Figure 1. A corn field with heavy hail injury. This field had high incidence of corn ear rots.
A number of corn fields scouted in the northeast and east-central counties were found with corn ear rots. Corn ear rots were mostly prevalent in areas that experienced hail storms recently (Figure 1). Corn ear rots are caused by a number of fungal pathogens and which one will develop depends on weather conditions.
Fusarium Ear Rot
Fusarium ear rot develops under hot dry weather and infection occurs at or after flowering. The fungus infects the corn ear through silk and wounds created by hail injury and insects. Occasionally, Fusarium stalk rot can develop systemically and cause ear rot. Several Fusarium species cause ear rot but the most common species are F. verticillioides and F. proliferatum. These Fusarium species overwinter in residue from corn and other plants.
The symptoms vary greatly depending on the genotype, environment, and disease severity. Individual infected kernels can be scattered in the ear (Figure 2). Under severe conditions, the entire ear may be consumed by the fungus. Infected kernels have whitish pink to lavender fungal growth.
This disease reduces yield and grain quality. The kernel can be completely consumed by the fungus and become contaminated with mycotoxins (fumonisins), which can be fatal to livestock (horses and pigs).
Figure 2. Corn ear with Fusarium ear rot. Photo by Emmanuel Byamukama.
Gibberella Ear Rot
Gibberella ear rot, also called red rot, develops under prolonged rainy weather late in the growing season. Its symptoms are characterized by a reddish mold that appears at or near the tip and grows down the ear (Figure 3). Gibberella ear rot is caused by Gibberella zeae. This pathogen overwinters on corn debris and has a wide range of hosts including small grains. The fungus infects ears through the silk and progresses down the ear. Corn following corn is more prone to Gibberella ear rot development. This fungus produces mycotoxins (deoxynivalenol and zeralenone) in infected grain.
Figure 3. Gibberella ear rot on corn. Photo by Emmanuel Byamukama.
Aspergillus Ear Rot
Aspergillus ear rot is the most important ear rot because it produces aflatoxins which are dangerous to humans and animals. Two common species Aspergillus flavus and Aspergillus parasiticus infect corn. Of these, Aspergillus flavus is the most predominant species. The fungus overwinters in soil and debris and infection is favored by hot, dry weather. The fungus is spread to silks by wind or insects. This disease can be important under drought conditions. Insect or hail damage predisposes the kernels to infection and Aspergillus ear rot development. In most cases only a few kernels on an ear are infected. Infected kernels have masses of olive to yellow green spores on and between them (Figure 4). Sporulation of the fungus is most evident on kernels that were injured. However the fungus can also be present on kernels without showing symptoms.
If Aspergillus ear rot is present in a field, the grain needs to be tested for aflatoxins. If concentrations are greater than 20 ppb, the grain cannot be sold or transported across state lines. The blending of corn to reduce aflatoxin concentrations is prohibited for interstate trade. If the grain is used for ethanol production, distillers’ grains obtained from that corn will have elevated aflatoxin levels.
Figure 4. Aspergillus ear rot symptoms. Photo courtesy of Robertson Alison.
Penicillium Ear Rot
Penicillium ear rot is caused by Penicillium spp and this ear rot is also important because of the mycotoxins associated with it. The most common mycotoxin is ochratoxin A. This mycotoxin is produced by Penicillium verrucosum. Penicillium ear rot is most common in ears with mechanical injury caused by insects or hail damage. Infected kernels have powdery green or blue-green mold (Figure 5).
Figure 5. A corn ear with both Fusarium spp infection (whitish mycelia) and Penicillium spp infection (greenish mycelia). Photo by Emmanuel Byamukama.
Diplodia Ear Rot
Diplodia ear rot develops in corn fields with history of this disease and when weather is wet and warm around silking. Diplodia ear rot is caused by Diplodia maydis (also known as Stenocarpella maydis). Ears are most prone to infection about three weeks after flowering when the silk dies off. Diplodia spores are spread through splashing rain. Infected kernels are dull gray to brown and Diplodia ear rot starts at the base of the cob rotting the husk as well (Figure 6).
Figure 6. A corn ear with Diplodia ear rot. Notice the “glued” rotted corn husks, a sign of Diplodia spp infection.
Sampling for Ear Rots & Mycotoxins
Scouting fields before harvest is important to determine the amount of ear rot in a field and consequently if there is a risk of mycotoxin contamination. Scout fields by peeling back the husks and inspecting at least 10 ears in a minimum 5 random stops throughout the field. If >10% of ears in a field have >10-20% moldy kernels, the field should be scheduled for harvest as early as possible. Care should be taken not to damage kernels during harvest. The grain should be cooled and dried to less than 15% moisture immediately after harvest. Grain from fields where ear rot was a problem should be stored in a separate bin to grain from fields where the ears were healthy. Corn ears suspected to have Aspergillus or Penicillium ear rots should be sent to a diagnostic lab to determine mycotoxin concentration.
Other resources on ear rots and allowed mycotoxin levels:
- FDA Mycotoxin Regulatory Guidance.
- Clay, D.E., S.A. Clay, and K. Reistma. 2009. South Dakota Corn Best Management Practices – Corn diseases chapter. SDSU, Brookings, SD.
- Munkvold, G. P. and White. D. G. 2016. Compendium of Corn Diseases. American Psychopathological Society Printing Press, Minneapolis, MN. 156pp.