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8/19/2013 - 8/25/2013

Soybean Diseases Starting to Wake Up

By Daren Mueller, Department of Plant Pathology and Microbiology

After taking much of the growing season off, soybean diseases are finally starting to wake up and make a bit of noise.


Soybean rust

Yes, you read correctly, a soybean rust update. It has been a few years. Dr. Carl Bradley has a nice article on the status and risk of soybean rust in Illinois. The article includes the most common look-alike diseases.

In general, soybean rust is building up in the southern states a bit sooner than previous years, and there are quite a bit of late-planted soybeans that could be affected in these southern states. As of now, most of the soybean rust finds are too far east for them to provide spores that could infect Iowa fields. But it is certainly worth keeping an eye on over the next several weeks because we do have late-planted soybeans that could be affected. For the latest distribution of soybean rust, visit the IPM PIPE webpage. If you suspect you have soybean rust, please send soybean leaves to the ISU Plant and Insect Diagnostic Clinic.


Sudden death syndrome

Sudden death syndrome (SDS) has been reported in several counties in southeastern Iowa. As a reminder, SDS typically shows up in patches, often at the edges of fields or in low-lying areas. See images for early and more advanced symptoms. Remember to split the stem to differentiate from brown stem rot, which will have the characteristically brown discolored pith.




With support from USDA and checkoff dollars, Dr. Leonor Leandro is collecting isolates of the fungus that causes SDS from across Iowa. If you have SDS in your field, we are very interested in collecting a few infected plants for research purposes. If you can mail a few infected plants, including the roots, to the address below or send an email to Dr. Leandro or myself to arrange for us to get the plants, it would be very much appreciated.

Dr. Leonor Leandro
351 Bessey Hall
Ames, IA 50011


Soybean vein necrosis virus

We continue to see soybean vein necrosis virus (SVNV) in Iowa, especially the southern half of the state. However, most fields that have the virus have fairly low levels. See the previous ICM article for information on how to identify this disease.


Daren Mueller is an assistant professor in the Department of Plant Pathology and Microbiology. He can be reached at 515-460-8000 or e-mail

Field Day at Iowa State University Ag Engineering/Agronomy Research Farm

By Mark Licht, ISU Extension and Outreach

A field day will be held on Aug. 30, hosted by the Iowa State University (ISU) Agricultural Engineering/Agronomy Research Farm, rural Boone, from 9 a.m. to noon with lunch and tours of the ISU Biocentury Farm following the program.

The event will focus on cover crops and corn production. The field day is free and the public is invited to attend.

ISU Vice President for Extension and Outreach John Lawrence will present information about the Iowa Nutrient Reduction Strategy. Sarah Carlson, Midwest Cover Crop research coordinator, will discuss establishing cover crops and selecting appropriate plants.

Bob Hartzler, ISU Extension weed specialist and agronomy professor, will present information on the effects of residual herbicide on cover crop establishment. Mark Licht, ISU Extension field agronomist, will offer tips about cover crop management in a corn-soybean rotation.

ISU Extension entomologist Erin Hodgson will share information about corn rootworm resistance and resistance management. ISU Extension corn agronomist Roger Elmore will update attendees on the state’s corn production status.

Following lunch, the ISU BioCentury Research Farm will offer tours of the facility, and Iowa Learning Farms will demonstrate the Conservation Station. Its rainfall simulator shows the effects of rainfall on several different surfaces, including intense tillage, perennial vegetation and permeable pavers for streets and driveways.

The field day is at the ISU Agricultural Engineering/Agronomy Research Farm, 1308 U Ave., Boone, at the intersection of Highway 30 and U Avenue between Boone and Ames. Iowa Learning Farms, Practical Farmers of Iowa, ISU College of Agriculture and Life Sciences and ISU Extension and Outreach are hosts of the field day.

Founded in 1985, Practical Farmers of Iowa is an open, supportive and diverse organization of farmers and friends of farmers, advancing profitable, ecologically sound and community-enhancing approaches to agriculture through farmer-to-farmer networking, farmer-led investigation and information sharing. Farmers in the network produce corn, soybeans, beef cattle, hay, fruits and vegetables, and more. For additional information, call (515) 232-5661 or visit

Iowa Learning Farms (ILF) takes a grassroots approach offering innovative ways to help all Iowans have an active role in keeping our state’s natural resources healthy and not take them for granted. A goal of Iowa Learning Farms is to build a Culture of Conservation, encouraging the adoption of residue management and conservation practices. Farmers, researchers and ILF team members are working together to identify and implement the best in-field management practices that increase water and soil quality while remaining profitable. For more information, go to

Iowa Learning Farms is a partnership between the Iowa Department of Agriculture and Land Stewardship, Iowa State University Extension and Outreach, Leopold Center for Sustainable Agriculture, Iowa Natural Resources Conservation Service, and Iowa Department of Natural Resources (USEPA section 319); in cooperation with Conservation Districts of Iowa, the Iowa Farm Bureau and the Iowa Water Center.


Mark Licht is an extension field agronomist. He can be reached at or 515-382-6551.

Palmer Amaranth Confirmed in Western Iowa

By Bob Hartzler, Department of Agronomy, and Rich Pope, Harrison County Extension

The presence of Palmer amaranth was recently confirmed in Harrison County near the Missouri River. The infestation was in two fields that have a history of land application of sludge. Because of the magnitude of the infestation, we believe the weed has been present for at least two growing seasons. We suspect the weed probably has spread to other fields in the area, but at this time we have not verified this.

Photo 1. Long, terminal inflorescence branches are a characteristic of Palmer amaranth. Its presence was recently confirmed in Harrison County.


The confirmation of Palmer amaranth at this site reinforces the need for thorough scouting of fields to make positive identification of the weeds present in individual fields. Although Palmer amaranth has a distinctive growth habit and is visibly different from waterhemp, casual observations are unlikely to differentiate the two species. 

Palmer amaranth has received much publicity due to its impact on crop production in other areas of the country. Although it is a difficult weed to manage, we believe that with integrated weed management programs Palmer amaranth should not pose insurmountable challenges for Iowa farmers. Select herbicides that are highly effective against Amaranthus species, waterhemp and Palmer amaranth respond similarly to most products. The use of full rates of preemergence herbicides and timely postemergence applications will be the backbone of management programs for most farmers. The use of post-applied residual herbicides (e.g., Warrant, Dual II Magnum, Zidua, etc.) in 30-inch row soybeans will further reduce selection pressure by postemergence herbicides. These are the same approaches we recommend for managing waterhemp.

We appreciate the watchful eye of the local farmer who suspected the escaped Amaranthus species might be Palmer amaranth and contacted us. We encourage continued vigilance for the presence of this weed and will appreciate being contacted when suspect populations are found. Remember, the simplest and most cost-efficient manner of managing Palmer amaranth, or any new weed species, is early detection and eradication before a permanent infestation is established. If found early, plants can be removed from the field before seed production establishes a permanent seed bank and persistent problem.


Bob Hartzler is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. He can be reached at or 515-294-1923. Rich Pope is a county extension program coordinator. He can be reached at or 712-644-2105.

Late-season Corn Development and Frost Probabilities

By Roger Elmore, Department of Agronomy

Cool August temperatures across Iowa slow growing degree day (GDD) accumulations. In addition, Iowa’s late corn planting dates this year obviously impacted the crop as well. These two factors affect corn yield potential.

I addressed GDD accumulations in a recent CropWatch blog posting.  Growing degree day accumulations clearly lag behind normal. Cool temperatures after silking not only slow GDD accumulation,  thus slowing crop development, but also can increase yield potential given specific conditions. The record yields of 2009 resulted from slow GDD accumulation after silking coupled with a late frost. On the other hand, warm temperatures after silking in 2010 reduced corn yield potential (See an ICM News article reporting this).

Earlier this season I addressed the potential impact of late corn planting on yields; see Crop Model Output and Field Research Data. The August 12th USDA yield forecast in part reflects this; Iowa’s USDA August forecast yield of 163 bushels per acre is almost 9 percent below 30-year trend-line yields(10 percent below the 30-year trend is “officially” drought). 


Dry matter accumulation and grain moisture during reproductive stages

Let’s address another question here: Will the corn crop mature before frost? My response to this question depends on when the first 28°F or colder frost occurs and the crop’s current development stage. Table 1 presents a timeline of corn development as well as kernel dry matter and moisture content during dent – R5. Physiological maturity (R6) is the point when maximum kernel dry matter occurs – normally around 35 percent grain moisture. Black layer formation occurs a bit later than R6, typically 28 percent ± 4 percent. Contrary to popular thinking, kernels do not lose dry matter after R6.

Full-size table


Based on data in Table 1, corn in early dent (R5) has about 60 percent grain moisture, accumulated about 45 percent of its dry matter, and needs another 33 days to mature. At three-quarter milk line, 97 percent of the dry matter is accumulated and it will take about two weeks to mature.


Freeze dates

Figure 1 shows the most recent 30-year dates for median first fall 28⁰F frost across the Midwest. The median date for portions of NW and NE Iowa ranges from October 1 to 10; that for SE IA range from October 21 to 30. The median first fall 28⁰F frost for rest of the state  ranges between October 11th and 20th (from MRCC).

Figure 1. Median fall 28⁰F freeze dates based on 1981-2010 averages. From MRCC. Full-size image


Mesonet provides tables of probabilities by specific locations for fall frost events with different temperature thresholds. These data are averages since 1951. Figures 2, 3, and 4, display probabilities of temperatures less than 29⁰F for Iowa’s nine crop reporting districts. For example: for SW Iowa(fig 2) the average date of the first hard freeze is Oct. 21. In addition, one year in five the freeze may be later than Oct. 28, and one year in 10 it may be Nov. 4 or later. On the other hand, note that one year in 10 the hard freeze is on or before Oct. 5.

Figure 2. Probability of temperatures less than 29⁰F for NW, WC & SW Iowa. The horizontal yellow line marks 50 percent probability. The vertical light purple lines point to the dates of the 50 percent probability of temperatures less than 29⁰F temeprtures for each of the three Crop Reporting Districts. Data from Mesonet. Full-size image


Figure 3. Probability of temperatures less than 29⁰F for NC, Central & SC Iowa. The horizontal yellow line marks 50 percent probability. The vertical light purple lines point to the dates of the 50 percent probability of temperatures less than 29⁰F temeprtures for each of the three Crop Reporting Districts. Data from Mesonet. Full-size image


Figure 4. Probability of temperatures less than 29⁰F for NE, EC & SE Iowa. The horizontal yellow line marks 50 percent probability. The vertical light purple lines point to the dates of the 50 percent probability of temperatures less than 29⁰F temperatures for each of the three Crop Reporting Districts. Data from Mesonet. Full-size image


The date with 50 percent probability of less than 29⁰F temperatures ranges with  northern CRD’s occurring earlier than the southern CRD’s – a range of 9 to 12 days earlier in the north in the three central and eastern-most CRDs. The northwest CRD has later frost dates then the west cenral CRD.

Fifty percent probability dates of temperatures below 29⁰F for the western parts of the central and southern CRD’s arrive six days earlier than the eastern parts of those regions. Those dates vary little across northern Iowa CRDs, October 14 to 18.

Warmer temperatures in the current short-term forecast may help accumulate GDDs faster. However, much of the state remains dry (see drought monitor). Warmer temperatures with dry conditions will stress the crop even more.

The critical issue of this whole season is the timing of the first 28⁰ F frost this fall. A later than normal frost encourages longer seed-fill period and higher yields. An early frost … well let’s hope it doesn’t happen!


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by e-mail at or (515) 294-6655.

This article was published originally on 8/26/2013 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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