Corn smut is a fungal disease that affects corn crops worldwide. Proper identification, treatment, and prevention of corn smut are essential for managing this destructive pathogen. This comprehensive guide provides farmers, gardeners, and field technicians with the information needed to effectively control corn smut.
What is Corn Smut?
Corn smut is a plant disease caused by the fungus Ustilago maydis that leads to the formation of galls on corn plants. It goes by several common names including common corn smut, boil smut, and huitlacoche.
The fungus that causes corn smut is an obligate parasite, meaning it requires a living host to survive. It infects all parts of the corn plant including the leaves, stalk, tassel, ears, and kernels. Once inside the plant tissues, the fungus proliferates and causes the plant cells to swell and distort, forming tumor-like galls.
These galls or “boils” initially appear white or pale green. As they mature, they turn gray/black and become filled with dark powdery spores produced by the fungus. The galls can vary considerably in size, from less than an inch to several inches across. They are most commonly found on the ears, stalks, and tassels.
Corn smut does not kill the infected plants outright but drastically reduces yield by inhibiting pollination and kernel development. Heavy infestations can weaken plants and increase the risk of stalk breakage. Unlike other ear rots, corn smut does not produce toxins.
The spores produced in the mature galls are spread by wind, rain, and insects, allowing the fungus to rapidly disseminate and infect new plants. The thick-walled spores can survive harsh conditions and remain viable in soil for years.
How Corn Smut Spreads
The main route of infection for corn smut fungus is through its airborne spores. When the spores land on susceptible corn tissue, they germinate and penetrate the plant. The mycelium then spreads intercellularly, causing the cells to enlarge and form galls.
Wind is the primary way the spores are dispersed over short and long distances. Wind can carry spores from smut galls on one plant to healthy plants up to a mile away.
Rain splash also propels spores from galls onto lower leaves and plant parts. Overhead irrigation has a similar effect.
Insects can pick up spores on their bodies and mechanically transmit the fungus to new plants. Sap-sucking insects like aphids may also facilitate infection through their feeding wounds.
The fungus can also spread via infected seeds. Both seed galls and asymptomatic kernel infections can lead to infected seedlings. However, this is a minor source of inoculum compared to wind and insect transmission.
In most cases, the spores infect the young, actively growing tissues of the corn plant. The ideal conditions for infection are moderate temperatures (70-80°F) and high relative humidity.
Symptoms of Corn Smut
Recognizing the symptoms of corn smut is the first step in identification and control:
- Small chlorotic or white bumps emerge on leaves, stalks, tassels, and ears.
- Galls rapidly enlarge, turning grayish-white, then grayish-black as they mature.
- Older galls become filled with black powdery spores in a semi-gelatinous matrix.
- Galls vary greatly in size, from less than an inch to several inches across.
- Ear galls may cover the entire ear, inhibiting kernel development.
- Infected plants show stunted growth, barren ears, and yield loss.
- Leaves near galls may turn yellow or die back.
- Severely infected plants can weaken and lodge (fall over).
Carefully scout fields during the growing season to detect initial symptoms on leaves and stalks. Examine developing ears for small white bumps that signal infection. Watch for worsening symptoms as galls expand and darken. Comparison to images of corn smut can confirm diagnosis in the field.
How to Identify Corn Smut
Being able to accurately differentiate corn smut from other ear rots is critical for proper treatment. The following tips will help identify corn smut:
- Observe the characteristic tumor-like galls on stalks, leaves, and ears. Healthy corn tissue swells and distorts.
- Look for spherical grayish-black powdery galls that rupture easily when squeezed.
- Note lack of tan-brown discoloration or rotting associated with other ear diseases.
- See if galls are filled with a mass of black powdery spores in a gel matrix.
- Check for stunting, poor ear formation, and yield impacts.
- Microscopic examination of spores can confirm Ustilago maydis as the causative fungus.
- Monitor weather conditions favorable to corn smut development.
- Eliminate other ear rots based on symptom differences.
- Review images of smut symptoms for comparison.
- Send suspect gall samples to a diagnostic laboratory for definitive diagnosis.
Examine ears carefully for the presence of characteristic smut galls. The powdery black spore mass inside galls distinguishes corn smut from other issues. Fungal culture or PCR testing can verify the pathogen.
Treatments for Corn Smut
While no treatment can cure infected corn plants, prompt fungicide application at initial growth stages may suppress smut development in the field.
Fungicides
Apply labeled fungicides like tebuconazole, prothioconazole, or metconazole at early vegetative growth to limit fungal infection of young plant tissues. Follow all label instructions.
Target fungicide sprays at thesilking/tasseling stage when the corn is most vulnerable. Repeat applications may be needed for full season control. Fungicides are most effective when combined with cultural practices that reduce inoculum levels and disease pressure in the field.
Surgery
Immediately remove and destroy any galls found on leaves or stalks to prevent spore spread. Carefully cut off smutted ears and tassels well below the infected area. Bury or burn the removed galls and tissue – do not compost. Sterilize tools after surgery using bleach solution.
Surgery can slow disease progression but is most useful before widespread infection when only a few plants show symptoms. It is not effective for advanced, severe infestations.
Crop Rotation
Rotate corn with non-host crops like soybeans, alfalfa, small grains, or sorghum for 2-3 years to reduce inoculum levels in the soil and disrupt the smut fungus life cycle. Avoid planting corn after corn.
Tillage
Plowing fields after harvest buries crop debris deep enough to prevent spore survival. Rotary hoeing helps deter fungal infection by drying out the top layers of soil.
Resistant Varieties
Planting smut-resistant corn varieties can prevent major crop losses. Look for hybrids rated as “highly resistant” or “resistant” to Ustilago maydis in seed catalogs. Confer with your seed company rep for local variety recommendations. However, no corn variety is 100% smut proof.
Preventing Corn Smut Infection
The best control strategy for corn smut combines cultural, chemical, and genetic tactics to block initial infection and limit disease development.
- Scout fields weekly and rogue out smutted plants as soon as symptoms appear.
- Allow proper plant spacing for good air movement.
- Avoid excess nitrogen fertilization that favors smut growth.
- Apply fungicides prophylactically before infection periods.
- Ensure thorough coverage when spraying.
- Rotate crops and avoid planting corn after corn.
- Use clean, treated seed tested for Ustilago maydis.
- Incorporate crop debris promptly after harvest to speed decomposition.
- Choose resistant corn hybrids suited to your growing region.
- Monitor weather and apply fungicide ahead of warm, humid conditions favorable to smut.
- Clean equipment thoroughly when moving between infected and non-infected fields.
An integrated approach using multiple preventive tactics consistently is key to effective, long-term corn smut control.
Impact of Corn Smut on Crop Yield
Although not typically deadly, corn smut can substantially reduce yields depending on infection severity:
- Smut galls rob nutrients and energy from developing ears and kernels.
- Ear galls directly decrease number and size of kernels.
- Stunted, diseased plants produce smaller ears.
- Up to 50% yield loss in susceptible varieties with heavy infestations.
- 5-10% loss more typical in fields with mild to moderate smut.
- Entire ears may be consumed if infection occurs early.
- Smutted plants have lower grain quality and test weight.
- Stalk breakage and lodging increase harvest losses.
- Need to harvest early before plants die accelerates field dry down.
Careful field scouting lets growers assess expected smut impacts on yield and make informed harvest decisions.
Is Corn Smut Dangerous?
Unlike fungal toxins in other grain crops, the corn smut fungus does not produce poisonous mycotoxins. The mature galls and spores contain no toxic substances hazardous to humans or livestock.
However, the tumor-like growths make the corn ears unmarketable for fresh or processed corn. The unpleasant appearance also devalues the crop for animal feed.
While not toxic, corn smut does render the grain unfit for human consumption. Livestock may refuse to eat heavily infected corn or waste infected parts of the ear. There are no restrictions on feeding unaffected parts of smutted ears to livestock as a grain source.
Barring extensive infection, corn smut itself does not threaten animal or human health. But grain from smutted fields may be contaminated by other mycotoxigenic fungi and should be tested for toxins before use as animal feed.
Is Corn Smut Edible?
In Mexico, the corn smut fungus has been prized as a delicacy for over 500 years. The Aztecs called it huitlacoche and considered it a great delicacy, even harvesting infected ears specifically for food.
When fresh, the immature galls have a nutty, earthy, mushroom-like flavor that enhances traditional recipes. Smut galls can be eaten raw in salads, sautéed as a side dish, baked into quesadilllas, or used to flavor soups. Canned and frozen huitlacoche are also sold.
However, corn smut is not considered palatable at the mature stage when the galls are filled with dry spores. The ideal time to harvest smut for cooking is when the galls are still soft, white, and growing – before they rupture.
In the U.S., entrepreneurs are working to create a niche market for the gourmet corn fungus. Small amounts of corn smut are edible for most people but always cook it thoroughly before eating. Those with corn allergies may react to huitlacoche.
Frequently Asked Questions About Corn Smut:
What causes corn smut?
Corn smut is caused by the fungal pathogen Ustilago maydis. Its spores infect corn plants and form galls that produce more spores, continuing the disease cycle.
What are the symptoms of corn smut?
Look for white or pale green bumps that develop into large grayish-black galls filled with powdery black spores. Stunting, yield loss, and poor ear fill also occur.
How does corn smut spread?
Wind, rain, and insects spread corn smut spores from galls on infected plants to healthy plants. Spores survive in residue and soil to initiate new infections.
When should I spray for corn smut?
Apply labeled fungicides at early vegetative growth through tasseling/silking when plants are most susceptible. Use resistant hybrids and crop rotation too.
How do I get rid of corn smut?
Rogue infected plants immediately, plow under crop debris after harvest, rotate corn, choose resistant varieties, apply fungicides prophylactically, and avoid excess nitrogen.
Can you eat corn smut?
In Mexico, immature corn smut galls are eaten as the delicacy huitlacoche. However, mature galls with dry spore masses may be distasteful and hard to digest.
Does corn smut affect yield?
Moderate to severe smut infestations can reduce yields by 50% or more. But light, sporadic infections cause little measurable loss if corn is harvested and dried normally.
Is corn smut toxic?
Unlike other fungal ear rots, corn smut does not produce toxic compounds. However, heavily infected corn may contain other dangerous molds and should be tested before feeding to livestock.
Can you feed smutty corn to cattle?
Lightly infected smutty corn can safely be fed to livestock. But animals may refuse corn with large galls. Test for other mycotoxins if smut is widespread in the field.
What is the best way to prevent corn smut?
Practice crop rotation, plow under debris, choose resistant hybrids, scout fields diligently, apply fungicides at susceptible growth stages, rogue diseased plants, and avoid excess nitrogen.
Conclusion
Corn smut is a prevalent fungal disease that can substantially decrease corn yields. Successful management relies on scouting for early symptoms, accurate diagnosis of infection, and prompt integrated treatment. Removing diseased plants, rotating crops, using resistant hybrids, and making timely fungicide applications are key to keeping smut under control.
While unsightly and damaging in the field, fresh corn smut galls are enjoyed as the delicacy huitlacoche in Mexican cuisine. However, the mature black spore stage is unpalatable. With care and vigilance, the impact of corn smut can be minimized through an integrated pest management plan using cultural, chemical, and genetic tactics.