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7/23/2012 - 7/29/2012

Possible Breakdown of SCN Resistance Topic of New Webcast

By Greg Tylka, Department of Plant Pathology and Microbiology

Iowa farmers produce soybeans profitably in fields infested with the soybean cyst nematode (SCN) by growing SCN-resistant soybean varieties that yield well and prevent large increases in SCN egg population densities. There are hundreds of SCN-resistant soybean varieties for Iowa. Almost all of the varieties contain SCN resistance genes from a single breeding line, called PI 88788. 

Because of widespread, repeated use of varieties with the same PI 88788 source of resistance, many SCN populations have developed increased reproduction on that type of resistance. It is no longer uncommon to see SCN females on roots of SCN-resistant soybean varieties in the Midwest (Figure 1). 

Figure 1. SCN females on roots of an SCN-resistant soybean variety with PI 88788 SCN resistance.


How does increased SCN reproduction on soybean varieties with the PI 88788 source of resistance affect soybean yields and the buildup of SCN egg population densities (numbers) in the soil? Results of Iowa State University field experiments reveal the answers.


Experimental results show PI 88788 resistance is still working

Each year, SCN-resistant soybean varieties are studied intensively in nine experiments throughout Iowa as part of the Iowa State University SCN-resistant Soybean Variety Trial Program. The research is funded by the soybean checkoff through the Iowa Soybean Association. Since 2007, more than half of the fields in Iowa where variety trial experiments were conducted had SCN populations with elevated (greater than 10 percent) nematode reproduction on the PI 88788 source of resistance. The 10-percent level of SCN reproduction is a critical level above which plants are not considered resistant. Some of the SCN populations in the experimental fields had 35 to 50 percent reproduction on PI 88788. 

But despite the increased SCN reproduction, soybean varieties with the PI 88788 source of SCN resistance in the experiments usually yielded the highest. Also, SCN population densities did not increase dramatically on the resistant varieties in the experiments.  These results were fairly consistent throughout the past five years (results available online at

Overall, the results indicate that SCN-resistant soybean varieties with the PI 88788 source of resistance are not “failing”. The resistant varieties are still producing profitable soybean yields and preventing large increases in SCN population densities.

Because SCN is considerably more damaging in hot, dry years than in growing seasons with more moderate weather, soybean varieties with PI 88788 SCN resistance may suffer significant yield loss due to nematode feeding this season in fields with SCN populations that have elevated reproduction on PI 88788. The 2012 growing season is the type of environmental conditions under which increased SCN reproduction on soybean varieties with PI 88788 SCN resistance may result in significant yield reductions.


New webcast puts situation in perspective

A new webcast explains the factors leading to the build up of SCN populations with elevated reproduction on PI 88788 and discusses the results of numerous variety trial experiments where high yields were produced by PI 88788 SCN-resistant soybean varieties in fields infested with such SCN populations. The webcast was developed and is hosted by the Focus on Soybean section of the Plant Management Network.  

The full-length webcast is 25 minutes long and a 4½-minute-long executive summary webcast highlights the “take-home” points of the full-length presentation. Both webcasts are available here.

The webcasts are available for viewing free to the public through November 30, 2012. But a subscription is needed to access most content on the Plant Management Network. 

Currently, soybean farmers and crop advisers can sign up for free, individual one-year subscriptions to the Plant Management Network sponsored by the United Soybean Board; simply establish a complimentary account by filling out this online form.


Greg Tylka is a professor with extension and research responsibilities in management of plant-parasitic nematode in the Department of Plant Pathology and Microbiology at Iowa State University. He can be reached at or 515-294-3021.

Phosphorus and Potassium in Silage Harvest of Drought-Damaged Corn

By John Sawyer and Antonio P. Mallarino, Department of Agronomy

The dry conditions this summer are resulting in quite variable corn growth and production potential. Some livestock producers are beginning harvest of corn damaged by drought conditions for silage. Corn silage harvest results in more phosphorus (P) and potassium (K) removal than grain alone because almost the entire plant is harvested. The increased amount removed with silage differs for P and K because the relative amount of P and K is different in corn vegetative parts than in grain. For P there can be approximately four times greater amount of P per ton of dry matter in grain than vegetative parts, but for K the opposite occurs, on average with almost three times greater amount of K per ton of dry matter in the vegetative parts.

The easiest approach for estimating P and K removal with silage harvest is to use existing estimates of the average corn silage removal amounts per unit of yield. From the ISU Extension and Outreach publication PM 1688 (A general guide for crop nutrient and limestone recommendations in Iowa), those values are 3.5 lb P2O5/ton and 8.0 lb K2O/ton (65 percent moisture based). On a dry matter basis, the values are 10 lb P2O5/ton and 23 lb K2O/ton. Of course the total removal amount per acre will vary according to the amount of silage harvested. These average concentration values could be correct with drought-damaged corn if the plant vegetation and grain are roughly affected in equal proportion.

However, drought conditions complicate estimates of P and K removal with silage harvested early because of largely unpredictable effects on dry matter production and nutrient uptake and accumulation. Depending on how moisture availability was during the season, there could be relatively more or less dry matter production than nutrient uptake, which would result in lower or higher concentration values, respectively. Data is very limited, however. Results from research in Kansas showed the nitrogen (N), P, and sulfur (S) concentrations were about the same for normal or drought-stressed corn, but the K concentration increased by about 50 percent.

An additional consideration is the growth stage of the corn when it is harvested. If the corn has not matured normally, but instead is harvested early or dies due to lack of moisture and high temperatures, it may really only be at an R1 (silking), R2 (blister), R3 (milk) or R4 (dough) growth stage at the time of silage harvest. In those cases, the full time period for nutrient uptake has not occurred and, in addition, the dry conditions could further reduce uptake. For each of those stages, and assuming little to no grain production, then the percent of normal full vegetative P and K uptake are for P: R1 50, R2 55, R3 55, R4 55 percent; and for K R1 75, R2 85, R3 85, R4 85 percent (adapted from ISU Extension and Outreach publication PMR 1009, Corn growth and development). Therefore, depending on the timing of the drought, the effects on P or K accumulation in the plant could differ greatly. Vegetative P and K concentration normally decrease as grain fill occurs due to translocation to grain, but that will be limited or non-existent in situations with little to no grain. One could multiply the percentage for the appropriate harvest growth stage by the normal P and K removal values for silage to estimate P2O5 and K2O amount per ton silage when there is little or no grain. For example, if silage is harvested at a R3 growth stage, then an estimated amount per ton silage (dry matter based) would be 10 x 0.55 = 5.5 lb P2O5/ton and 23 x 0.85 = 20 lb K2O/ton.

Because of the great deal of uncertainty, a more accurate estimate of P and K removal can be obtained by sending silage samples to a laboratory for analysis. The Iowa State University Soil and Plant Analysis Laboratory and all private laboratories are equipped to analyze plant tissue for P, K and other nutrients. You need to be sure the sample is representative by making a composite sample from material taken from different silage loads.

No matter the method used to estimate silage P and K concentration, the amount of silage harvested per acre has the greatest impact on nutrient removal. Therefore, it is important to have a good estimate of silage harvested and the moisture content.

For P and K, even if estimates of removal with silage harvest are off somewhat, soil sampling and testing in the fall can help with longer-term management in drought-affected fields. In extreme cases where no or very limited crop is harvested, then P and K will recycle from plants and be available for the next crop.


John Sawyer and Antonio P. Mallarino are professors in the Department of Agronomy with research and extension responsibilities in soil fertility and nutrient management. They can be contacted at and

Mid-season Soybean Disease and Fungicide Update

By Daren Mueller and Nate Bestor, Department of Plant Pathology and Microbiology

While the hot, dry weather across the Midwest has affected soybean production, it has also negatively affected soybean diseases. In general, there is very little disease across Iowa in soybean. There have been a few fields with Septoria brown spot and Cercospora leaf blight, but it has been a quiet year for soybean diseases.

That has not stopped the questions about foliar fungicides. With the price of soybean and fields being sprayed for spider mites or Japanese beetles, questions around “throwing in a fungicide” have been coming in.

Through funding from the soybean checkoff, we have completed 21 site years of trials looking at the responses to fungicides, insecticides and tank mixes of fungicides + insecticides. Except for a few site years, the soybeans have been mostly disease free. And many of the locations have had little or no insect pressure.

One thing is for certain, none of the previous trials have had any inkling of drought stress. Really, the only environmental stress we have seen in previous fungicide trials is too much moisture. Table 1 summarizes the response to fungicides and fungicides + insecticides. The products summarized in Table 1 include a strobilurin fungicide, which are Headline, Stratego YLD and Priaxor. The addition of a fungicide to an insecticide application has resulted in a lower yield response of 1.7 bu/ac.


Table 1. Mean yield response of fungicides alone (vs. control) and fungicide-insecticide tank mixes (vs. insecticide alone) applied at growth stage R3 in trials from 2008-2011 in Iowa.


Table 2 summarizes the economic response at $10 and $16 per bushel. Even considering a lower total application cost for fungicides as part of a tank mix, fungicides “thrown in” with insecticides only have a 46 percent chance of getting an equal return on your investment.


Table 2. Economical analysis of fungicides applied alone and fungicides mixed with insecticides at growth stage R3 in studies conducted in Iowa from 2008-2011. Analyses are based on grain prices at $10 and $16 per bushel.


A final thought on fungicides: Remember to scout for frogeye leaf spot if you have made a fungicide application. We are monitoring for fungicide resistance for the frogeye leaf spot pathogen so if you suspect you have it, please send samples to our Plant and Insect Diagnostic Clinic.


Charcoal rot

Even with rain last night across much of the state, one soybean disease that may be showing up is charcoal rot. Charcoal rot is caused by the fungus Macrophomina phaseolina. Infection can occur early in the growing season, yet symptoms may not become evident until mid- to late-season.


Identifying charcoal rot

There are a few characteristic symptoms to distinguish charcoal rot from other problems. Microsclerotia (see Figure 1) are probably the most characteristic sign. These structures are very dark and round and are how the fungus survives. The fungus infects roots and moves to stems, filling tissues with microsclerotia, which clog vascular tissue, causing wilting, yellowing, and stunting of the plant. And this is much more apparent in drought-stressed areas. A second characteristic is gray discoloration when you split the lower stems (Figure 2).

Figure 1. Black fungal structures called microsclerotia can be found in stem tissue infected by the fungus that causes charcoal rot. Photo by Alison Robertson.


Figure 2. Gray discoloration of stems is symptomatic of charcoal rot. Photo by XB Yang.

Even with these two signs of charcoal rot, this disease may still be hard to diagnose because general symptoms are similar to drought stress. As for management, foliar fungicides are not effective. The microsclerotia can survive for several years, so cultural practices are not effective. There is a limited selection of resistant cultivars and planting at lower populations to reduce competition for water among plants can help.


Daren Mueller is an extension specialist in the Department of Plant Pathology and Microbiology. He can be reached at 515-460-8000 or e-mail Nate Bestor is a research associate in the Department of Plant Pathology and Microbiology. He can be reached at 515-294-1741 or e-mail

ISU Extension and Outreach to Host Central Iowa Corn Stover Harvest Meeting

By Mark Licht, Extension Field Agronomist

New central Iowa opportunities exist for harvesting corn stover in support of cellulosic ethanol plant biomass feedstock needs. Iowa State University Extension and Outreach will host a meeting on Aug. 15 to address emerging opportunities related to stover harvesting. The Corn Stover Harvest Meeting will begin at 1 p.m. in Christy Hall, Story County Extension Office, Nevada.

Stover harvesting can impact economic, agronomic and water quality production factors. This meeting will address many of the positives and negatives of stover harvesting and discuss how stover harvesting can be managed within corn production in central Iowa. Crop producers, land owners, crop consultants, agronomists, service providers and others with interest in stover harvest and its removal from farm fields are encouraged to attend.

University speakers include: Matt Darr, agricultural and biosystems engineering assistant professor; Mahdi Al-Kaisi, extension soil management specialist; John Sawyer, extension soil fertility specialist; Antonio Mallarino, extension soil fertility specialist; Chad Hart, extension agricultural marketing specialist; Kapil Arora, extension agricultural engineering specialist; Mark Licht, extension field agronomist; and Kelvin Leibold, extension farm management specialist. Industry speaker on the agenda is Dennis Penland, DuPont business development manager.

By attending the meeting, participants will
• understand the benefits and constraints related to stover harvest based on the research information available to date,
• learn about the cellulosic ethanol program being implemented by DuPont, and
• participate in a panel discussion with producers involved in stover removal from their farm fields.

Prior to the start of the meeting, a noon lunch will be sponsored by DuPont Pioneer, provided at no cost to the participants, but pre-registration is required. Participants will have the opportunity to interact with industry representatives during lunch and after the meeting adjourns.
There is no fee to attend this meeting. Registration is required to make sure appropriate number of handout copies can be made along with a correct meal count for lunch. The registration form is available online or by contacting the Story County Extension Office, 220 H Avenue, Nevada or calling 515-382-6551. Please use a separate form for each registering individual. Register by 5 p.m. Thursday, Aug. 9. Walk-in registration the day of the meeting will only be accepted if space is available. Participation at the meeting is limited to 150 registrants.


Mark Licht is an ISU Extension and Outreach field agronomist. He can be reached at 515-382-6551 or e-mail

Take Precautions When Feeding Drought-Damaged Corn as Silage

By Steve Ensley, Department of Veterinary Diagnostic and Production Animal Medicine

When a corn plant is stunted, or not growing normally, nitrates can build up in the plant. Typically nitrogen that is taken up by the corn plant is converted to amino acids – the building blocks of protein – but in the stressed plant this conversion does not occur. And that’s why producers need to know how to handle and feed drought-damaged corn in silage form, according to Steve Ensley of Iowa State University’s Veterinary Diagnostic and Production Animal Medicine (VDPAM) department.

“The nitrate level can be high enough in stunted plants that if harvested as silage the nitrate can be toxic to livestock that consume the silage,” Ensley said. “The most common problem is when drought-stressed corn is green chopped and fed to livestock without going through ensiling.”

Ensiling will lower the amount of nitrate in the plant. Nitrogen availability to the plant, which will depend on recent rain, fertilizer applied and other factors, will affect the level of nitrate in the plant. If producers are planning on making silage from drought-damaged corn, the only way they can be sure about the level of nitrate is to do some sampling and testing of the corn silage before feeding.

“Producers have several options to help determine whether nitrate toxicity might be a problem,” Ensley said. “Assess the corn field that will be harvested to determine how much of the field has stunted corn that’s not developing a normal ear. Take samples either by taking grab samples of silage cut by a forage chopper or by cutting several entire plants by hand that represent the various types of corn in the field.”

A quick test that will screen for the presence of nitrates in stalks without having to chop the stalk can be done by ISU Extension beef and dairy program specialists. A drop of diphenylamine in sulfuric acid on the surface of a stalk split in two will turn a blue-black color if nitrate is present. However, this does not provide a concentration of nitrate. Availability of this screening test varies. In some cases, the screening test may be a part of drought meetings where the beef or dairy field specialists are present.

Other options include getting a representative sample of several stalks and performing a strip test, which can help determine the concentration of nitrate with sending in a sample. ISU Extension beef and dairy program specialists also will have capability for this, but due to the time required to prepare the sample for analysis the availability will be more limited than the screening test. Samples also can be sent to commercial feed analysis labs, or your veterinarian can send samples to the ISU veterinary diagnostic lab.

For more information on nitrate toxicity in drought-damaged corn silage, see the fact sheet “Nitrate Toxicity” on the Iowa Beef Center website. Producers also can contact their ISU Extension beef program specialist or dairy program specialist. Additional drought-related information is available on the ISU Extension website and the Iowa Beef Center website.


Steve Ensley is a senior clinician in the Department of Veterinary Diagnostic and Production Animal Medicine. He can be reached at 515-294-1950 or e-mail

Symptoms of SCN Damage Apparent Early, Yield Loss Likely Great in 2012

By Greg Tylka, Department of Plant Pathology and Microbiology

As if the direct effects of Iowa’s hot, dry growing season on crops were not damaging enough, the soybean cyst nematode (SCN) probably will be more damaging this year than in the past two decades due to the lingering drought conditions.


Symptoms of damage appearing sooner in 2012

Damage from SCN does not always cause aboveground symptoms, especially in years with adequate to excess moisture. But when symptoms occur, they usually appear in mid to late July (see 2011 ICM News article here). 

This season, stunting and yellowing of soybean foliage caused by SCN feeding has been apparent throughout the state since late June, which is much earlier than usual. 

Symptoms of SCN damage can be relatively mild when SCN population densities, or numbers, are low (Figure 1).  But stunting and foliar yellowing can be severe in very dry soils and when SCN population densities are high (Figure 2). The earlier those symptoms appear in the growing season and the more severe they become, the greater the yield loss that occurs with SCN.

Figure 1. Mild yellowing and stunting of plants caused by SCN.


Figure 2. Severe yellowing of foliage and stunting of plants caused by SCN. (Photo by Jim Fawcett)


Yield loss with SCN-resistant soybean varieties in 2012

Soybean yield losses from SCN have been minimized over the past 20 years through use of resistant soybean varieties. Good SCN-resistant soybean varieties keep nematode reproduction in check and produce profitable yields in fields infested with the nematode, at least under most conditions.

Almost all SCN-resistant soybean varieties available for the Midwest have resistance genes from a breeding line called PI 88788. Because of repeated exposure to the same type of resistance, many of the SCN populations in Iowa fields have increased reproduction on varieties with PI 88788 SCN resistance genes. 

SCN populations with elevated reproduction on PI 88788 resistance were first noticed in Iowa seven or eight years ago.  But to date, the increased SCN reproduction has not severely reduced yields of resistant varieties or caused large increases in SCN population densities in Iowa. 

Unfortunately, greater yield loss and larger increases in SCN population densities on SCN-resistant soybean varieties are possible for 2012 because root stunting from SCN feeding makes plants even more drought stressed and also, there seems to be greater reproduction of the nematode in dry soils (as discussed in an ICM News article earlier this year).


Management of SCN requires persistence

Successfully managing SCN is not an achievement to be accomplished in one growing season; it is a continual process. To ensure profitable soybean production in SCN-infested fields long-term, a sustained, integrated approach is necessary. 

Every field in Iowa in which soybeans will be grown should be checked for SCN. This can be done by checking roots for SCN females (see ICM News article here) or by collecting soil samples to test for SCN (see ICM News article here). 

Fields that are known to be infested with SCN and have had resistant soybean varieties grown should be sampled periodically to assess SCN population densities. Knowing if SCN population densities are decreasing, increasing or staying the same in a field over a period of six years or so will indicate how aggressive of an approach to management is needed to maintain profitable soybean production in the field in the future.

Growing nonhost corn in alternating years with SCN-resistant soybean varieties is key to keeping SCN numbers from increasing. Growing varieties with various sources of SCN resistance, if possible, reduces the selection of SCN populations with increased reproduction on the very common PI 88788 source of resistance. Finally, there are new nematode-protectant seed treatments that can be used in conjunction with SCN-resistant soybeans for added defense against the nematode.

More information about the biology and management of SCN can be found at and the Plant Health Initiative’s website.


Greg Tylka is a professor with extension and research responsibilities in management of plant-parasitic nematode in the Department of Plant Pathology and Microbiology. He can be reached at or 515-294-3021.

This article was published originally on 7/30/2012 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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