Crop Update Newsletter 8-3-18

Iowa State University Extension

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Extension Crop Update
This newsletter, and previous issues from recent years, can be found on-line at:
http://www.extension.iastate.edu/plymouth/info/cropupdate.htm

August 3, 2018
Volume 21, Issue #10

Prepared by
Joel DeJong,
Extension Field Agronomist
Click on photo for contact info

Corn Growth and Development – It appears to me that corn development is ahead of normal. I went to the U2U Growing degree calculator today, selected Sioux County, assumed a May 5 planting date with a 108 day hybrid just to see what date it predicts for maturity. By the way, it uses weather station data until today, then a short term forecast followed by average weather past the short-term forecast. That model predicts black layer on September 10. If so, it looks like harvest will be early this year. Go to that website, select your county, planting date, and hybrid maturity days to see the prediction for your fields.

Focus on Nitrogen Day at the NW Research Farm – August 21, 9:30 to noon. Topics include Nitrogen Application Management, including research on rate selection, split applications, time of application, N additives, manure N, and N in wet years. We will also review progress towards the Nutrient Reduction Strategy N goals, what research is being done at the NW Research Farm, and look at edge of field practices to reduce N contribution to surface waters. Extension Field Agronomist Paul Kassel, Ag Engineer Kris Kohl and I will lead the sessions. Register by e-mailing or sending me your intentions. There will be a $10 registration fee collected – but you need to register in advance (by Aug. 17) so I can plan for meals for participants at the conclusion of the event. To register, contact me via text (712-540-1085) or e-mail (jldejong@iastate.edu).

Soybean Gall Midge Larvae – Western Iowa has received several reports of field edges with soybean plants wilting, or snapping off easily. At first glance, it seems to resemble phytopthora root rot, but these plants often look like they have lesions near ground level. If you peel back the skin of the soybean plant you can often find little orange larvae, and even smaller white ones, that look to be feeding on the stem. Vegetative and reproductive plants can be infested. They only seem to feed near the soil surface, causing swollen and discolored stems.

Erin Hodgson has just successfully raised these larvae into adults, and hopes to learn exactly what specie it is. Then, we hope we can understand its life cycle, and maybe be able to develop a way to manage that pest. So far, seed treatments haven’t proven very successful, and insecticide sprays have given mixed results. We have a lot to learn. If you see this pest in your field, let me or Erin Hodgson (ewh@iastste.edu) know. See her recent article titled “New Soybean Pest in Iowa: Soybean Gall Midge” for additional discussion and pictures.

Soybean Aphids have been easier to find in the northern counties of the region, but still at low numbers from almost all reports I have heard – or observed. They remain hard to find at all in southern counties of the area. But, keep scouting. Remember, the treatment threshold is the 250/per plant on 80% of the plants, numbers we have mentioned for years – still plenty of time before they reach that 650 (economic loss level) number. Or for a scouting alternative, use the “speed scouting” method. To print off a copy of the speed scouting form, with directions, go here.

Corn Aphids – are beginning to appear in some cornfields. That might be earlier than normal, but the maturity of corn is also ahead of normal in many fields. Unlike soybeans, more than one aphid variety can infest cornfields. Aphids suck on the sap from plants, so the stress from their damage is worse under dry to drought conditions. They also excrete honeydew, which can cover leaves. That honeydew can get a sooty mold, interfering with photosynthesis. Although no true thresholds exist, it is thought that 500 aphids per plant (at least 80% of the plants in a field) might benefit from an insecticide application. ISU would like more data on response to insecticides, so if you spray, please leave an untreated strip (not at the field edge), and share the harvest data with me. I will also share it with our Extension Entomologist, Erin Hodgson. For a list of additional considerations about treating, see this recent ISU ICM article written by Dr. Hodgson titled “Check Your Corn for Corn Aphids.”

Potato leafhoppers in alfalfa – This pest seems quite aggressive this year. Potato leafhoppers have piercing-sucking mouth parts. They cause physical damage when probing to feed and inject saliva that plugs vascular tissue. Initially, alfalfa leaf tips will turn yellow, which is commonly referred to as "hopperburn". This is yellowing starting from the outside tip of the leaflet, in a sort of V shape. Heavily infested plants will be stunted, particularly new stands and regrowth after cutting. In some cases, large leafhopper populations can significantly reduce tonnage of the current crop, as well as the following crop. See this 2014 ICM News article for more details.

Check for SCN In Your Soybean Fields – Soybean Cyst Nematodes continue to be a large yield robber in Iowa. In the NW Corner of the state where I work, not all areas have severe issues with this pest – but some do! Identifying it early and managing the pest can reduce the yield loss that might occur. I would encourage you to dig some roots in every field you are scouting for insects this month to see if you can find those cysts on your soybean roots. Unfortunately, our genetic resistance is becoming less effective in some areas. For help on identifying what a cyst on a soybean root looks like, and additional discussion, click here to read the article titled “SCN Females Seen on Roots as Early as Ever in 2018.”

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