February 22, 2021

Identification of Midwestern Corn Disease

The potential for corn seeds and plants to become infected with a disease is greatly determined by genetics, use of fungicidal seed treatments, environmental conditions, time in the growing season, and the presence of casual agents (fungal or bacterial pathogens, viral vectors, and nematodes).

Newly planted seeds are immediately subjected to pathogenic fungi and bacteria that inhabit the soil. In general, these pathogens have a greater potential for causing damage when soils are wet and cold; however, some pathogens prefer warm conditions. Infected seedlings may show signs of damping-off or rotting, stunting, yellowing leaves or loss of color, and deformities.

Susceptibility to diseases depends largely on the level of genetic resistance to a disease. Some seed products can have nearly complete resistance while others are very susceptible. When selecting seed products for planting, the historical disease presence should be considered, and seed selections made accordingly.

Foliar fungal diseases may become apparent anytime during the season depending on the presence of the pathogen. Many fungal pathogens can overwinter on or in infected residue and can contact plants via splashing rain or wind. Some fungal pathogens that are unable to overwinter can be carried from warmer southern areas into the Midwest on wind currents. Depending on the disease, foliar fungicides may provide control.

Bacterial diseases can also overwinter on infected residue. The bacterium can contact plants through water movement in the soil, splashing rain, wind-driven rain, and mechanical means such as cultivating when plants are wet. Infection occurs when bacterium enter the plant through wounds or natural tissue openings such as stomates.

Viral diseases rely on a vector such as aphids or thrips to infect a plant. As the vector feeds, infected juices are injected into plant tissue. Scouting for insects that have the potential to vector viral diseases and applying a timely insecticide may help protect plants from becoming infected via insect feeding. Caution should be taken not to spray insecticides without proper identification as many beneficial insects can be killed.

Additional harvest-time diseases include stalk and ear rots. Stalk rots can result in lodging that can diminish harvest speed, increase harvesting equipment wear and tear, decrease grain quality, and increase the potential for personal injury. Ear rots can affect grain quality, test weight, and marketability. Some ear rots produce toxins that can be harmful or even lethal to livestock and humans.

Nematodes are often discussed under the disease umbrella; however, they are microscopic parasitic worms that live outside (ectoparasitic) or within (endoparasitic) corn roots. Nematode infestations may go unnoticed, particularly under favorable growing conditions, or cause stunting, nutrient deficiencies, and create entrance wounds for other pathogens.

Click below on Seed and Seedling, Foliar, Viral, Stalk, Ear Rots, or Corn Nematodes to identify and learn about the corn diseases on your farm.

Fusarium

Figure 1. Fusarium-infected corn roots. Note discoloration on the mesocotyl.

Identification, Characteristics, and Diagnosis:

Caused by at least six Fusarium species.

Initially, infected seedling roots and the mesocotyl may be shriveled and have tan to reddish-brown lesions, which later become brownish to dark black and rotted (Figure 1).

Favored by wet and cool (55° F or lower) soils.

Most notable in early planted and reduced tillage fields.

Management:

Utilize fungicide-protected seed.

Plant into warm soils that are favorably dry.

Planting depth should not be too deep.

Penicillium

Figure 2. Corn seedling roots infected with Penicillium fungi. Picture courtesy of William M. Brown, Jr., Bugwood.org

Figure 3. Corn seedling with symptoms of Penicillium seedling blight. Picture courtesy of William M. Brown Jr., Bugwood.org.
Identification, Characteristics, and Diagnosis:

Caused by the fungus Penicillium oxalicum.

A bluish-green mold develops on seed causing roots to deteriorate (Figure 2).

Seedling leaves turn yellow and may die (Figure 3).

Favored by wet and cool (55° F or lower) soils.

Most notable in early planted and reduced tillage fields.

Management:

Utilize fungicide-protected seed.

Plant into warm soils that are favorably dry.

Planting depth should not be too deep.

Pythium

Figure 4. Darkened mesocotyl near the soil surface caused by Pythium. Picture courtesy of Don White, University of Illinois

Identification, Characteristics, and Diagnosis:

Caused by several Pythium species.

Favored by wet and cool (55° F or lower) soils.

Symptoms include dark, slimy lesions that cause roots or the mesocotyl to shrivel and the outer layer of roots may be rotted while inner tissues remain white and intact (Figure 4).

Most notable in early planted and reduced tillage fields.

Can infect anytime from planting to mid-season.

Management:

Utilize fungicide-protected seed.

Plant into warm soils that are favorably dry.

Planting depth should not be too deep.

Rhizoctonia

Figure 5. Corn seedlings inoculated with Rhizoctonia solani AG-4 HG-II. Picture courtesy of Srikanth Kodati, University of Nebraska.
Identification, Characteristics, and Diagnosis:

Caused by the fungus Rhizoctonia solani.

Initial symptoms include brown lesions on the mesocotyl and roots of seedlings and young plants (Figure 5).

Later symptoms on older plants include reddish-brown, sunken cankers on crown and brace roots.

Plants may be stunted, chlorotic, or have no above ground symptoms.

Occurs more often in irrigated fields.

Favored by temperatures between 46 to 82° F.

Management:

Utilize fungicide-protected seed.

Plant into warm soils that are favorably dry.

Planting depth should not be too deep.

Bacterial Foliar Diseases

Fungal Leaf Blights

Corn Lethal Necrosis (CLN)

Figure 27. Corn lethal necrosis.

Identification, Characteristics, and Diagnosis:

Occurs when Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) (formerly Maize dwarf mosaic virus (MDMV-B)) or Wheat streak mosaic virus (WSMV) infect the same plant.
The combination of MCMV and SCMV is more frequent.
Symptoms of the combination appear as a bright greenish-yellow leaf mottling that lasts until the end of the growing season (Figure 27).
Plants are susceptible at all stages of growth.
Coloration is usually not noticed until late June or early July and only occurs on the leaves that emerge after infection occurs.
Plants infected late in the season may exhibit poorly filled ears, have premature husk death, and lack leaf mosaic symptoms.

MCMV may be vectored by corn rootworm beetles (Diabrotica species) (laboratory research) and/or thrips (Franklinella species) (Hawaiian breeding nursery). Research has also noted a possible soil-borne mechanism for overwintering.

SCMV is vectored by greenbugs (Schizaphis species) and corn leaf aphids (Rhopalosiphum maidis).

WSMV is vectored by the wheat curl mite (Aceria tosichella (K.)).

Corn lethal necrosis has been identified in North Central Kansas and South Central Nebraska.

Management:

Scout for and manage the occurrence of greenbugs.

Crop rotation can greatly reduce the incidence of MCMV.

Plant seed products that demonstrate higher levels of tolerance.

High Plains Virus (HPV)

Figure 28. High Plains virus. Picture courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org

Identification, Characteristics, and Diagnosis:

Vectored by wheat curl mite (Aceria tosichella (K.)).

In the United States, the disease is found primarily in Eastern Nebraska to Western Idaho and from the Texas Panhandle to Central South Dakota.

Symptoms can vary depending on genetics, environment, timing of infection, and the complex of viruses.

Symptoms can include chlorotic spots on the leaves, stunting, and purple leaf tips and margins. Chlorotic spots form on lower leaves first and progress up the plant (Figure 28). Spots tend to form along vascular bundles in a linear fashion.

Usually occurs along with Wheat streak mosaic virus because they share the same insect vector.

Management:

Scout for and manage the occurrence of wheat curl mites.

Plant seed products that demonstrate higher levels of tolerance.

Maize Chlorotic Mottle Virus (MCMV)

Figure 29. Maize chlorotic mottle virus lesions, Gothenburg, NE. (9-17-2019)

Identification, Characteristics, and Diagnosis:

Symptoms include light-greenish mottling (alternating light- and dark-green areas) of leaves (Figure 29).

Symptomology resembles those of Maize dwarf mosaic virus (MDMV).

Largely a South American disease until discovered in North Central Kansas. Has also occurred in South Central Nebraska and Hawaii.

MCMV may be vectored by corn rootworm beetles (Diabrotica species) (laboratory research) and/or thrips (Franklinella species) (Hawaiian breeding nursery). Research has also noted a possible soil-borne mechanism for overwintering.

The disease has the potential to dramatically affect yield potential.

Favored by high temperatures and wet conditions.

Management:

Scout for and manage the occurrence of corn rootworm beetles.

Crop rotation can greatly reduce the incidence of MCMV.

Plant seed products that demonstrate higher levels of tolerance.

Maize Dwarf Mosaic Virus (MDMV)

Figure 30. Maize dwarf mosaic virus. Picture courtesy of Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org.

Identification, Characteristics, and Diagnosis:

Initial leaf symptoms of susceptible plants are pale, chlorotic spots and short streaks ranging from 1/64 inch to 1/16 inch in width.

Symptoms include light-greenish mottling (alternating light- and dark-green areas) of leaves (Figure 30).

Stunting and chlorosis develops.

Symptomology resembles those of Maize chlorotic mottle virus (MCMV). Laboratory testing should be completed to confirm the virus.

MDMV is vectored by corn leaf aphids (Rhopalosiphum maidis) and possibly greenbugs (Schizaphis species).

Plants are barren and predisposed to root rot and other infections.

In late-season infections, symptoms are found only on upper leaves.

The virus overwinters mainly in rhizomes of Johnsongrass (Sorghum halepense) but can be found in many species of grasses.

Management:

Scout for and manage the occurrence of corn leaf aphids.

Manage and control Johnsongrass.

Plant seed products that demonstrate higher levels of tolerance.

Sugarcane Mosaic Virus (SCMV)

Figure 31. Sugarcane mosaic virus. Picture courtesy of Jeffrey W. Lotz, Florida Department of Agriculture and Consumer Services, Bugwood.org.

Identification, Characteristics, and Diagnosis:

Formerly known as Maize dwarf mosaic virus strain B (MDMV-B).

Mostly a threat in Asia, Africa, Europe, and South America.

Initial leaf symptoms of susceptible plants are pale, chlorotic spots and short streaks ranging from 1/64 inch to 1/16 inch in width (Figure 31).

Symptoms include light-greenish mottling (alternating light- and dark-green areas) of leaves.

Stunting and chlorosis develops.

Symptomology resembles those of Maize chlorotic mottle virus (MCMV). Laboratory testing should be completed to confirm the virus.

SCMV is vectored by corn leaf aphids (Rhopalosiphum maidis).

Overwintering host unknown (Johnsongrass (Sorghum halepense) is not a host).

Management:

Scout for and manage the occurrence of corn leaf aphids.

Plant seed products that demonstrate higher levels of tolerance.

Wheat Streak Mosaic Virus (WSMV)

Figure 32. Corn leaf with Wheat streak mosaic virus. Picture courtesy of Tamra Jackson-Ziems, University of Nebraska.

Identification, Characteristics, and Diagnosis:

Most commercial corn products are not affected by WSMV. However, some inbred lines may be susceptible.

Vectored by the wheat curl mite (Aceria tosichella (K.)).

Infected leaves initially show a yellowish mosaic pattern of parallel discontinuous streaks that become mottled yellow (Figure 32).

Plants infected early are stunted, discolored, and rosetted.

Leaf edges tend to roll toward the mid-rib. The leaf tip of a new leaf may be trapped in the rolled leaf below causing a loop effect.

Infection depends on the population of wheat curl mites, nearness of virus-infected plants, temperature (75 to 80° F), and moisture that promotes plant growth to help maximize mite reproduction.

Occurrence within a field is generally higher next to wheat stubble or grassy areas that harbor mites.

Usually occurs along with High Plains virus because they share the same vector.

Management:

Scout for and manage the occurrence of wheat curl mites.

Volunteer host plants of wheat curl mites (cereals, stubble, other grasses) in adjoining fields should be destroyed two weeks before planting and three to four weeks before sowing in the field to be planted.

Rotation to non-host crops.

Overview

Stalk rots can be caused by bacteria, fungi, viruses, and physiological interactions. Regardless of the cause of the stalk rot, infected stalks can have reduced nutrient and water flow to developing kernels, causing yield loss. Lodged stalks can make harvesting very difficult. Severely lodged plants may:

not be harvestable, resulting in lost yield,
cause a harvest-time fire hazard as broken stalks pile up on the corn head,
restrict the flow of harvested ears into the combine throat when broken stalks block the throat entrance,
add to foreign material in grain when excessive fiber flows through the combine,
create a potential injury situation when plugged stalks require removal,
produce volunteer corn in the next crop.

As harvest approaches, fields should be scouted for stalk and root strength. Each seed product within a field should be checked separately. The pinch or push methods can be used to test standability by randomly selecting 100 plants in a zigzag pattern across the seed product and squeezing the lower nodes or pushing the stalks to a 45° angle. If more than 10 to 15% of the stalks can be crushed or broken, considerable lodging is possible. Though grain moisture content may be higher than desired for harvest, an early harvest should be considered to help reduce the potential of having to harvest lodged plants.

Anthracnose Stalk Rot

Figure 33. Anthracnose top dieback.

Figure 34. Anthracnose stalk rot.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Colletotrichum graminicola that overwinters on infected corn residue.

The fungus also causes Anthracnose leaf blight; however, the leaf blight phase does not cause stalk rot or vice versa.

The stalk rot phase may initially appear in the plant’s upper stalk (known as Anthracnose top dieback). The top may appear bleached while the lower plant remains green (Figure 33).

The characteristic symptom of Anthracnose stalk rot is shiny, black blotches on the lower stalk internodes (Figure 34).

Infected stalks can collapse easily when squeezed and discoloration extends into the pith.

Hair-like appendages (setae) may develop and be seen with a hand lens in the infected area.

Favored by cloudy, warm, and humid conditions after silking; however, infection can occur earlier in the season.

Infection can occur through the roots or when the pathogen is splashed by rain or wind-blown onto stalks.

Stalk breakage may occur higher on the stalk compared to other stalk rots.

Management:

Residue management can help speed decomposition and reduce spore production.

Plant resistant seed products.

Scout for symptoms and prioritize harvest based on stalk strength and the potential for lodging.

Rotate to non-host crops.

Manage residue to promote decomposition.

Reduce potential stress through fertility, drainage, pest, and population management.

Bacterial Stalk Rot

Figure 35. Bacterial stalk rot.

Identification, Characteristics, and Diagnosis:

Caused by the bacterium Erwinia chrysanthemi pv. zeae.

Primarily occurs in irrigated fields where the water source is from ponds, lakes, or streams that can potentially be a source for the bacterium.

Infection can occur any time during the growing season when the whorl or leaf tissue can harbor contaminated irrigation water.

Symptoms include a foul odor resulting from the rotting of internodal tissue that appears dark brown, mushy, and slimy (Figure 35).

Lodged plants appear twisted and likely remain bright green.

Favored by warm, wet weather.

Management:

Avoid using water sources (lakes, ponds, streams) for irrigation that may harbor infecting bacterium.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Charcoal Stalk Rot

Figure 36. Charcoal stalk rot.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Macrophomina phaseolina.

Characterized by a shredded pith that has a silvery-black and speckled appearance. The coloration resembles charcoal dust (Figure 36).

The fungus invades the plant through the roots.

Favored by hot (90° F or higher), dry conditions, especially during grain-fill.

Management:

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

If possible, irrigate to reduce the effects of heat and drought.

Time planting to help avoid seasonal dry weather patterns.

Select seed products that demonstrate higher tolerance.

Rotate to non-host crops.

Manage residue to promote decomposition.

Diplodia Stalk Rot

Figure 37. Diplodia stalk rot pycnidia on the stalk surface.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Stenocarpella maydis that overwinters in residue.

Characterized by dull-green leaves, disintegrated pith, and small, black, flask-shaped spore-producing structures (pycnidia) on the stalk surface (Figure 37).

Infects stalks through the roots, crown, and lower stalk.

Pycnidia cannot be easily scraped from the stalk and give the stalk a sandpaper-like texture.

Favored by dry weather before silking followed by warm, wet weather after silking.

The fungus can also cause ear rot.

Management:

Select seed products that demonstrate tolerance.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Rotate to non-host crops (corn is the only known host).

Manage residue to promote decomposition.

Reduce potential stress through fertility, drainage, pest, and population management.

Fusarium Stalk Rot

Figure 38. Stalk on the right shows symptoms of Fusarium stalk rot.

Identification, Characteristics, and Diagnosis:

May be caused by three fungi, Fusarium verticilliodes, F. proliferatum, and F. subglutinans, that overwinter in residue.

Symptoms including yellowing of the lower stalk and dull-green leaves often appear near the dent (R5) growth stage (Figure 38).

A white fungal growth may appear on the stalk rind.

The pith has a pink or salmon coloration.

Stalks deteriorate and collapse when squeezed and may break off at lower nodes.

Favored by dry weather prior to silking and warm, wet weather after silking.

The fungus can survive for long periods in soil.

Can be confused with Gibberella stalk rot.

The fungus can also infect the ears (Fusarium ear rot), roots, and leaf nodes.

Management:

Select seed products that demonstrate tolerance.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Rotate to non-host crops.

Tillage may or may not help reduce fungal survivability.

Reduce potential stress through fertility, drainage, pest, and population management.

Gibberella Stalk Rot

Figure 39. Gibberella stalk rot with characteristic pinkish nodes and deteriorated pith.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Fusarium graminearum (also called Gibberella zeae) that overwinters in residue.

Symptoms include the development of tiny, round, blue-black fungal structures (perithecia) on the stalk surface near the first internode that can be easily removed with a fingernail.

The pith deteriorates, leaving only the stringy vascular tissues, and has pink-red coloration (Figure 39).

Favored by warm, wet conditions.

Infection can occur through roots or leaf sheaths and plant wounds from insects and other sources.

Can be confused with Fusarium stalk rot.

Can survive on residue or in the soil for many years.

The fungus can also cause Gibberella ear rot, and Fusarium head blight of wheat and barley.

Management:

Select seed products that demonstrate tolerance.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Rotate to non-host crops; wheat/corn or barley/corn rotations should be avoided.

Tillage may or may not help reduce fungal survivability.

Reduce potential stress through fertility, drainage, pest, and population management.

Physiological Stalk Rot

Figure 40. Lodging resulting from physiological stalk rot.

Identification, Characteristics, and Diagnosis:

Caused by the plant’s inability to produce sufficient amounts of carbohydrates because of the interaction between stresses due to weather, fertility, compaction, leaf diseases, plant population, planting date, kernel fill, and other stress factors.

Leaf tissue dies prematurely resulting in an inability to fulfill the plant’s need for energy.

To sustain kernel fill, the kernels draw upon stored energy (carbohydrates) in the stalks, causing the stalks to deteriorate.

Large areas or entire fields can lodge (Figure 40).

Stalks can be easily crushed when pressed and can become rope-like in appearance.

Harvesting can be hazardous because stalks can break off and/or can be pulled out of the ground easily. Broken stalks can pile up on the corn head and soil attached to the roots can flow with the grain into the combine.

Management:

Select seed products with very good stress and disease tolerance.

Maintain fertility.

Use best management practices to help reduce stress factors.

Scout fields prior to harvest and use the pinch or push method to determine stalk strength. Schedule fields with weak stalks for early harvest.

Physoderma Stalk Rot

Figure 41. Snapped stalk at a node due to Physoderma stalk rot. Note the rust-colored sporangia by the stalk rind.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Physoderma maydis that can survive in crop residue for 2 to 7 years.

The fungus can cause leaf blight (see Physoderma leaf spot) and stalk rot.

Plants with Physoderma stalk rot infection often appear healthy; however, stalks are weakened and can easily snap at a node when pushed.

Nodes where breakage occurs are black, rotted, and may have rust-colored sporangia near the rind (Figure 41).

The disease is associated with very wet weather and temperatures ranging between 73 to 90° F.

Because of survival in residue, the occurrence is more likely in continuous corn and conservation tillage systems.

Management:

Select seed products that demonstrate tolerance.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Rotate to non-host crops.

Manage residue to help speed decomposition.

It is not known if foliar fungicides applied to help protect plants from Physoderma leaf spot can help reduce the development of Physoderma stalk rot.

Pythium Stalk Rot

Figure 42. Pythium stalk rot. Photo courtesy of Alison Robertson, Professor at IA State University.

Identification, Characteristics, and Diagnosis:

Caused by fungus-like Pythium aphanidermatum that overwinters in residue.

Infection is generally confined to the first internode above the soil line where the stalk becomes brown and mushy (Figure 42).

Lodged plants appear twisted and remain green.

Infection can occur from V2 to R6 growth stages.

Favored by extended periods of hot (above 90° F), wet, and humid weather.

Poorly drained fields are more susceptible.

Management:

Select seed products that demonstrate tolerance.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Manage residue to promote decomposition.

Reduce potential stress through fertility, drainage, pest, and population management.

Red Root Rot

Figure 43. Red root rot.

Figure 44. Root crown showing infection from red root rot. Picture courtesy of Steve Koenning, North Carolina State University.
Identification, Characteristics, and Diagnosis:

Caused by the fungus Phoma terrestris in association with Pythium and Fusarium species.

Characterized by a red or pink discoloration of the root system and a deep-red to purple discoloration of lower stalk tissue (Figure 43). The root crown can be discolored and rotted (Figure 44).

Root deterioration causes plants to wilt, turn grayish green, die prematurely, and lodge.

The fungus overwinters on infected residue.

Symptoms usually occur late in the growing season just prior to maturation; however, symptoms can occur around mid-silking.

Favored by moderate temperatures (75 to 80° F).

Reddish discoloration resembles infection from Gibberella or Fusarium stalk rots; however, red root rot discoloration is a deeper and darker red.

Associated with high yield environments (excellent fertility, productive soils, high plant populations, and irrigation).

Management:

Select seed products that demonstrate tolerance.

Scout for symptoms and prioritize harvest timing based on stalk strength and the potential for lodging.

Rotate to a non-host crop such as soybean.

Rootless Corn Syndrome

Figure 45. Rootless corn syndrome.

Identification, Characteristics, and Diagnosis:

Caused by agronomic factors that limit nodal root development.

Factors include furrow or sidewall compaction in the planting slot, hot and dry soil conditions during early root development (V2 to V4 growth stages), shallow planting depths, loose or cloddy soil conditions, fertilizer burn, and herbicide injury.

Seed furrows that open after planting can also contribute to the syndrome.

Nodal roots are important for absorbing water and nutrients to support plant development; lack of nodal roots can cause the seedling to wilt and possibly die.

Loss of root mass can subject the plants to root lodging, particularly during high winds and excessive rain events (Figure 45).

Yield potential can be reduced because nutrient and water uptake are reduced. Physiological stalk rot may develop.

Management:

Avoid planting when soil conditions favor compaction of the seed furrow.

Seed planting depth should be monitored.

Row cultivation can place soil around the roots which can help promote new root growth, provide support, and help aerate the soil.

Select seed products with very good root strength.

Prior to harvest, pull plants upward to determine root strength. Fields with a high percentage of easily pulled plants should be a priority for harvest.

Overview

The presence of ear rots and kernel moisture content greater than 15% is not a good combination because diseased kernels can have reduced test weight, grain quality, and ultimately, yield potential. Some ear rots require hot, dry weather and others are associated with moist to wet and cooler conditions. Wounds created by insects, mechanical damage, and hail can provide openings for pathogen entrance into the kernel.

If ear rots are found during routine scouting, identification of the causal agent is important for marketing and storage purposes, and infected fields should become a harvest priority. Identification is important because ear rot diseases such as Aspergillus and Gibberella can produce mycotoxins that are harmful and potentially deadly to humans and livestock. Infected grain should be tested by a toxicology lab to determine if mycotoxins are present. If present, appropriate marketing and/or feeding measures should be instituted based on FDA guidelines. Please see What You Need to Know about Aflatoxin for additional information on mycotoxins associated with ear rots.

Best management practices for ear rot management should begin prior to planting because little can be done to protect ears after infection except harvesting at higher moisture content and drying grain to below 14% within 48 hours. Strategies that help minimize crop stress can help reduce the potential for ear rot development.

Factors that can help reduce crop stress include:

Soil test and fertilize accordingly.

If available, irrigate to help reduce drought conditions.

Use insect-protected corn products (particularly for European corn borer (Ostrinia nubilalis), earworm (Helicovepa zea), and western bean cutworm (Striacosta albicosta (Smith))).

Use foliar disease-resistant corn products.

Use fungicides to help manage foliar diseases.

Avoid continuous corn.

Utilize tillage to manage infected residue.

Aspergillus Ear Rot

Figure 46. Aspergillus ear rot.

Identification, Characteristics, and Diagnosis:

Caused by Aspergillus species.

Characterized by gray-green or light-green powdery mold starting at the ear tip (Figure 46).

Considered to be more of a storage mold because of its ability to grow at 15% moisture content.

Capable of producing an alflatoxin which is toxic to livestock and humans.

Favored by hot (above 90° F), dry conditions and after damage to silks or kernels from insects, hail, birds, or other factors.

Cladosporium Ear Rot

Figure 47. Cladosporium ear and kernel rot.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Cladosporium herbarum.

Characterized by gray to black or very-dark-green powdery mold that may appear in streaks scattered across the ear (Figure 47).

Mold can be rubbed off the kernel surface.

Wounds from insects, hail, frost, and other factors provide infection entrance points.

Favored by wet weather during grain fill. However, it is more common in hot (above 90° F), dry conditions.

The fungus can grow at grain moisture content as low as 16%.

Diplodia Ear Rot

Figure 48. Diplodia ear and shank infection.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Stenocarpella maydis.

Characterized by a white to gray mold that can cause the entire ear to appear brown.

Infection usually begins at the ear base and moves toward the tip, growing between kernels (Figure 48).

Infection can also start at the tip of the ear, especially following insect or bird damage.

Pycnidia (fruiting bodies) can form on husks and at the base of kernels causing ears to be “mummified” or completely covered in white mycelial growth, which causes kernels to be stuck to husks and cobs.

Cobs can also be infected via the ear shank, causing yield loss due to poorly developed ears and/or kernels.

Occurs most often in reduced tillage and continuous corn systems.

Favored by warm, dry conditions prior to silking followed by wet conditions after silking.

Not known to produce toxins; however, there has been some association with diplodiosis (lack of voluntary muscle coordination and/or weakness or partial paralysis) in cattle and sheep.

Fusarium Kernel and Ear Rot

Figure 49. Fusarium ear and kernel rot.

Identification, Characteristics, and Diagnosis:

Caused by fungal species of Fusarium.

Characterized by a white to pink mold infecting random kernels around the ear and/or causes a starburst pattern on kernel caps (Figure 49).

Infection may occur from late-vegetative growth stages to three weeks after mid-silk.

Entry points for infection include kernel growth cracks and damage from insects, hail, mechanical, and other injurious factors.

Warm, wet weather after silking favors development.

Fumonisin toxin, which is toxic to livestock (particularly horses), can be produced.

Gibberella Ear Rot

Figure 50. Gibberella ear rot.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Gibberella zeae.

Characterized by a bright-pink coloration that can vary from red to white (Figure 50).

Mold growth usually begins at the ear tip and progresses toward the base.

Ears become “mummified” because the mold covers the ear and causes kernels to be stuck to husks and cobs.

Favored by cool (average daily temperatures below 72° F) temperatures and seven or more days of rain within a period of three weeks after silking.

Potential mycotoxins include vomitoxin or deoxynivalenol (DON) and zearalenone, which are deadly to livestock.

Nigrospora Ear and Cob Rot

Figure 51. Shredded cob from Nigrospora ear rot.

Figure 52. Black fungal spores in the pith of a Nigrospora infected ear. Picture courtesy of Don White, University of Illinois.
Identification, Characteristics, and Diagnosis:

Caused by the fungus Nigrospora sphaerica, synonym N. oryzae.

Characterized by very small, jet-black spore masses covering kernel tips and cob pith (Figure 51).

Kernels appear bleached and often have whitish streaks starting at the tips and extending toward the crowns and may have a gray mold growth (Figure 52).

Infected ears are easily broken into small pieces during harvest because of rotted pith.

Infection usually starts at the ear butt.

Often associated with premature plant death from frost, hail, drought, and leaf, stalk, and root diseases.

May appear more often in low fertility situations.

Penicillium or “Blue Eye” Ear and Kernel Rot

Figure 53. Penicillium ear rot. Picture courtesy of Tamara Jackson-Ziems, University of Nebraska.

Identification, Characteristics, and Diagnosis:

Caused by fungal species of Penicillium.

Characterized by blue-green mold growing between kernels, usually at the tip of the ear (Figure 53).

Infection generally occurs on kernels damaged by hail, insects, frost, or other factors.

Capable of infecting non-injured kernels with moisture content of 14%.

Trichoderma Ear and Kernel Rot

Figure 54. Trichoderma ear rot.

Identification, Characteristics, and Diagnosis:

Caused by the fungus Trichoderma viride.

A dark-greenish mold grows on and between husks and kernels and sprouting can occur when infection is severe (Figure 54).

Favored by excessive rain and insect, hail, or mechanical damage to the ear.

Overview

Nematodes are microscopic, thread-like worms, which can be found in virtually any field where corn is grown. While some species of nematodes can be beneficial, plant-parasitic nematodes are those which feed only on plants and may reduce yield potential by limiting the plant’s ability to take in water and nutrients. Nematodes can also create openings for other pathogens to enter the plant and cause disease. The presence of nematodes in a corn field can vary from no obvious symptoms to severe injury and considerable yield loss.

The presence of nematodes can vary depending on soil type, environmental conditions, and the crop planted. Under favorable conditions, most parasitic corn nematodes can complete their lifecycle (egg, four juvenile stages, and egg-producing adult) in about one month and may complete four or more lifecycles in one growing season. Mature females of some species, such as root-knot nematode, can increase population densities dramatically by producing hundreds of eggs.

Nematodes are classified by where and how they feed on the plant. Endoparasitic nematodes live within the roots, while ectoparasitic nematodes live in the soil outside of the root.

Corn Cyst

Figure 55. Corn cyst nematode egg sacs. Picture courtesy of Jonathan D. Eisenback, Virginia Polytechnic Institute and State University, Bugwood.org.

Scientific name: Heterodera zeae

Endoparasitic nematodes with brown, lemon-shaped egg sacs on corn roots (Figure 55).

Presence identified in Maryland and Virginia.

Favorable temperature for establishment is 86° F. Does not adapt well to cooler environments.

Damage to corn can be severe when the crop is under moisture stress and the temperature is hot.

Plant hosts include sorghum, barley, oats, rice, sugarcane, wheat, and several species of grassy weeds.

Another corn cyst nematode was identified in Tennessee in 2006. The same nematode was found in 1978 in Tennessee feeding on goosegrass (Eleusine indica). Little is known about the damaging potential of this nematode.

Dagger

Scientific name: Xiphinema spp.
- Ectoparasitic nematodes that can cause stunting and chlorosis.
- Found in sandy and silty loam soils.
- Known for their long lifecycle as they reproduce once per year and can live up to 5 years under favorable conditions.
- Tillage may be helpful for control; however, tillage equipment should be cleaned before leaving an infested field.
- Has a wide range of plant hosts.
- Estimated damage threshold is 30 to 40 nematodes per 100 cm³ of soil.

Lance

Scientific name: Hoplolaimus spp.
- Endoparasitic.
- Mostly found in sandy soils but can be found in many other soils.
- Very common and have a wide range of hosts; therefore, crop rotation is ineffective.
- Estimated damage threshold is 60 nematodes per 100 cm³ of soil.

Lesion or Root-lesion

Scientific name: Pratylenchus spp.
- Endoparasitic.
- Likely the most important corn nematode in the Midwest.
- Small migratory endoparasites that can be found across a wide range of soil types.
- Damage can range from small water-soaked areas to severe root necrosis.
- Damage threshold is 1,000 nematodes per gram of root and populations can be 10,000 to 84,000 nematodes. Threshold numbers may vary based on environmental conditions.

Needle

Scientific name: Longidorus spp.
- Ectoparasitic nematode and one of the most devastating species.
- Highly restricted to sandy soils.
- Their feeding on root tips stunts the lateral roots and destroys the fibrous root system.
- Damage thresholds are as low as 1 nematode per 100 cm³ of soil; 25 nematodes per 100 cm³ of soil can cause severe damage.

Scientific name: Paratylenchus spp., Gracllacus spp.
- Ectoparasitic.
- Small nematodes that cause little damage by themselves.
- Common in fine-textured soils.
- May contribute to noticeable damage when present with other nematode species.
- Threshold unknown.

Ring

Scientific name: Criconemoides spp.
- Ectoparasitic nematode that is adapted more to sandy soils but can be widespread.
- Optimum reproduction conditions include a pH near 7 and soil temperatures between 75 to 80° F.
- Sensitive to dry soils and low pH.
- Damage potential for corn is low.
- Estimated damage threshold is 100 nematodes per 100 cm³ of soil.

Root-Knot

Figure 56. Root-knot nematode damage on corn roots. Picture courtesy of Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org.

Scientific name: Meloidogyne spp.

Endoparasitic nematode causing small galls on corn roots (Figure 56).

Soybean is also a host; therefore, rotation to soybean is not an option.

Damage potential for corn is low unless corn is stressed.

Estimated damage threshold of 100 nematodes per 100 cm³ of soil.

Favored by ample moisture and warm soil temperatures.

Sheath

Scientific name: Hemicycliophora spp.
- Ectoparasitic.
- Damage threshold is unknown.

Spiral

Scientific name: Helicotylenchus spp.
- Considered an ectoparasitic nematode; however, some species may be endoparasitic.
- Found in most soil types.
- Identifiable by characteristic spiraling when relaxed or dead.
- Has a wide range of plant hosts.
- Estimated damage threshold of 500 to 1000 nematodes per 100 cm³ of soil.

Sting

Figure 57. Sting nematode damage on corn roots. Picture courtesy of Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.

Scientific name: Belonolaimus spp.

Ectoparasitic.

Found in sandy soils that are irrigated.

Estimated damage threshold of 1 nematode per 100 cm³ of soil.

During feeding, a toxic chemical is injected into the roots.

The root mass can be greatly reduced, become stubby, and have dark, characteristic shrunken lesions on the root tips (Figure 57).

The appearance of stubby roots is also associated with stubby-root nematode (Paratrichodorus spp.), stunt nematode (Tylenchorhynchus spp.), and dinitroaniline herbicide injury; however, in these situations the characteristic lesions from sting nematode feeding are absent.

Has a wide range of crop hosts including sorghum, soybean, and wheat. Wheat may mature prior to the occurrence of damaging populations of sting nematodes; however, a secondary crop may experience damage.

Stubby-Root

Scientific name: Paratrichodorus spp.
- Endoparasitic.
- Associated with sandy soils.
- Damaging populations have occurred in southern states.
- Root systems become stunted, blunt, and swollen. The appearance of stubby roots is also associated with sting nematode (Belonolaimus spp.), stunt nematode (Tylenchorhynchus spp.), and dinitroaniline herbicide injury. Sting nematode feeding can be differentiated by the presence of dark, shrunken lesions.
- Estimated damage threshold of 40 nematodes per 100 cm³ of soil.

Stunt

Scientific name: Tylenchorhynchus spp., Quinisulcius spp.
- Ectoparasitic.
- Associated with varied soil types.
- Estimated damage threshold of 100 nematodes per 100 cm³ of soil.

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Web sites verified 8/28/19.

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Identification of Midwestern Corn Disease

Click below on Seed and Seedling, Foliar, Viral, Stalk, Ear Rots, or Corn Nematodes to identify and learn about the corn diseases on your farm.

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