Abnormal Corn Ears
This symptom is associated with an unknown problem that occurs before the silking stage. Silk development is interrupted; thus ovules are not receptive for pollination and kernels do not form. The main causes related to this issue are unknown but can be associated with a condition that took place before flowering, around V10-V15. The response differs among hybrids.
Banana-shaped ears develop when entire or partial kernel rows are aborted. The ears bend, taking the shape of a banana because of an uneven kernel number on the different sides of the ears. The causes are not clearly known, but could be associated with severe weather, herbicide misapplication, heat and drought stresses, and conditions before or at pollination.
Illustration 4 Examples of disease problems in corn ears. Disease problems in corn ears (pictures taken after silking stage) as the outcome of poor husk coverage.
Ear molds often occur when there is too much rain at pollination, drought during the grain fill period, insect damage to the ear, or consistently wet weather during the dry-down period. The greenish-yellow mold on this ear indicates the presence of Aspergillus.
Removing the husks and inspecting these ears reveals a white mold growing in the kernel channels starting at the base of the ear. In some cases, Diplodia can mummify the entire ear.
A pink to reddish mold that starts forming at the tip of the ear is symptomatic of Gibberella ear rot. Gibberella is capable of producing vomitoxin, also known as deoxynivalenol (DON).
Fusarium ear rot can produce a whitish pink to lavender fungal growth on kernels. As seen here, a "starburst" pattern is often present. Fusarium ear rot is associated with the presence of the mycotoxin fumonisin.
Ear stunting is also referred to as "beer can ears" or "blunt ear syndrome." Ear size is abnormally limited, which offers a physical restriction for the final number of kernels, affecting primarily the total number of kernels per row. The occurrence of this abnormality may be related to the misapplication of chemicals, but the primary cause is still unknown. Ear size is defined before flowering, so any stress (biotic or abiotic) conditions affecting the crop during the mid- to late-vegetative stages could influence the size of the ear.
Exposed ears occur when the ear keeps elongating beyond the end of the husks. When this happens, the upper part of the ear is partially or completely exposed, which makes the ear more susceptible to any biotic or abiotic stresses (weather and pests in general). The cause of this, as suggested by Robert Nielsen, Purdue University, seems to be primarily related to the weather before or at pollination (heat and/or drought stresses) combined with a reprieve of those conditions after flowering. The combination of heat and drought early this season, followed by an unusually cool and wet pattern, has been the cause of exposed ears in Kansas.
Poor Kernel Set
Poor kernel set relates to lack of pollination, fertilization failures, or abortion after the ovules are fertilized, among other factors. The degree of severity ranges from just a few missing kernels to an ear showing mostly the cob with just a few kernels visible. Poor pollination or fertilization failure (due to pollen tube failure, desiccated and nonfunctional silks, no viable pollen) can be primarily related to warm temperatures and insufficient water supply during flowering. Other factors also can cause a similar pattern in corn ears, such as herbicides, nutrient deficiency (nitrogen and phosphorous), and insect feeding. Any biotic or abiotic factor that affects pollen shed or silk development and extrusion from the husks will interfere with the timing of pollen shed or silk development, which will cause fertilization problems and affect kernel set.
Ears sometimes become malformed after misapplication of a herbicide, in this case glyphosate and AMS (ammonium sulfate).
The pinched appearance of these ears resulted from an application of ALS grass herbicide applied after the label-recommended stage. The off-label herbicide application disrupts the normal doubling of kernel rows at the V9 stage.
Malformed ears from late application of glyphosate and a growth regulator herbicide can result in severe grain yield reduction.
Tassel ears occur when the tassel and the ear are present within the same structure. The tassel ears tend to appear at the top of the plant. The physiological explanation of this syndrome is not firmly known but seems to be related to the environment. This phenomenon seems to be more frequent in tillers, in plants close to the borders of the field, and under very low plant density levels with more isolated corn plants. The husks are absent, so the ear is not protected, and will be subjected to the influence of the weather, insects, and diseases.
Tip dieback symptoms become evident after pollination and are related to a failure in kernel set at the tip of the ear. The physiological explanation for these types of ears is related to poor pollination of fertile ovules, nonfertilized ovules where pollen is shed when silks were not yet extruded from the husks, and kernel abortion in the weeks after pollination until milk stage, R3. Any biotic (corn rootworm, Japanese beetles, foliar diseases, among several other factors) or abiotic (heat or drought stresses and nutrient deficiencies--specifically with nitrogen) stress can promote asynchrony between tasseling and flowering, which prevents undeveloped or late-developed ovules and silks from being pollinated. In addition, conditions before flowering can affect the kernels at the tip of the ear. These kernels are the most sensitive to stress conditions.