Abnormal Corn Ear and Kernel Development

Pinched Corn Ears

Pinched corn ears are characterized by a normal length and a portion of the ear base that may have up to 50% more rows than the remainder of the ear (Figure 1).¹ Kernel row number is determined during the V7 to V10 leaf collar stages, with a doubling of the row number occurring around V9.¹ Severe stress during this growth period—particularly a late post-emergence application of an ALS (acetolactate synthase) herbicide—can cause the kernel row number to be reduced from the point in time of the application. Sulfonylurea herbicide and organophosphate soil insecticide interactions may also cause pinched ears.¹

^{Blunt Ear Syndrome}

Blunt ear syndrome is also referred to as “beer can ears” or ear stunting. It is characterized by reduced ear length, normal (for the corn product) kernel rows at the base of the ear, and usually a barren ear tip (Figures 2, 3, and 4). Husks are slender and pointed, which restricts silk emergence.

The reduced kernel number suggests that biotic (living) or abiotic (non-living) stress occurred before ear size determination was complete. Possible causes include cold shock, a foliar fertilizer application, or a chemical application during the V8 to V12 (pre-tassel) growth stages using a late post-emergence herbicide (ALS, PGR or plant growth regulator, glufosinate, glyphosate), a fungicide (strobilurin), an insecticide, or spray adjuvants.² Research at Purdue University suggests the syndrome can be caused by an application of a nonionic surfactant (NIS) prior to tasseling with or without a foliar fungicide.^3,4 The greatest risk for occurrence appears to happen from V12 to V14 growth stages (about one to two weeks before pollination).⁴

Figure 3. Blunt ear syndrome ears.

Incomplete Kernel Set

Incomplete kernel set is characterized by ears with few and sparse kernels. Ineffective pollination is the most common cause of poor kernel set and can be a result of heat and drought stress delaying silk emergence, which may lead to poor synchrony with pollen shed (sometimes referred to as “missing the nick”). A combination of excessive heat and low humidity can also desiccate silk, reducing the ability of the silk to receive pollen. Other potential causes of poor kernel set may include tassel wrap, herbicide injury, silk clipping by insects such as corn rootworm beetle or Japanese beetles, excessive rainfall during pollination, or diseases such as corn smut (Figures 5, 6, and 7). Additionally, a phosphorus deficiency can interfere with pollination.¹

Zipper Ears

Zipper ears are characterized by partial or missing kernel rows on the underside of the ear due to kernel abortion (Figures 8 and 9). The missing rows can cause the ear to bend like a banana. The specific cause is unknown, though potential contributing factors include drought stress, severe defoliation during late silk and early blister growth stages, high plant population, extensive covering of underside ear silks by upper ear silks which may block fertilization, and differential heating around the ear circumference.^1,7 Pesticide misapplication may also be a cause.

Unfilled Ear Tips or Tip Dieback

Unfilled tips can be caused by poor silk fertilization which results in kernel abortion. Unfertilized ovules and aborted kernels may appear dried and shrunken (Figure 10). Aborted kernels may have a yellow color (Figure 11).¹ Drought, nitrogen deficiency, high temperatures, hail damage, foliar diseases, and cloudy weather during early kernel development may be potential causes.

Unfilled ear tips can be genetic, as some corn products tend to have an unfilled tip. Additionally, ample moisture after pollination can cause the cob to lengthen beyond previously fertilized kernels (Figure 12).

Chaffy Ears

This abnormal development is characterized by ears that have gaps between shrunken, lightweight kernels and (Figure 13). Ears can be poorly filled. Premature plant death due to frost, drought, disease, high temperatures, and insects are among the causes. Additionally, high plant population, severe potassium deficiency, and hail damage may result in chaffy ears.¹

Drought Damaged Corn Ears

Drought damaged ears have characteristics of other abnormal ear types. Tips can be unfilled, row numbers can be reduced, kernel set can be incomplete and scattered, kernels can be light in weight and chaffy, and ear size can range in size from very small (nubbin) to medium depending on when the drought event began. Other causes for nubbin ears can include nitrogen deficiency and high plant populations.¹

Bouquet Ears

Several ears appearing on the same node are referred to as bouquet ears (Figure 14). The cause may be related to heat stress during early ear formation and development (V5 to V15 growth stages) and the misapplication of pesticides (herbicides, fungicides, insecticides) prior to tasseling Additionally, any factor that disrupts apical dominance of the primary ear can result in the formation of multiple ears.⁵

Insect and Mite Injury to Corn

Insect (adults or larvae) and mite feeding on silks, kernels, and developing ears can cause ears to have missing kernels, bruised or damaged kernels, malformed ears, and reduced grain quality.

Silk feeding by corn rootworm beetles and Japanese beetles can cause ears to have missing kernels because of unsuccessful ovule fertilization (Figure 7).

Stink bugs pierce through the husk on immature ears and inject a toxin that causes the cob and husk to cease development on the pierced side. The other side continues to grow, creating a hooked or banana shaped ear (Figure 15) to form. Exposed kernels from the reduced husk coverage become food for birds, other insects, and are exposed to environmental issues. Piercing through milk-stage ears can cause individual kernels to shrink, discolor, and develop feeding scars.¹

Direct feeding on corn ears by corn earworm, European corn borer, and Western bean cutworm can cause kernels to be highly damaged or missing (Figure 16). Additionally, because of the damage, individual kernels or parts of the ear may develop kernel or ear rot.

Kernel red streak is a reddish discoloration that usually occurs in ear-tip kernels caused by the toxin secreted by feeding wheat curl mites (Figure 17). Coloration can vary by corn product, and some corn products may have a genetic characteristic for reddish kernels.

Hail Damage to Corn

Hail can cause kernels to be bruised, discolored, missing, and to have fungal growth. The amount of injury largely depends on the state of kernel development at the time of the hail event and the size of the hail stone (Figure 18). Large hail stones can crush immature kernels and break the seed coat, which allows the milk to flow onto surrounding kernels. The sugar content of the milk becomes a medium for fungal growth which can damage additional kernels. Hail damaged ears may have a void of kernels where a hail stone hit (Figure 19).

Corn Ear Smut

Corn ear smut can be found on any part of the corn plant throughout the growing season. Smut spores can enter corn tissues after injuries from hail, insects, or wind. The smut can completely engulf or partially cover an ear (Figure 20). Smut can also be found on “tassel ears” which are prone to develop on corn tillers (Figure 21).

Clear or Translucent Kernels

This condition is characterized by the appearance of random kernels filled with clear liquid interspersed among normal kernels on a normal-sized ear (Figure 22). The abnormal kernels collapse during kernel maturation, leaving behind a shriveled shell. The syndrome has been attributed to a late glyphosate herbicide application.¹