Volume 24 Issue 4 | May 6, 2021
This newsletter and the advice herein are free. You usually get what you pay for.
Temperature and rainfall continue to be concerns. The cool temperatures have slowed crop and pest development.
From April 26 to 28, black cutworm moth arrived in large enough numbers to indicate there is some risk for damage to susceptible crops in parts of southern Minnesota. See the latest Crop News blog post or Black Cutworm Reporting Network for more info.
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.
- Most corn is planted, and soybean planting is wrapping up in the southwest and most of southern MN. Planting was delayed in some areas that received a bit more rain.
- Although quite a bit of the crop went in early, cool weather has slowed emergence.
- 45 degree-days (50ºF base) were accumulated April 10th to May 5th, 35 DD at the 2” soil depth.
- A welcome May 5th rain fell in southern Minnesota where the SWROC picked up 0.28”. Parts of central and southeast Minnesota received the highest amounts. Some, but not all, of these rainfall amounts were enough to lessen pre-emergent herbicide concerns. Unfortunately, rainfall in some areas was very light and may not have been enough for moistures to meet, and some of west central, southwest, and south central Minnesota missed out on this round (Figure 1). To the north, spring rains delayed the start of planting somewhat, but it has been a very dry planting season for some (Figure 2).
- Wild buckwheat is emerging and a few green foxtail have emerged.
Things to watch
Scout early season stand problems in corn and soybeans
Evaluating crop stands, or the lack thereof, is an important task early in the growing season. Stand counts can be effectively combined with other early season scouting tasks. Only large-scale disasters can be detected at 60 mph. Determining the cause of missing corn plants often requires detective work.
The following hints may provide some help in detecting and determining the cause of stand losses.
Scout soon after emergence to determine if an acceptable stand is present and to detect potential problems, such as cutworms, in time to act. Once corn and soybeans can be rowed, scouting should begin in earnest. When an unacceptably low population occurs, the earlier a replant decision can be reached the better. If all goes well, you won’t need these but in the case you need to determine if replanting is economically justified, refer to Corn grower’s guide for evaluating crop damage and replant options and Soybean damage and replanting for information.
Check the whole field
Check corn and soybean plant populations throughout the field. Problems tend to develop in portions of the field that are farthest away from the field approach or road. Before entering the field, look over the entire field for areas that appear different. Check them when scouting. Scout the field with an adequate scouting method (X or M patterns work well). Make a close examination of plant population in several areas throughout the field.
Look for patterns
Is the problem throughout the field? Is there a pattern to the missing corn? Are individual rows missing or emerging erratically? In areas without corn, determine if seed was planted. Despite modern electronics, areas of a field can mysteriously be planted without seed. Sometimes planter settings are made correctly or not adjusted for field conditions.
Check planting depth
While planting too deep causes emergence problems with soybeans more often than corn, both soybeans and corn can struggle if planted too shallow, particularly when dry. Uniform emergence is as important, or more important, than plant spacing. Counting rows can determine which planter row(s) was malfunctioning.
Poor stand or seedling vigor can sometimes also be traced to compaction and other field preparation problems. Patterns relating to wheels or tillage equipment might be observed. Misapplication of herbicide, herbicide carryover, and excessive N or K fertilizer in contact with the seed provide identifiable symptoms.
Stand losses caused by insects and plant disease do not follow straight lines and the damage is usually not throughout the entire field. Localized or spotty areas of the field being affected provide a clue that a stand problem is disease or insect related. Examine entire plants as well as the appearance and location of problem areas in the field before drawing conclusions. Most corn seed and some soybean seed are treated with a combination of several fungicides and insecticides. These do not, however, make your germinating corn seed and seedlings invincible.
Emergence and early season stand issues from many of the causes above can often be related to soil type and soil moisture.
Weather and field history
Several fungi, including Pythium spp. and Fusarium spp., can cause pre-emergence and post-emergence “damping-off”. A rotted, discolored, or water-soaked appearance of the seed or portions of the seedling can indicate the presence of a pathogen. Seedling diseases are often associated with cold, wet soils such as poorly drained low areas. Seed and seedling disease can also be secondary to damage caused by other factors.
Damage from several insects tends to be worse when cool, wet soil conditions slow corn germination and emergence. Field history can provide clues to the species causing stand reduction. For example, if a poor stand is associated with heavy manure application or other high organic matter conditions, suspect seed corn maggot. A recent history of sod implicates wireworms, white grubs, and some cutworms.
Several species of cutworm attack Minnesota corn. Unlike the insect and disease problems previously mentioned, cutworm damage can be controlled effectively with insecticide rescue treatments. Early detection is important. Look for leaf feeding. Most species will feed on leaves before they are large enough to cut plants. Knowing which species is present, the stage of cutworm and corn development, and amount of damage will help determine if an insecticide treatment is necessary.
Modern seed corn often has better early season vigor than its predecessors do. Infrequently, however, some seed lots have poor germination (especially cold germination) and emergence. Poor seed should be suspected only if planting, insect, and disease problems have been eliminated.
Consider the nature of the problem if the remaining stand dictates replanting. For example, a seed treatment may be appropriate when planting back into an insect problem. Keep a record of where the loss occurred as problems can repeat themselves in future years.
There are many additional factors that can cause poor corn stands and a combination of factors often occurs. Fortunately, things usually go well.
- Very early planted corn has started to emerge
- Dry topsoil (subsoil in some cases) is a concern in parts of Minnesota - a reversal from previous planting seasons.
- Although we had picked up enough black cutworm moths to present some risk to crops, the migration of armyworm into Minnesota has been low.
- Soybean planting is wrapping up in SW Minnesota.
- Some soybean emergence has been observed.
- Chlorpyrifos is back in the news. The Ninth U.S. Circuit Court of Appeals has given the EPA 60 days from April 29 to decide whether to write a new rule for use of the insecticide or ban crop tolerances. This has a potential impact on insect management in several Minnesota crops. Although losing chlorpyrifos as a management tool reduces options and affects control costs, we have options to manage soybean aphid and other insect pests.
Soybean aphid watch
We will need to wait for some soybean growth to get a feel for how soybean aphids managed last winter’s brief cold snap and this spring’s sub-freezing temperatures on buckthorn.
Other soybean pests
Seed corn maggots are forecast to be active for the next week (https://www.vegedge.umn.edu/scmdd). Egg-laying seed corn maggot flies are attracted to fields where livestock or green manures were recently incorporated. Consider an insecticide seed treatment if planting into these conditions.
- Spring growth at the SWROC approximately 12 inches.
- Cold temperatures and wind have limited the effectiveness of sweep net sampling most days.
- Pea aphids and tarnished plant bugs are present in low populations at the SWROC.
- I have not found alfalfa weevil adults but that should happen soon in the southern part of the state. If they are already present, SWROC populations are still very low.
- Potato leafhopper migrants have not yet been observed.
- Early planted oats and wheat at the SWROC are now one – two leaf stage.
- Cereal rye and to a lesser extent winter wheat are in the stem elongation stage.
- Armyworm captures in light traps and pheromone traps have been low so far this spring.
- Check winter rye and winter wheat for cereal aphids. Although insect migrations into southern Minnesota have been relatively light so far this spring, English grain aphids were first found in sweeps of a cereal rye cover crop study at the SWROC on April 21. At least some of these spring arrivals from the south have avoided freezing their cornicles off during the sub-freezing temperatures of April 19-21, 25 and May 5. Based on the presence of nymphs, these immigrant populations are now reproducing. Bird cherry-oat aphids, another common aphid in Minnesota cereals, have not been found. Current SWROC populations are not economically threatening. The treatment threshold for seedling to heading stage small grain crops is 85% of the stems with more than one aphid. Based on weather patterns, southern Minnesota is most likely to have received migrating aphids to this point.
Some points of interest
- Events: Advancing Ag Series webinar - May 21
- Other news: MN Crop News: Evaluating small grain stands
As always, I appreciate hearing and seeing your observations!
Good Scouting and...