Anthony Hanson, UMN Extension crops IPM educator; Jeff Coulter, UMN Extension corn agronomist; Dean Malvick, UMN Extension plant pathologist
The following information was provided during the July 17, 2024 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Claire LaCanne that featured Drs. Jeff Coulter and Dean Malvick.
Corn development
Jeff Coulter discussed what to expect with corn development with the wet conditions this year. This has been a difficult year with too much water resulting in potential nitrogen issues and variability in development in areas where growth was delayed due to saturated soil. This staggered development also complicates management decisions.
Growers have been asking Coulter if applying extra nitrogen is worth it in these conditions. If additional nitrogen is to be applied, it should be done no later than V14.
Pollination status is also a common question this time of year. Weather conditions have been favorable for pollination unlike previous years, but ear tip dieback after pollination is the main concern right now as plants may abort kernels. Temperatures should moderate and reduce stress on crops in mid-July.
However, stunted corn may not produce even an ear this year due to delayed development or may not reach maturity if an ear is produced.That means the fall outlook for harvest will be a mix of mature versus wet immature corn throughout a field depending on how staggered development was.
In terms of hybrid selection, early planted corn did best this year, but these plants had cold stress to contend with. If a grower planted early, longer maturity varieties can perform well, but shorter maturity varieties are a better choice on fine textured soil apt for planting and growing delays like we experienced this year. Drought tolerance has also increased across varieties. Earlier maturity varieties may also be more beneficial if a grower is planning to manage for drought conditions as these varieties may be able to reach maturity with what little water remains in the soil earlier in the year.
Corn and soybean diseases
For the second half of the discussion, Dean Malvick gave updates on disease concerns given the weather we’ve seen so far this year.
Corn diseases
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| Figure 1. Common rust on a corn leaf. Photo: Dean Malvick. |
Common rust (Fig. 1) appears to be pretty widespread this year, and temperatures in the 70s could make it more prevalent. It rarely causes significant yield loss in commercial hybrids, but it could be confused with southern rust or possibly tar spot when scouting. There have also been reports of northern corn leaf blight reports around the region.
Malvick was also asked where tar spot is showing up this year. National website. Corn IPM Pipe. Updated maps can be viewed at the Corn ipmPIPE website: https://corn.ipmpipe.org/tarspot/
So far this year, tar spot has been found in west-central Wisconsin, but no confirmed fields yet in Minnesota. This disease can be more noticeable as it develops later in the season. The optimal timing for fungicides is R1 or R2 if the disease is present, but Malvick cautioned against excessively proactive applications as fungicides can run out of protective power if applied too early.
Soybean diseases
Phytophthora root and stem rot has been a concern in soybeans this year with saturated soils and with reported failures of resistance (i.e., Rsp3a traits). Conditions were also ideal for Rhizoctonia root and stem rot earlier in the season, but we are now past soybean growth stages where damage primarily occurs.
Bacterial blight is another cool, wet weather disease. It is typically seen on younger leaves on upper parts of the plant, while Septoria brown spot, which looks similar, is normally found in older leaves near the bottom of the plant.
White mold has had ideal conditions, especially going forward into the week of July 22 with lots of dew and humidity. If considering a fungicide, target early R2 stage soybeans since timing for protection is important.
Some diseases have not been prevalent yet, but are worth keeping an eye out for. Frog eye leaf spot has not been common because of dry weather the last two years, but there is a higher risk this year for this newer disease to the area. Sudden death syndrome has had ideal conditions this year, but symptoms likely won’t show until early August.
Outlook and other planning
Malvick was asked if fungicides pay when used for yield improvements. He said independent data from UMN and from other states in the region show that fungicides are most likely to have a returns on investment (ROI) when significant levels of leaf disease develops, and preventative use for yield benefits when a disease is not present has had very inconsistent ROI.
Other questions included how cover crops may affect diseases. They may have potential to help reduce white mold by shading soil, and the fungus may grow before beans are susceptible. A thick layer of residue can also stimulate fungal development or even inhibit it when layers are extremely thick, but more study would be needed to evaluate potential benefits on a wide scale. However, cover crops can increase seedling diseases in corn. This is a contrast to soybean systems where cover crops do not appear to increase soil-borne pathogens and disease risk in soybean in otherwise low-disease situations. There is a lot of variability in these systems, so it is also difficult to make broad statements about how diseases are affected.
Malvick was also asked if pathogen populations will be higher next year because of this wet year so far? It could increase pathogen load for some fields, but some pathogens spread via wind or are so prevalent already they only need the right conditions, so risk may not necessarily be higher next year.
Additional Resources
For identifying disease, send information and photos to the Digital Crop Doc (https://extension.umn.edu/crop-production/digital-crop-doc) or visit the UMN Plant Disease Clinic’s page for more information on sending in samples: https://pdc.umn.edu/
Corn growth and development: https://extension.umn.edu/growing-corn/growth-and-development
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
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