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Reproduction or maximizing potential grain fill is a corn (Zea mays) plant’s ultimate priority. The developing kernels are the primary recipients of photosynthetic energy, sometimes at the expense of stalk, leaf, and root health. Late-season stresses (insect feeding, disease, drought) that reduce photosynthesis can cause plant carbohydrates to move from stalks and leaves into the grain and result in poor stalk strength and a slow harvest.
Insects: Insect damage can occur in the form of stalk tunneling, loss of leaf tissue, and direct ear and kernel feeding. Insecticidal controls can be difficult when larvae tunnel into or are protected by plant tissues. Treatment thresholds can vary by state and locations; therefore, local information should be sought prior to spraying. Major insects of concern and their damage include:
Fall armyworm (Spodoptera frugiperda) (Figure 1)- Leaf destruction.
Figure 1. Fall Armyworm
European corn borer (Ostrinia nubilalis) (Figure 2)- Stalk and shank tunneling and early and mid-season on whorl feeding.
Figure 2. European corn borer and stalk tunneling.
Western bean cutworm (Figure 3, top) (Striacosta albicosta), Corn earworm (Helicoverpa zea) (Figure 3, middle)- Damage to ear and kernels.
Figure 3. Western bean cutworm (Top), and Corn earworm (Middle). Corn earworm picture courtesy of Mario Carrillo. Grasshopper damage (Bottom).
Grasshopper- Leaf feeding (Figure 3, bottom), large chunks of kernels removed around blister and milk stages, and damage to ear shanks and husks.
Southwestern corn borer (Diatraea grandiosella)- Lower stalk tunneling and girdling (Figure 4, Page 2, 1st).
Corn leaf aphid (Rhopalosiphum maidis)- Leaves may wilt, curl, and show yellow patches. Sugar-rich honeydew excretions can cover the leaves, tassels, and silks (Figure 4, Page 2, 2nd and 3rd).
Spider mites- Spider mite populations can increase rapidly under hot, dry conditions. Yellow spots and fine webbing are characteristic on leaves. High populations and favorable weather result in plant death (Figure 4, page 2).
Figure 4. (Left to Right): Southwestern corn borer, Corn leaf aphids, Banks grass mite life stages (Mite picture courtesy of Dr. Ed Bynum, Texas A&M AgriLife Extension).
Diseases: Corn plants that have suffered physical damage or have been stressed by drought are especially prone to problems during harvest and storage. Physical damage can provide entry points for disease-causing pathogens. The extent that corn plants are affected by disease varies based on genetics, weather conditions, and physical damage. Scouting to evaluate where diseased or stressed plants may exist in a field can help determine harvest order. Harvesting stressed fields first can help reduce the impact on harvesting and grain condition.
The pinch and push tests can be used to assess lodging risk. While walking through fields, select around 100 random plants and either 1) pinch the stalk at one of the lower nodes to check for rigidity or 2) push the plant to about a 30 degree angle, let go, and see if it returns to original position. If the plant crushes with the pinch test or fails to return with the push test, stalks are likely diseased or cannibalized and should be scheduled for an early harvest. Common stalk rots that may infect a field include Anthracnose, Diplodia, Fusarium, and Gibberella (Figures 5,and 6).
Figure 5. Common stalk rots (Left to Right): Anthracnose, Gibberella, Diplodia, and Charcoal.
Figure 6. Common ear rots (Left to Right): Diplodia, Gibberella, and Fusarium.
One condition that develops frequently is the growth of saprophytic fungi on dead plant tissue (Figure 7). Black fruiting bodies can virtually engulf leaves, husks, and stalks. During harvest, a cloud of black dust can be seen.
Figure 7. Saprophytic fungi growing on dead plant tissue.
Common ear rots that may be identified in a field include Diplodia, Gibberella, Fusarium, and Penicillium (Figures 5 and 6). Ear rot development depends greatly on favorable environmental conditions for each rot, physical damage, such as hail, and insect feeding. Disease infected grain should be dried to around 12% for long-term storage and potentially separated from other grain to help prevent pathogens and associated mycotoxins from infecting other grain. Aspergillus flavus (Figure 8) can produce a deadly aflatoxin and requires special treatment when present.
Figure 8. Aspergillus flavus (Left) can produce a deadly aflatoxin and be confused with non-toxic Penicillium (Right).
Diplodia ear rot symptoms include bleached husks, white mold over kernels beginning at the base, and rotted ears with tightly adhering husks. Small, black fungal bodies called pycnidia are often found on husks, kernels, and cob tissues. Gibberella ear rot symptoms include reddish kernel discoloration, usually beginning at the ear tip. Typical symptoms of fusarium ear rot include scattered individual kernels or groups of kernels with whitish-pink to lavender fungal growth. Penicillium ear rot infection usually begins at the ear tip and primarily occurs on ears with mechanical or insect damage. Powdery green to blue-green mold develops on and between kernels. Infected kernels may become bleached and streaked. Aspergillus ear rot symptoms include olive-green or yellowish-tan fungal growth on and between kernels.
Sources: :Boyd, M. and Bailey, W. 2001. Southwestern corn borer management in Missouri. G7111 University of Missouri. Jackson-Ziems, T. 2013. A guide to late season corn stalk and ear rot diseases. CropWatch. University of Nebraska. http://cropwatch.unl.edu. Lauer, J. 2008. Mid-and late-season yield enhancing management practices for corn. Agronomy Advice. University of Wisconsin. http://corn.agronomy.wisc.edu. Nielson, R.L. 2013. Stress during grain fill: a harbinger of stalk health problems. Corny News Network. Purdue University. www.agry.purdue.edu. O’Day, M., Becker, A., Keaster, A., Kabrick, L. and Steffey, K. 2006. Corn insect pests: a diagnostic guide. M166. University of Missouri. Thelen, M. 2012. Late-season scouting of corn and soybean fields is worth the effort. Michigan State University. http://msue.msu.edu/news. Thomison, P. and Paul, P. 2010. Monitor corn fields for stalk lodging and late season “intactness.” C.O.R.N. Newsletter. The Ohio State University. http://corn.osu.edu. Western bean cutworm. 2009. Purdue University Field Crops IPM. http://extension.entm.purdue.edu/fieldcropsipm/. Web sites verified 6/28/18. 140808131637