

Nigrospora oryzae is the fungus that causes Nigrospora ear rot. It is a weak parasite that is most prevalent late in the growing season on corn that has been stressed or damaged by other diseases or environmental factors. Signs of Nigrospora ear rot include gray fungal growth on corn kernels and small, black spores (Figure 13). Corn cobs with Nigrospora ear rot can be prone to shredding during shelling.

How to distinguish Nigrospora ear rot from major ear rots:

Nigrospora ear rot can be distinguished by the small black spores caused by the fungus. These spores, unlike the pycnidia of Diplodia ear rot, can be removed by rubbing them with your finger.

Penicillium Ear Rot (Penicillium spp.)

Several Penicillium species can cause Penicillium ear rot. Although Penicillium ear rot is associated with the mycotoxin ochratoxin, not all Penicillium species produce the mycotoxin. Signs of Penicillium ear rot include a blue-green powdery growth on and between corn kernels (Figure 14). Penicillium ear rot is typically found on ears that have been injured mechanically, or by insects or hail.

Colonized kernels can appear streaked and bleached, and if the moldy growth gets into the kernel embryo it results in the kernel having a blue-green appearance, sometimes called blue-eye of corn. Fungal growth usually appears at the ear tip, but can also be found on the injured portions elsewhere on the cob. Wet, humid conditions after grain-fill favor infection. The fungus also may continue to spread in grains stored at greater than 17 percent moisture.

How to distinguish Penicillium ear rot from major ear rots:

Penicillium ear rot can be distinguished from the other ear rots by its blue hue. Additionally, it is the only ear rot that results in blue eye of corn.

Trichoderma Ear Rot (Trichoderma spp.)

Several different species of Trichoderma fungi can cause Trichoderma ear rot. The disease can affect injured ears (from insect feeding, heavy storms, etc.). Some (but not all) of the fungi that cause Trichoderma ear rot can produce a mycotoxin in a group of molecules called trichothecenes.

A common sign of the fungi that cause Trichoderma ear rot is dark, blue-green spores that grow on and between the kernels of an infected ear. Trichoderma ear rot can also cause corn kernels to germinate prematurely (called sprouting or vivipary), which is distinct to Trichoderma ear rot (Figure 15). Ears affected by Trichoderma ear rot may weigh much less than uninfected ears from the same field.

How to distinguish Trichoderma ear rot from major ear rots:

Trichoderma fungal spores tend to be brighter green than those of Aspergillus flavus, and they tend to grow in between the kernels, whereas the A. flavus fungus tends to grow from a relatively confined area. Additionally, the major ear rots do not typically cause kernels to germinate prematurely.

Corn Smut (Ustilago maydis)

Ustilago maydis is the fungus that causes corn smut. When it infects corn, it causes the kernels to grow into gray, tumor-like fungal masses, called galls (Figure 16). As these masses develop, they form dark black spores inside. When the fungus dries out, the mass will open to release the spores. 

How to distinguish corn smut from major ear rots:

The gray galls U. maydis forms are distinctive structures that none of the ear rots form. Although ear rot fungi may mummify corn kernels or cover them with sooty spores, they will not form galls. The corn smut fungus can produce galls on any plant part (brace roots, leaves, stalks, tassels).

Black Corn (various fungi)

Several species of saprophytic fungi can cause black corn. The fungi are weak parasites that colonize ears that are already stressed (from insect feeding, hail, lodging, etc.).

Signs of black corn infection typically include black or dark-colored fungal growth, primarily on senescing tissue (leaves, husks, etc.). The color can vary, and if conditions are favorable for fungal growth, they may infect the kernels.

How to distinguish black corn from major ear rots:

The fungi that cause black corn tend to infect senescing tissue in addition to the kernels. Infected ears tend to be black and sooty, primarily on the husk rather than on kernels and cobs (as with ear rots).

Acknowledgments

Authors

Kiersten Wise, University of Kentucky; Tom Allen, Mississippi State University; Martin Chilvers, Michigan State University; Travis Faske, University of Arkansas; 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-Madison; and Albert Tenuta, OMAFRA

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.

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.