- Insect and disease damage to corn ears and stalks can cause losses in yield and quality.
- Environmental or chemical stress during the vegetative and early reproductive stages of corn can cause abnormal or malformed ears.
- Weed monitoring can help assess weed management effectiveness and planning.
- The end of season corn stalk nitrate test can help determine the effectiveness of nitrogen management practices.
Insect, disease, and environmental stress during grain fill can compromise corn yield potential and quality.1 Monitoring the crop through harvest can help guide decision making during grain fill and provide insight for managing the harvested crop and planning for next season. Late season stress can reduce the plant’s photosynthetic capacity and trigger remobilization of carbohydrates from stalks and leaves to feed developing kernels which may weaken the plant structure and overall health at the same time.
Stalk and Ear Diseases
Stress, particularly hot, dry weather, and hail damage during late vegetative and reproductive phases of growth can increase susceptibility to foliar diseases and stalk rots. Stalk rot is frequently favored by excellent growing conditions early in the season that encourage kernel set and development followed by stress during grain fill. Carbohydrate remobilization, unbalanced soil nutrients, compaction, and insect injury contribute to the infection of corn plants by a complex of stalk rot fungi. Common stalk rot diseases include: anthracnose, diplodia, fusarium, gibberella stalk rots, and charcoal rot (Figure 1).
Scouting to determine the infestations in a field and the lodging potential can help determine field harvest order and grain handling practices to minimize the impact from diseases.
The pinch and push tests can be used to assess lodging risk in your fields. Select around 100 representative plants while walking through fields and use one of the following tests to determine lodging risk:
- Pinch plants at one of the lower internodes above the brace roots. If the stalk collapses easily, it has likely been compromised by stalk rot, making it more prone to lodging.
- Push plants to around a 30 degree angle. If they fail to flex back, stalks may be compromised by stalk rot and at risk for lodging.
Different environmental conditions favor the development of different ear rots. For example, Aspergillus ear rot is favored by hot, dry conditions while gibberella ear rot is favored by cool, wet weather during silking. The fungi that cause corn ear rots are often favored by late-season humidity and rain following pollination. Delayed planting or conditions that slow grain drying in the field and delay harvest can lead to an increased incidence of ear rot diseases. Fields with stalk rots may also be at a greater risk for developing ear rots. Common corn ear rots include aspergillus, fusarium, gibberella, diplodia, and penicillium ear rot. Ask your local representative or agronomist for more information about ear molds, including mycotoxins and aflatoxins.
Aspergillus ear rot symptoms include olive green or yellowishtan fungal growth on and between kernels. 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. Typical symptoms of Fusarium ear rot include scattered individual kernels or groups of kernels with whitish-pink to lavender fungal growth. Gibberella ear rot symptoms include reddish kernel discoloration, usually beginning at the ear tip. 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.
Stalk and Ear Feeding Insects
Stalk and ear shank damage can be caused by Southwestern or European corn borers, in addition to ear feeding damage. Southwestern corn borers lay eggs in the ear zone and young larvae feed on ear shoots, kernels, and the cob, before tunneling down the stalk to the base of the plant.2 European corn borers larvae feed on pollen and silks then enter the ear by tunneling through the shank and cob. Injury can be found at both ends and along all sides of the ear.
Ear feeding insect larvae look similar but have characteristic feeding patterns (Figure 2).3 Corn earworm larvae enter the ear primarily through the silk channel, unlike European corn borer and fall armyworm, which enter through the husks or cob. Larvae feed at the tip and along the sides of the ear near the tip. Fall armyworm, unlike corn earworm, burrow through the husk or base of the ear and feed along the sides of the ear. Western bean cutworm larvae are not cannibalistic like corn earworm and may have multiple larvae feeding in an ear. The larvae enter ears through silk channels or the husk, feeding on kernels at the tip, base and sides of the ear. Insect feeding damage in ears can increase susceptibility to ear molds.
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 the stress corn undergoes. Pesticide applications can be a source of stress on corn if application occurs outside of product label conditions or other stresses increase corn susceptibility.
Late-season weed monitoring should document weed escapes, new weeds, and the distribution of species and infestations to help plan pre- or post-harvest herbicide applications and adjust the weed management plan for the next crop.
- Identifying weed species and the extent and distribution of weed infestations in a field can help determine the right herbicides and application timing to keep tough-to-control weeds in check next season.
- Weeds with late season germination and survival and/or prolific seed production, may require multiple herbicide modes of action and sequential applications to cover the emergence characteristics and competitiveness of the weed community in a field.
- Weeds present at harvest may indicate that a postemergence (POST) herbicide application that includes a residual may need to be part of the weed management plan.
- The POST herbicide program may need a different mix of herbicide modes of action to cope with the weed spectrum.
- A pre-emergence (PRE) herbicide program alone may not provide the diversity and longevity of herbicide action necessary to manage the weed situation without a sequential POST application.
The end of season corn stalk nitrate test can help determine the effectiveness of nitrogen management practices by sampling the lower portion of corn plants.4 Sample an 8-inch segment of the stalk from about 6 to 14 inches above the soil 1-3 weeks after physiological maturity. Contact your agronomist or University Extension for fact sheets outlining procedure for sampling and interpretation of data.
Late season crop monitoring can help assess management practices and begin the planning process for the next crop.5
1Nielsen, R.L. 2013. Stress during grain fill: A harbinger of stalk health problems. Corny News Network. Purdue University. www.agry.purdue.edu (verified 8/11/14).
2Sloderbeck, P.E., et al. 1996. Southwestern corn borer. Kansas State University.
3Cook, K. 2005. Corn earworm, European corn borer, armyworm, or western bean cutworm: Which one is causing the injury I’m seeing on my corn ears? University of Illinois.
4Laboski, C.A.M. 2010. Considerations when using the end-of-season corn stalk nitrate test. Wisconsin Crop Manager 17(26):113-114.
5Thelen, M. 2012. Late-season scouting of corn and soybean fields is worth the effort. Michigan State University Extension.