Volume 25 Issue 12 | July 15, 2022
This newsletter and the advice herein are free. You usually get what you pay for.
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 http://swroc.cfans.umn.edu/weather.
Cicadas have started singing here the past couple of days and the cicada killer wasps are active too.
Corn has started to tassel but there will be a wide range in silking dates. The forecast heat and dry weather may delay pollen shed in some of the early tasseled fields.
While most fields look good, wet soils have been hard on some fields where IDC, SCN, and root rots are causing yellowing. Plant development is behind in most fields this year with most soybeans in the R2 stage.
Alfalfa
Potato leafhoppers will be the primary concern from now into August. But populations here have been unusually low.
At the SWROC, populations of red-legged grasshopper nymphs are by far the most abundant. We do not have economic infestations here yet, but borders of alfalfa have the highest numbers. Nymphs of our latest hatching species, the differential grasshopper, are also present.
Alfalfa weevil
I have been receiving questions from West Central and Central Minnesota about alfalfa weevil larvae still present well into July. In some fields, there appears to be a relationship with topography or soil type. This is not the first year that this has occurred, and it presents management issues.
Curiosity aroused, I checked some of the alfalfa at the SWROC. As of July 13, we still had 1st instar alfalfa weevil larvae in SWROC alfalfa. 1st to 3rd instar larvae and some adults were found. I could not find any cocoons and pupae. Here too, populations seemed to have declined after 1st cutting but have recently increased with numerous young larvae present.
There might be several reasons for weevils persisting late in the season: The presence of more than one alfalfa weevil biotype, the proportion of eggs laid last fall vs. this spring, changes in weevil overwintering behavior, biocontrol changes, developmental changes in a part of the population developing pesticide or other management resistance (not confirmed in MN), or simply temperature-dependent development differences among microclimates and individuals. I hope we can get some research started to find the answers.
Small grains
It’s the home stretch. It looks like some of the upcoming forecast night temperatures have been lowered a bit which might help.
We have wheat stem maggot, but we are also finding numerous white heads caused by other, yet-to-be-determined, causes.
Although the SWROC has not seen frequent rains some diseases have progressed. Varieties differ but bacterial leaf streak in wheat has increased. Crown rust has also recently increased in some late maturing oat varieties with several reports from areas with heavier rainfalls.
Corn
Corn rootworm
Root feeding larvae are still present, and the 1st adult male rootworms are beginning to emerge now.
It may be less than 2021, but we are seeing significant root injury in research plots at the SWROC… so enough western corn rootworm eggs survived the winter to cause problems. We have not had the severe winds as some areas to the south and lodging is still minimal. As hot dry weather continues, corn plants with severely injured roots will struggle.
It is still on the early side to know the extent of 2022 root injury but not too early to note fields with problems for future management.
Watch early sweetcorn and early-silking field corn as they can attract large numbers of beetles, particularly northern corn rootworms. Northern corn rootworm populations are the wildcard in 2022. Will we see more Bt RW fields with unexpected damage from northern corn rootworm – extended diapause or otherwise?
Soybeans
Expect to see some foliar diseases such as bacterial blight, Septoria, and frogeye leaf spot show up in areas with heavier rainfall. After last week’s rains, yellow flash showed up. There are several possible biotic and abiotic causes that need to be sorted out. Make sure you know what disease you are looking at and weather forecasts before making management decisions.
Do insurance fungicide applications pay? Gives a brief summary of several years of corn and soybean foliar fungicide studies in southern MN.
Wet soil, heat, and soybeans
Some parts of SW MN received heavy rain last week. On the 11th, I received the first call about soybeans wilting and dying. While the affected area is more widespread, I looked at a few of these fields in Nobles and Jackson counties on Wednesday. Bleached leaf symptoms were visible from the road and for the most part, the pattern followed draws and low areas within the field.
Death of leaves and upper nodes appeared to have occurred rapidly. Affected leaves and petioles remained attached to the stem. Leaves were green, did not display chlorosis, but did show gray areas, possibly water soaked at one time. The lower taproot and lateral roots were dead but not greatly discolored. The outside of the damaged roots sloughed easily, leaving the stele. Many plants were not completely killed and were putting out new roots near the soil surface. Some have reported varietal differences.
While the general appearance of the plants resembled root disease, these symptoms did not completely match those of any of our more common root rot diseases and might not be caused by a pathogen.
The symptoms may be the result of high temperatures and saturated soils. Did high respiration rates and anaerobic soil conditions cause rapid root death and moisture stress of plant saturated soils? Are the symptoms caused by a pathogen? Samples are en route to the Plant Disease Clinic to help answer these questions.
In other fields, the signs of Fusarium spp. fungi associated with wet soils and/or SCN are present in some fields.
Soybean aphid watch 2022
Low populations of aphids are being reported from more areas of Minnesota, with parts of Central and Southeast MN perhaps, having higher populations. Remember, aphids can colonize small, sheltered, fields more rapidly. Over the next few weeks, aphids will take advantage of the shelter provided by growing canopies and sap quality as soybeans progress from the R2 through R4 stages.
Large soybean aphid populations on scattered plants or in pockets within the field often collapse as winged females are produced and leave. Spraying too early can reduce biological control and open the field up to infestation by winged aphids moving short or long distances between fields. Scattered plants with single or few aphids indicate these were recently deposited by winged adults.
In one area of the SWROC planned for insecticide efficacy studies, 53% of the plants are infested with aphids but plants still average less than 10/plant. Populations can build rapidly once 50% or more of the plants are infested.
Will 2022 finally be another soybean aphid year? Will the forecast warm, dry weather change things? The size and number of areas with large infestations producing alates will be the ultimate driver. Stay tuned.
Soybean gall midge
Some wilting plants and brittle stems are present along the edges of the Rock County fields where the Koch lab and I are conducting research. At this point, the infestations appear as low or lower than 2021. The situation may be different in other fields. If history repeats, we will see soon see larvae from 1st generation adults in soybeans. Late planting may have limited infestation success in some areas.
Thank you to all who keep me apprised of pest status around the state.
Happy trails,
Bruce