Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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May 20, 2010



Seedling Diseases


The recent cool, wet weather may result in increased seedling diseases in both crops this spring. When checking plant stands, it’s important to do some digging and check the health of the below ground portions of the plant.


In corn, seedling rots often affect the mesocotyl (part between the seed and nodal roots). Check to see if it is firm and white or becoming brown and mushy. Fungicide seed treatments help to protect the developing corn seedlings, but with the extended period of cool weather, seedling diseases can still develop. See the Alison Robertson and Gary Munkvold article for some additional discussion and pictures of seedling rot at


For soybeans, emerged or not, evaluate the hypocotyls and roots.  The hypocotyls should be firm. Emerging soybean fields should still be watched for a short while after emergence just to be sure that damping-off is still not an issue.  If germinated seedlings were infected during the cold spell, it’s possible to have a “post-emergence” damping-off as discussed in the following publication:



With the recent rains and related flooding, especially south of Highway 92, many are asking the question, "How long can crops be under water and survive?"  Corn and soybeans can normally only survive complete submersion for 4 days (77 degree F. day air temperature) and most forages can survive for 1 - 2 days.  I have seen survival for considerably longer periods of submersion, however.  The cooler the air temperature, the longer the plants can survive.  Plants NOT totally submerged will survive considerably longer.  By the time the water has receded and the field dries out, it will be easy to see whether the crop has survived or not.

A light rain shortly after the water recedes / drains might be beneficial to wash off the mud on plants.

Flooding can lead to greater disease problems on all crops.

A more detailed discussion of the effects of flooding on corn can be found at, and a more detailed discussion of the effects of flooding on soybean can be found at





Black cutworms should begin cutting soon if eggs were laid during the early May black cutworm moth flight.  Based on temperatures since then and using National Weather Service forecasts through May 25 and normal temperatures thereafter, cutting in eastern Iowa should begin along Highway 34 on Thursday, May 27, along Interstate 80 on Friday, May 28, along Highway 30 on Sunday, May 30, and along Highway 20 on Monday, May 31.  It is always good to start scouting a few days before the predicted onset of cutting.  Note that there was an earlier significant moth flight in Benton County as well as scattered low-level catches elsewhere which may be resulting in some cutting now, so it is probably wise to be looking now.  I have seen and heard of low levels of leaf feeding but am not aware of any significant cutting at this time.


Of course, any changes in predicted temperatures may alter the onset of any cutting from the early May moth flight.  You can monitor black cutworm growing degree days for east-central and southeast Iowa at   A more comprehensive overview of the 2010 black cutworm situation is posted at  Please note that the dates in the overview are earlier than listed here because the cool temperatures since the article was written have slowed black cutworm development.


Most commonly, stalk borers hatch in ditches, fence lines, and waterways, where they move into tall grass, such as brome. As the stalk borers develop, they outgrow the grass plant, killing the top of the plant, and they then move into nearby crop land, generally corn.  As each dead stem represents one stalk borer that may migrate into the field, observing the seed head stems of tall grasses for signs that they have been killed will provide some guidance on the need to spray the edge rows of corn to kill the stalk borers as they migrate into the field.


Stalk borer migration from grassy areas into adjacent corn rows should begin Wednesday, May 26 along Highway 34.  Using National Weather Service Temperature forecasts through May 25, 2010 and then normal temperatures thereafter, migration should begin along Interstate 80 on Friday, May 30, along Highway 30 on Thursday June 3, and along Highway 20 on Monday, June 7.  Weather anomalies between now and then may change that.


To monitor Growing Degree Days for stalk borers and for more information on managing stalk borers, see



Late Spring Soil Nitrate Test


Late Spring Soil Nitrate Test samples should be pulled to a one foot depth when the corn is 6-12” tall at the whorl. Soil samples should be sent to a lab immediately after sampling. Results can help to fine-tune nitrogen management.


It is best to use a systematic method rather than a random method to pull the samples. Pull the first sample in the corn row, the next 1/8 of the distance between rows, the next 1/8 the distance between rows, etc. until you have worked your way across the rows. Do this at least twice for a total of 16 cores. This way you won’t by chance happen to be over or under representing areas that have differing amounts of nitrogen (i.e., anhydrous bands, manure bands, starter fertilizer, etc.).


For more details see the publication “Nitrogen Fertilizer Recommendations for Corn in Iowa” at






Plant Populations


Even though optimum seeding rates have been increasing every year for corn, recommended seeding rates for soybeans have been going in the other direction. Recent work by Palle Pedersen has shown that the optimum final stand for soybeans is 100,000 plants per acre. Optimum seeding rates will vary depending on the seeding method used and germination of the seed, but it will seldom pay to seed at higher than 125,000-140,000 seeds per acre. Because of soybean’s ability to compensate for lower stands by branching out and producing more pods/plant and more seeds/pod, yields do not decrease much until populations get below 75,000 plants/acre. See Palle’s fact sheet “Optimum Plant Population in Iowa” at for more information.


Seed Treatments?


As we get later into May with warmer soil temperatures, it becomes less likely that fungicide seed treatments for soybeans will pay off. Insecticide seed treatments do a nice job of controlling overwintering bean leaf beetles, but they are a greater problem with the early planted soybeans which are already in the ground. The cold winter should also have reduced our bean leaf beetle populations.  Insecticide seed treatments are not very effective for controlling soybean aphids, because the treatment does not last long enough for killing aphids in August.




Farm Progress Hay Expo, Strawberry Point

June 16-17


For details about the program, exhibitors, etc. go to:


Spring Field Day & Special Session For CCAs

SE IA Research Farm – Crawfordsville

June 24


The Spring Field Day of the Southeast Research and Demonstration Farm (Crawfordsville) will be on the afternoon of Thursday, June 24, 2010 at the farm.  Certified Crop Advisor (CCA) credits will be available.  The field day will feature Erin Hodgson on new advances in soybean aphid management, Matt Helmers on Tile Drainage Research results, Emily Heaton, on Miscanthus and other Cellulosic Ethanol Crops, and Johns Sawyer on Nitrogen Management and Cover Crops. In addition, Certified Crop Advisors can obtain additional hours of credit (including soil and water) by attending a special session in the morning (9:00 a.m.) followed by the afternoon tour (1:00 p.m.).  Soil drainage will be featured in this special session. More details are at



Muscatine Island Research Farm Field Day and 75th Anniversary, Fruitland

June 29


The Muscatine Island Research and Demonstration Farm was founded 75 years ago.  A special field day will include many special events in addition to the traditional field day.  If you have an interest in horticulture, be sure to attend.  Information will appear soon at



Northeast ISU Research Farm Field Day, Nashua

June 30, 1:00-4:00


Field day speakers include:   Ken Pecinovsky, Farm Superintendent, Robert Hartzler, ISU Extension Weed Scientist, Alison Robertson, ISU Extension Plant Pathologist, John Sawyer, ISU Extension Soil Fertility Specialist, Chad Ingels, ISU Extension Program Specialist, and Brian Lang, ISU Extension Agronomist. CCA Credits available for a fee.  More details are at




If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: May 20, 2010
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