Volume 4, Issue 6
June 23, 2021
This newsletter is an update on current topics and events in agriculture in Boone, Dallas, Jasper, Madison, Marshall, Polk, Story, Tama, and Warren counties.

Meaghan Anderson
Extension Field Agronomist

ISU Extension & Outreach
1421 S. Bell Ave. Ste. #107
Ames, Iowa 50010
Office:  515.337.1601
Cell:     319.331.0058

Upcoming Events

Pesticide applicator testing information available at this link. Click links for online pesticide applicator testing, IDALS test sites, and college test sites.

1st and 3rd Wednesday of each month at noon - Conversations About Carbon - online webinar

June 24 - Iowa Swine Day - Ames

June 29 -
Herbicide Resistance Mgmt. Field Day - Kanawha

June 29 - Story County Pasture Walk - Ames

June 30 - 

July 8 -
ISU Weed Science Field Day - Ames

July and August - Farmland Leasing Meetings - Various locations, contact your county extension office to register!

Handy Links

Central Iowa Crop Update
Updates from an agronomist and weed nerd in central Iowa.

In this issue:

  • Assessing Hail Damaged Crops
  • Fungicide Use on Hail Damaged Crops
  • Insects incoming - what should we be scouting for?
  • Herbicide Field Days coming up!

Assessing Hail Damaged Crops

Hail has swept through several parts of central Iowa with the recent storms. While the rain was much-needed, we did not need or want the damage that comes with hail. Colleague Rebecca Vittetoe posted a useful blog a couple days ago about assessing hail injury and evaluating whether to use a fungicide on it you can read here. I've got some notes on assessing hail damage below. Please call if you need another set of eyes or have concerns about crop recovery or replant questions.

Step one in assessing hail damage is to always call your crop insurance agent first. Step two is making some assessments of the crop yourself. For both corn and soybean, it will take several days for regrowth and plant death to be obvious, so it may be best to look at fields later this week or early next week for a more effective evaluation.

Assessing corn damage

The growing point of corn reaches ground level around V6 and most of the corn damaged was between V8 and V10 (using leaf collar staging method). Corn cut off at or slightly above the ground level is no longer a viable plant if corn plants were this large. Corn cut off above the growing point should produce new leaves and try to outgrow old tissue.

Corn with an intact and healthy growing point will survive, but there are considerations for the rest of the season.
  • Evaluate corn yield potential from original stand and planting date - this is not what you seeded in the field, it is what the stand was
  • Take several stand counts across the damaged field to evaluate stand loss and use resources listed below to determine potential loss in yield potential
    • Some plants may be in a "questionable" category for survival and production of a harvestable ear - this might include plants caught up in old tissue or deeply bruised plants. I think we'll be surprised by how many will make it out of this bind, but marking several plants to monitor would be a good thought for future reference.
    • Plants with exposed stalks that are damaged (NOT shallow leaf sheath bruising) may suffer from stalk rot or standability issues later in the season and should be monitored closely to determine whether early harvest is necessary
    • Plants in north-south rows may have different levels of damage than those in east-west rows and might need evaluation separately
  • Evaluate defoliation of surviving plants - this means tissue dead or lost from plant; if it is still green, it's still photosynthesizing!
    • NOTE: when using Figure 6 in "Hail on Corn in Iowa", add approximately 2 leaves to your "leaf collar estimate" of stage (i.e. if your corn had 7 leaf collars (V7), use V9 for your potential yield loss estimates in Figure 6)
      • This is because those figures use crop insurance staging which counts more leaves on a plant than the collar method.
Additional resources on assessing hail damage in corn:

Assessing Hail Damaged Crops Continued...

Assessing Soybean Damage
The growing point of soybeans is above ground as soon as the plant emerges. At each location on the stem where a leaf attaches, an axillary bud exists that can grow if the top bud is broken off the plant.  This means soybeans have an incredible ability to recover from lost tissue, as long as an axillary bud still exists on the plant. If soybean plants are cut-off below their cotyledons (close to ground level), they will not survive.

If soybeans are completely defoliated, stands may be slower to recover and this makes stand evaluation more difficult without giving them some more time to resume growth of
axillary buds; 7-10 days should be enough to see if regrowth is occurring or plants are dead.
  • Evaluate potential maximum yield from original final stand and planting date. Again, this isn't the 150,000 seeds planted per acre, it is the final stand in the field.
  • Soybean yield loss can come from stand loss, defoliation on plants, and loss of nodes or breaking of stems on individual plants
    • Vegetative stage soybeans are considered to have 0% loss in yield potential from even total defoliation; many soybeans were still in vegetative stages or were just this week reaching R1
    • We are most concerned about stands thinned to very low numbers and nodes lost from plants
    • Like corn, damage to stems is difficult to quantify, but may result in lodging later this season
  • To evaluate stand: count plants in a known length of row in several areas (at least 5) across the field or hail damaged area. You could do a small area like 3 feet of row or evaluate the full length of 1/1000th of an acre. More counts = more accurate assessment. Separate plants into "will survive", "won't survive", and "questionable" categories. These categories can help in estimating a % stand loss from the original final stand in the field.
    • Plants should begin producing new leaves at axillary buds that will be visible within the next week.
    • Soybean stands generally need to be below 50,000-60,000 evenly-spaced plants at this late in the season to receive a yield potential increase from replant. Soybeans have an incredible ability to make up for stand loss, as long as some plants are still there!
      • If many of the surviving plants fall into the "questionable" category, replant or filling in may pay from a yield perspective
    • Again, resources below are helpful for estimating potential yield loss from stand loss and node loss on soybean plants, but cannot easily assess potential lodging issues from stem damage
  • The value of canopy to help suppress late season weed pressure is also not easily quantified. Soybean fields should be monitored for weed emergence that requires control.
  • If replanting, don't bother tearing up the current stand, but do try to plant across rows (at an angle or perpendicular to rows) and try to increase the stand closer to 140,000 plants per acre (i.e. don't plant a full rate).
    • Find a shorter maturity variety well-adapted for the area (0.5 to 1.0 shorter).
Additional resources on assessing hail damage in soybeans:  Assessing forage and small grain damage
  • Small grains (oats, wheat, rye, etc.): For those with small grains, Joe Lauer at the University of Wisconsin put together a good resource for evaluating hail damage, which can be found here.
  • Forages: For those with hail damage to forage crops like alfalfa or red clover, Dan Undersander and Dan Wiersma at the University of Wisconsin explain how to evaluate the damage in this resource here.

Fungicide Use on Hail Damaged Crops

One of the first questions that typically comes up after a storm is whether treating plants with a fungicide will help them recover. Fungicide application decisions should be made in basically the same manner as if no leaf tissue had been damaged. A misconception is that hail-damaged crops are more susceptible to disease and would thus benefit from a fungicide more than an undamaged crop. It's also important to remember that the cost of fungicide does not change, but the yield potential of the crop is often reduced following a hail event.

With the exception of smut on corn, fungal diseases like frogeye leaf spot, grey leaf spot, northern corn leaf blight, and tar spot do not require wounds to enter the plant; the diseases enter the plant in other ways. None of our foliar fungicides claim efficacy on smut. Bacterial diseases do enter plants via wounds, but fungicides would offer no protection from them.

If you're interested in reading up more on this topic, I suggest referencing the two ICM Blogs "Would a fungicide benefit hail-damaged crops?" (2018) and "Fungicide Use on Hail Damaged Corn and Soybean" (2020).

Insects incoming - what should we be scouting for?

Insect issues haven't been too problematic to this point in the season, but they're showing up now. Keep an eye out for the ones I've highlighted below and let me know if you're seeing other issues!

Corn rootworm - Larvae have hatched in central Iowa and can be found feeding on corn roots in fields. Scout fields, especially long-term continuous corn or fields with known Bt trait performance issues, for larvae and feeding on the roots. Now is a great time to look as feeding is fresh and easily identifiable. Erin Hodgson wrote a nice blog last summer on how to scout for corn rootworm larvae using either black plastic or a 5 gallon bucket.

Corn rootworm larvae found in Boone County on June 22.

Soybean aphid - It seems a bit early for soybean aphid, but the first ones were spotted in central Iowa soybeans late last week. Check out Ashley Dean's blog on the find for more information and read this article for information on speed scouting for aphids. Here's a speed scouting resource sheet for you as well.

Soybean gall midge - this is a relatively new pest to central Iowa, only known to be in Boone, Dallas, and Madison Counties, along with counties to the west. We have no known effective management option in soybean once they are discovered but trust many researchers are working to find effective management options for us! Let me know if you see any suspicious death of soybeans near field edges or find reddish larvae feeding on soybean stems. See this article for more information and images of what to look for.

Japanese beetle - The first Japanese beetles have been spotted out in central Iowa, so we'll soon see them in corn and soybean fields. This pest very rarely would reach infestation levels to warrant treatment in soybean or in corn (aside from maybe field edges in corn). Read more in this article about thresholds for treatment and identification.

Potato leafhoppers (alfalfa, soybean) - I spotted my first "hopperburn" (image below) in a central Iowa alfalfa field, which means leafhoppers are probably present and need to be scouted for in other alfalfa fields. First should be monitored weekly following the first cutting of alfalfa until the end of the growing season. This pest was abundant in alfalfa last summer due to the dry conditions and I wonder if we're in for another similar case this year. This 2019 ICM News article from Erin Hodgson and Ashley Dean contains information on scouting and treatment thresholds for potato leafhoppers.

Characteristic "hopperburn"  V-shaped yellowing on alfalfa leaflets spotted in Boone County on June 22.

Herbicide Field Days coming up!

My colleague Angie Rieck-Hinz and I are hosting two herbicide resistance management field days coming up in June. Note that the McCallsburg location will be corn plots and the Kanawha location is soybean plots.
June 29 - Join us for a Herbicide Resistance Management Field Day at the south farm of the Northern Research Farm from 1-3 p.m.! This field day is the sister event to the June 15 field day looking at herbicide resistance management in corn.  We will tour soybean herbicide plots to talk about using effective modes of actions, effective rates and appropriate timing strategies. We will also look at impacts of cover crops on weed pressure.

July 8 - Hold this date for the ISU Weed Science Program Field Day! This will be a public field day with plot tours focused on corn and soybean herbicide programs, herbicide resistance screening updates, harvest weed seed control, cereal rye termination timing, and other research updates from the ISU Weed Science Program. Please call or email me at 319-331-0058 or to register for the free lunch!

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