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Hot temperatures can reduce viability, production, and release of corn pollen. Environmental or chemical stress during the vegetative and early reproductive stages (V5 to R3) of corn can cause abnormal or malformed ears. Distinctly different symptoms develop depending on the timing, type, and severity of corn stress.
An individual corn tassel can shed pollen for a week with peak shed occurring on the second or third day.1 Hot temperatures coupled with dry weather can hasten pollen shed. Each tassel produces millions of pollen grains providing adequate pollen in most conditions.1,2 Pollen production may be reduced by continuous hot weather occurring a few days before and during pollination as photosynthesis may not be efficient and carbohydrate production can be reduced.3
Figure 1. Incomplete kernel set.* .
Pollen shed from anthers is called dehiscence and occurs when there is a drop in humidity as temperatures begin to rise.3 Typically, pollen shed begins in the morning when temperatures are not as high with a second ‘flush’ of pollen shed sometimes occurring with cooler temperatures in late afternoon. Humidity and temperature conditions can affect dehiscence, and a decrease in pollen shed has been reported when temperatures are greater than 86 °F.1
Until all anthers have shed pollen from the tassel, new pollen continues to mature from anthers. Pollen grains have a thin outer membrane and remain viable for 18 to 24 hours in favorable conditions.2 Viability is reduced to a couple hours or less in extreme heat.3 Temperatures of 100 °F or greater cause extreme heat stress that can sometimes desiccate pollen before it can successfully fertilize an ovule or kernel.1 High temperatures can also reduce viability before pollen grains are shed by anthers.3
Figure 2. Zipper ear.*
Exposed corn silks can desiccate prematurely when humidity is low and temperatures exceed 95 °F. Silks generally emerge at a rate of 1 to 1.5 inches a day and continue until fertilized. High temperatures do not significantly affect the rate of silk elongation.4 Consistent and timely silking is needed to allow for a viable silk and pollen grain to unite. Silks that become less receptive to pollen and senesce before pollen shed can lead to ears with reduced kernel set.
Kernel set can be irregular on ears when pollen shed and silk elongation are not synchronized. This is more common when moisture stress accompanies heat stress. Pollination can be successful during stretches of high temperatures if adequate moisture is supplied to the plant. Additional conditions that facilitate successful pollination include cooler nighttime temperatures, healthy soil structure, and corn products with close timing between midpollen shed and mid-silking date.
Figure 3. Blunt ear syndrome.*
Abnormal corn ear development can begin at the V5 corn growth stage, or when the 5th fully expanded leaf and collar occurs and ear shoot tissue development is initiated. Any stress during the V5 growth stage through early corn development can cause abnormal growth and development of reproductive plant tissues. Corn may experience one or more stresses, such as drought, extreme temperature, saturated soil, nutrient deﬁciencies, disease or insect injury, and misapplied pesticides. Many stresses result in speciﬁc malformations of the corn ear. Symptom recognition will help diagnose the causal factors of malformed ears.
Figure 4. Tip dieback.*
Ears will have a limited number of kernels. Causes range from uneven crop development, an inadequate supply of pollen during pollination, severe drought, high temperatures, phosphorous deficiency, herbicide injury, or insect feeding and silk clipping.
Partial or missing kernel rows on the underside of the ear due to kernel abortion. Differential kernel formation can result in bending of the ear (banana ear). Potential causes include drought stress, crop defoliation after pollination, or herbicide damage to reproductive tissues (pollen, silks, ovules) from misapplication of herbicides beyond the labeled crop height.
Corn with blunt ear syndrome can have a nearly normal ear size and number of kernels per row at the basal end of the ear, but kernels are greatly reduced toward the middle of the ear and barren at the tip (Figure 3). Husk length and the number of kernel rows are usually normal. The reduced number of kernels suggests stress occurred prior to the completion of ear size determination. Research suggests that late post-emergence applications of herbicides (ALS, PGR, glufosinate, glyphosate), fungicides (strobilurin), insecticides, foliar fertilizers, and spray adjuvants during the V8 to V12 (pre-tassel) growth stages may be possible causes of blunt ear syndrome problems.
Symptoms of tip dieback are the result of kernel abortion at the tip of the ear which is associated with poor silk fertilization. Underfertilized ovules and aborted kernels may appear dried up and shrunken but aborted kernels may have a yellow color. The cause of tip dieback is stress during early kernel development as the result of drought, nitrogen deficiency, high temperatures, foliar diseases, or cloudy weather.
Sources: 1 Nielsen, R.L. 2016. Tassel emergence and pollen shed. Corny News Network. Purdue University Extension. https://www.agry.purdue.edu/. 2 Thomison, P. Corn pollination. Ohio State University Extension. https://agcrops.osu.edu/. 3 Hoegemeyer, T. 2011. How extended high heat disrupts corn pollination. University of Nebraska-Lincoln. https://cropwatch.unl.edu/. 4 Elmore, R. 2005. How high temperatures and stress affect corn pollination. Iowa State University. https://crops.extension.iastate.edu/. 5 Thomison, P. and Geyer, A. 2007. Abnormal corn ears. ACE-1. The Ohio State University. http://u.osu.edu/. 6 Nielsen, R.L. 2003. Ear initiation & size determination in corn. Corny News Network. Purdue University. https://www.agry.purdue.edu/. 7 Nielsen, R.L. 2007. Symptomology of arrested ear development in corn. Corny News Network. Purdue University. https://www.agry.purdue.edu/. * Photos courtesy of Dr. Peter Thomison, Ohio State University, from the publication Abnormal Corn Ears. ACE-1. Web sources verified 06/04/18. 180604114140