7/9/21
Greetings,
Crop weather
Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.
At least temporarily, the weather has shifted toward more moderate temperatures and some precipitation. While many areas received rain over the past week, some received much more or less than others. Soil moisture is still critical in some areas. The SWROC received 0.50 inches after July 1 that are not accounted for in Figure 1. During the beginning of July in 1988 and 2012, soil moistures were about an inch less than 2021.
For May 1 to July 5, we are well behind average for growing season precipitation 3.23 inches vs 8.17 long-term average. We are slightly ahead of average for growing degree days for the same period 1090 vs. 996.
Corn
Some corn has started to tassel, and many fields will tassel over the next week.
This continues to shape up to be a bad corn rootworm year. Early emerging northern and western beetles are now present. The western corn rootworm beetle populations are still predominately males, and some larvae are still present. Yet to occur is a large percentage of rootworm beetle emergence, and some root damage.
There will likely be more reports of unexpected damage to Bt-Rootworm hybrids than usual this year. While many of you are out visiting corn fields, it is important that the appropriate seed company is informed of damage to any of their hybrids with Bt-RW traits.
There seem to be more observations of damage to rotated and short-term corn this year. This is also a cause for concern if larger extended diapause northern corn rootworm populations are back. Extended diapause will further complicate the already messy rootworm management problem. Ken Ostlie and I would greatly appreciate hearing of any fields where predominately northern corn rootworm populations are damaging corn with Bt-RW traits.
Because of extended diapause, it might be worth including some rotated corn fields if you are scouting or using sticky traps to monitor rootworm populations. With the help of MN Corn Research and Promotion Council funding, we have distributed quite a few yellow sticky traps this year. For those who are trapping RW beetles for your company, or on your own farm, we would like to include your trap data if you are willing to share. Contact me for details.
It looks like we have some good corn rootworm pressure in studies at the SWROC. I am in the process of trying to set up a field day or two for corn rootworm management in August.
It is time to make treatment decisions for univoltine European corn borer and it will soon be time to begin scouting for 2nd generation egg masses should be completed see: degree days for insect pests.
For scouting and economic threshold information including an ET calculator see https://extension.umn.edu/corn-pest-management/european-corn-borer-minnesota-field-corn.
I am starting to see the first cereal aphid adults and small nymphs (Figure 2) in corn at the SWROC. I would not be anxious to treat these populations. Historically, large aphid populations in corn have been indicators of stalk rot issues later.
Tiny, black, red-headed flea beetles are abundant in corn and soybeans. Despite their distinctive feeding to corn and soybeans, the damage is seldom, if ever, economic in these crops. Include defoliation from flea beetles with that from other insects in the generic 20% whole plant defoliation threshold for reproductive stage soybeans. In corn, heavy defoliation of the ear leaf and above is of concern but flea beetles most often feed on corn’s lower leaves. Common cocklebur (Figure 3) seem to be a flea beetle favorite.
Soybean
The earliest emerging soybeans in SW Minnesota are now V8 and above and R3 stage. Foliar diseases are currently at very low levels in drier areas. Except for SCN infestations and IDC, root health is good.
On the other hand, things are different in areas where heavy rainfall occurred, particularly if water stood for any amount of time. I visited some fields in Waseca County where this was the case. In addition to obvious drowned low spots areas, waterlogged soils showed up as as pale soybean foliage and areas of dead and injured plants. In the latter areas, many plants had started to recover, at least before the most recent rain.
Patterns in the field related to tile lines, wheel tracks, edges of depressional areas, and tillage; all of which would affect soil moisture.
Above-ground symptoms were wilting and yellowing and dead leaves. In one field, leaves had dropped leaving the central petiole (Figure 3).
Below ground, most roots showed a clear demarcation between dead and clean, white root tissue. This boundary was usually at a consistent depth within individual affected areas and likely related to the soil depth where saturated soils and/or low oxygen occurred. Where the outer layer of the roots had sloughed from dead root tissue, red to purple fungal mycelia were visible - indicating a Fusarium species fungus was likely present. Above the dead tissue, new adventitious roots were developing.
In wet years, or when excessive rainfall is widespread, plant symptoms like these are often just attributed to drowning and are not that unusual in flooded soils.
In the past, when I have submitted plants with similar symptoms, Fusarium spp. were usually isolated. In this case, Fusarium sp. and possibly other fungi are involved. It is only an academic question if the fungi invaded before or after root tissue death. I’ll leave it to the plant pathologists to make that call.
Herbicide injury symptoms resembling dicamba injury are widespread. Remember, the affected plants are likely to yield just fine. To follow up on my comments from last week, some dicamba susceptible soybean fields may be seeing symptoms caused by other than dicamba. In some cases, the reason for cupped/puckered soybeans might be more complicated than dicamba drift. In some fields where only parts of the field had received an Enlist, Liberty or other herbicide application, the soybeans that were not sprayed did not show symptoms. Some have reported some Enlist and GT27 varieties display more pronounced symptoms than others.
More detective work is still needed to sort this all out. In the meantime, be good to your neighbors, they may have had nothing to do with your injured beans.
Soybean gall midge
We have seen no further emergence from 2020 soybean fields. On July 9, we captured the first adult male from this year’s soybean. At the SW MN research sites, the percentage dead and wilting plants is currently lower in 2021 that similar dates in 2019 and 2020. Please contact me if you find soybean gall midge in your Minnesota soybean fields.
Soybean aphid watch
Soybean aphids have been found in many areas of Minnesota, but populations remain low. Some small pockets of aphids, often tended by ants, can be found in many fields but the percent plants with aphids remains low (a high of 2% plants with aphids at the SWROC). The larger soybean canopies will help protect aphids from high temperatures, wind, and rain.
Economic aphid issues, if they occur, are likely to be late in the season. We are in a period of weather suitable for soybean aphid reproduction so things may change quickly. As always, I like to hear your observations on soybean aphid populations.
Alfalfa
Watch for building pea aphid populations. Mark Hoekel reported populations starting to build in some WC MN fields. Most, if not all, potato leafhopper populations are low.
Small grains
Pest management is over for 2021. Avoid/minimize weed seed set after fields are harvested.
Grasshoppers
Watch CRP acres for grasshopper populations. Most issues are NW, WC and C MN but I would not ignore hoppers in other dry areas as well.
Meetings and other topics of interest
- July 15 Corn-soybean weed management field day. Rosemount Research and Outreach Center
- July 20-21 UMN Field school. St. Paul Campus: z.umn.edu/fieldschool
- July 28 Advancing Ag Series: Edge of field practices for achieving water quality goals
- August 17 Cover crops field day. SWROC
- TBD Pest management field day. SWROC
Happy trails,