Setting the record straight on Tar Spot

Remember that game of telephone we played as kids?  One person says something in to the ear of another and after passing through 10 people or so the starting message, “I like peanut butter” ends up as, “John licks turtles.”  Sometimes that can happen with information pertaining to plant diseases.  Lately there have been some interesting things said about tar spot on corn in the community.  To help clarify, and set the record straight, I published an article on my blog, which can be accessed here.

Corn Hybrid Response to Tar Spot

Hybrid resistance is a key component for managing many plant pathogens.  To access a new sheet on hybrid response to tar spot in corn click the following link: Corn Hybrid Response to Tar Spotfin.docx.

We are currently rating multiple variety trials in affected areas across states to generate more hybrid-specific data.  More information will be available soon.

Tar Spot in corn- requesting your help

Tar spot is a relatively new disease in corn.  It was first described in Illinois and Indiana in 2015, and was first located near DeKalb.  Tar spot has been detected to some degree in Northern Illinois each year since.  However, typically infections are sparse and the disease does not come in until later in the season.  Consequently, yield loss due to this disease has been minimal, and the disease mostly considered an oddity.

However, in parts of Latin America, where the disease is known as Tar Spot Complex, severe yield losses can occur.  In this case, two pathogens are involved.  One fungus produces the black tar spots we typically see, and another produces toxins that can cause varying degrees of foliar  blight and necrosis.  Our colleagues at CIMMYT in Mexico are currently working on identifying the toxins involved and how they may relate to  virulence.  It is important to note that there is very little known about tar spot complex, how the pathogens interact with oneanother, the epidemiology of the disease, and how the pathogens interact with their corn host.  In addition, it is possible that this disease may act differently in Midwest production systems, as hybrid genetics, production practices, and environments differ from those in Latin America.

Tar Spot on corn with Black raised “bumps” and small necrotic fisheye symptoms. N Kleczewski and J Donnelly

This season we have seen this disease take off in Northern Illinois, as well as Southern Wisconsin, Michigan, and parts of Indiana.  Symptoms vary from from the traditional black raised bumps, to bumps with necrotic fisheye lesions, to spots on leaves that blight and drydown.  Some fields have light infection, whereas others have over 30% leaf severity through the highest leaf of the hybrid.   Early this summer, prior to this outbreak,  we started working with colleagues in other states and CIMMYT  to better understand the tar spot pathogens and improve our abilities to detect and manage this disease if needed.  One item that we need for this project are samples.  If you have fields with symptoms of tar spot, particularly those with necrosis associated with the lesions, please send care of Dianne Plewa at the University of Illinois Plant Disease Clinic.  The website with address and contact information is located at the following address:  Please include the county of origin, if a fungicide was applied, and the hybrid, if possible.

In addition, we are working to assess potential variety response and yield impacts of this disease.  If you would like to participate in the effort, please contact me at 217-300-3253 or email me at  I also can be reached on Twitter @ILplantdoc

Spray Decisions for Frogeye Leaf Spot on Soybeans

Many people have asked about the need to make a fungicide application for frogeye leaf spot on soybeans this season.  I have posted a new article on the Illinois Field Crop Disease Blog which reviews this pathogen, how it works, and some new tools that may help you with these important decisions.  Find the article by clicking here.

Is the 2018 soybean crop as good as it looks?

If the appearance of the soybean crop going into late July predicts how it will yield, the 2018 crop in Illinois is going to be a high-yielding one. The crop in Illinois was rated at 78% good + excellent (G+E) as of July 22. Conditions across the US soybean growing regions are somewhat variable, but the 2018 crop is in good condition overall.

As I did with corn in the Bulletin on July 6, I examined Illinois soybean crop ratings in the second half of July over the past decade to see how well they predict yield. The drought year 2012 was in a category by itself, with a G+E rating of less than 10% by late July. Ratings were only about 50% G+E in 2015, but in six of the past ten years—2008-2011, 2013, and 2017—ratings were around 60% G+E in late July. In 2014, 2016, and now in 2018, ratings were high, at 75 to 80% G+E.

With a few exceptions, soybean crop ratings have tended either not to change much after mid-July or to drift up slowly. Exceptions included 2008, when crop conditions rose by about 20 points from late July to mid-August, and 2011, when ratings dropped about 20 points during the same period. In 2017, ratings drifted down after June, from 70% G+E in early July to less than 60% by late August.

Illinois soybean yields were 50 bushels per acre or less in years with late July crop ratings of 60% G+E or less, with the exception of 2010, when the crop yielded 51.5 bushels. In both 2015 and 2017, mediocre late-July ratings failed to predict high yields, of 56 in 2015 and 58 in 2017. In 2014 and 2016, high ratings did predict high yields, of 56 and 59, respectively. Overall, high soybean crop ratings by late July tend to predict high yields, while low to medium ratings can sometimes be offset by August weather, resulting in average or even above-average yields. Because high crop ratings in late July predict high yields, we expect high soybean yields in 2018.

Another factor favoring soybean crop prospects this year is the rapid pace of crop development. Planting did not start early this year but it was finished early, and with May and June warmer than average, the crop was off to the races, with 44% of the crop flowering by the end of June. This continued into July, with 44% setting pods by July 15 and 66% setting pods by July 22. The 5-year average shows 24% of the crop setting pods by July 22, and about 50% by August 1. So the 2018 crop reached 50% podsetting about two weeks earlier than normal.

An early start to podsetting should be favorable, as long as there’s enough water to keep the crop in good shape. The amount of time between podsetting and loss of green leaf color (both recorded by NASS) estimates the duration of the seedfilling period, which is well-correlated with yield. On average, the Illinois reaches 50% leaf color change by about September 15, or about 45 days after 50% podsetting. The 2018 crop needs only to fill seed through the end of August to reach 45 days. With somewhat higher temperatures and longer days in August than in September, having seedfilling begin and end two weeks early should be favorable for seedfilling rate and yield.

We don’t very well understand what signals the end of seedfilling, but both temperature and daylength have some influence. The number of pods that are filling, maturity rating of the variety, and other factors have some effect as well. If temperatures remain normal and the crop has enough water, the need to have days shorten to a certain length before seedfilling stops should mean a somewhat longer filling period this year; this should add yield. But with the early start, seedfilling should be adequate for good yields this year even if it ends by early September.

Other positive signs for the soybean crop this year are the excellent plant stands in most fields, the excellent condition and color of the canopy, and the large number of pods already formed and still forming. The canopy is outstanding in most fields now, and we expect it to take on a darker green color as podfilling gets up to full speed over the next two weeks. I have not heard much about fertilizer nitrogen application this year, but with the canopy in such good shape, it seems unlikely that the crop will need extra N. A fair number of fields have received fungicide, and probably insecticide as well, though we can probably call the “race” between canopy development and insect feeding in favor of the crop this year.

Soybean plants have grown tall due to warm temperatures and adequate water in most fields. Plants are at or close to their final height in fields where seedfilling has gotten underway. Periods of drier weather and warm temperatures have provided enough competition for water to keep the canopy in most fields from getting heavy enough to cause internal shading that can limit pod formation or seedfilling rates. We can probably expect some lodging, especially in 30-inch rows where plants are already tall. Moderate lodging as pods fill is a signal that pods are heavy, so is not a concern, especially if plants just “lean” without affecting light interception.

Counting pods and seeds on plants in mid-season is neither a lot of fun nor an overly accurate way to estimate soybean yield potential. But with pods setting early this year, it’s a little easier to see how yield potential is shaping up. Pod numbers and number of seeds per pod appear to be very good in most fields. We’d like to see 4-5 pods filling at each middle node, and 30 to 50 pods per plant. A field with 130,000 plants per acre, 40 pods per plant, 2.8 seeds per pod, and 2,700 seeds per pound at harvest projects a yield of 90 bushels per acre. While we don’t expect such yields in most fields, we have seen yields this high or higher in some fields in each of the past four years. Based on what we see now, we expect to see this in some fields in 2018 as well.

Ewing Demonstration Center Celebrates 50 years of Continuous No-till Research at Agronomy Field Day on July 26

The University of Illinois Extension will host the Ewing Demonstration Center Agronomy Field Day on Thursday, July 26, 2018 at 9 a.m.  Every growing season presents challenges to production, and this year is no exception!  We are happy to host this summer field day to share with local growers current, ongoing agronomy research in southern Illinois, including cover crop trials on corn and soybeans, nitrogen management in corn, weed management in soybean, and our continuous no-till field, now in its 50th year of continuous no-till production.

We are highlighting our 50th year of continuous no-till production in our field day this year.  This no-till trial area was established in 1969 by George McKibben, the “Father of No-Till”, long-time agronomist and researcher at the Dixon Springs Ag Center in southern Illinois.  This plot has been cropped utilizing no-till production of corn and soybeans ever since.  The “zero-till” system as it was first called, was researched to “save the soil” that was lost over the many years of intensive tillage required to raise grain crops on the sloping hills of southern Illinois with the planting equipment available at the time.

In honor of this milestone, we will have the original “zero-till planter” on display.  This planter was modified and built in the early 1960s at the Dixon Springs Ag Center and used there and also at the Ewing Demonstration Center and other research sites.  The demonstrated success of this zero-till planter and production system was one of the inspirations that led companies like Allis-Chalmers and John Deere to start engineering and producing no-till planting equipment.  Also, joining us for the field day will be Donnie Morris, retired farm mechanic and engineer who built this planter, along with other retired Extension advisors and educators that worked at the Ewing Demonstration Center over the years.


The topics to be discussed at Field Day include:


Looking Back at 50 Years of Continuous No-till

  • Current and Retired Staff, University of Illinois

Insect Management in Corn and Soybean

  • Nick Seiter, Research Assistant Professor, University of Illinois

What We Have Learned After 48 Years of Continuous No-till

  • Ron Krausz, Manager SIU Belleville Research Center
  • Sarah Dintelmann, Undergraduate Assistant ,Weed Science, SIU

Managing Cover Crops in Corn and Soybean

  • Nathan Johanning, Extension Educator, University of Illinois

Intro to Corn Genetics:  Why is Sweet Corn Sweet?

  • Talon Becker, Extension Educator, University of Illinois


Please join us for Agronomy Field Day to help celebrate this milestone in crop production!  The field day is free and open to anyone interested, and lunch will be provided.  Certified Crop Advisor CEUs will also be offered (Soil & Water – 2.0; IPM – 0.5, Crop Management – 0.5).  The Ewing Demonstration Center is about 20 minutes south of Mt. Vernon located at 16132 N. Ewing Rd; Ewing, IL 62836, on the north edge of the village of Ewing, north of the Ewing Grade School on north Ewing Road.  Watch for signs.

To help us provide adequate lunch and materials, please RSVP to the University of Illinois Extension Office in Franklin County at 618-439-3178 by Tuesday, July 24.  For additional information on the field day, contact Marc Lamczyk at the Franklin County office or

Tips on making fungicide application decisions in field crops

We are at that time in the season where many people will be making final decisions regarding fungicide applications in soybeans and corn.  I wrote an article with tips and other items to consider when making fungicide decisions and on farm trials on the Illinois Field Crop Disease Blog, which can be found by clicking HERE.  

Soybean Cyst Nematode: Race Shifts and Grass Cover Crops as a Potential Alternative Control

Authors: Talon Becker and Nathan Kleczewski

Among the various soybean pests, Heterodera glycines Ichinohe, known by most as soybean cyst nematode (SCN), continues to be a persistent cause of yield loss for soybean producers1.  SCN has been found in every county in Illinois, as well as much of the eastern United States, Puerto Rico, and parts of Hawaii and Canada2 (Figure 1).

Figure 1.

Map of the known distribution of the soybean cyst nematode, Heterodera glycines, in the United States and Canada from 1954 to 2017. Known infested counties are indicated in red. Map © C. C. Marett and G. L. Tylka, Iowa State University, 2017.


It is widely understood that the recommended control measures for this pest include rotation to non-host crops, often corn in this part of the country, and use of resistant varieties.  However, what is not generally understood is the importance of rotating the source of the resistance genes introgressed into those resistant varieties.  If a given field is planted year after year with soybeans that derive their SCN resistance from the same source (i.e. PI 88788), even if it is in a rotation with a non-host crop, a “race shift” in the SCN population may occur.  The term “race shift” is derived from the terminology used to describe and differentiate SCN populations.  Diagnostic tests were developed in the 1970s3, and updated in the late 1980s4, which classified SCN populations into “races” based on their ability to reproduce on a specified set of resistant soybean cultivars, called “indicator lines”.  This classification system was updated again in the early 2000s5 with the use of HG Types, instead of races, for SCN resistance classification.  The updated system also included an updated list of indicator lines (Table 1).  HG Type is still determined by a given SCN population’s ability to reproduce on the indicator lines at a threshold of 10% compared to the standard susceptible check variety.  For example, an SCN population with the HG Type of 1.2.4 would be able to reproduce on PI 548402 (Peking), PI 88788, and PI 437654 with at least 10% efficiency compared to the susceptible check.

Table 1.

Number   Indicator Line
1 PI 548402 (Peking)
2 PI 88788
3 PI 90763
4 PI 437654
5 PI 209332
6 PI 89772
7 PI 548316 (Cloud)
Indicator lines for HG Type classification of genetically diverse populations of Heterodera glycines. Niblack et al., 2002

By exposing the SCN population in a given field to the same resistance or control mechanism, one imposes a selective pressure on that population.  What must be understood is that in any population, genetic diversity exists.  In a given SCN population, there is likely genetic diversity associated with the nematode’s ability to feed/reproduce on a given SCN resistant cultivar.  Even if the genetic advantage is marginal and only present in a small proportion of the population, over several generations, the gene or genes responsible for the nematode’s ability to overcome the soybean’s resistance mechanism become enriched in the population.  For example, if a population of SCN is made up of individuals from HG Types 1, 2, and 4, in equal proportion, but only individuals from HG Type 2 can reproduce well on PI 88788 soybeans, several seasons of using soybeans with PI 88788-derived resistance, even in a corn/soy rotation, would likely result in an increase in the proportion of the population that would be characterized as HG Type 2 (Figure 2).

Figure 2.

A. First season growing soybean variety with PI 88788-derived SCN resistance. B. Second consecutive soybean crop, following rotation to corn, using a variety with PI 88788-derived SCN resistance. C. Third consecutive soybean crop, following rotation to corn, using a variety with PI 88788-derived SCN resistance.  SCN population has shifted to be a majority HG Type 2.  This figure is meant to illustrate the concept of “race shift” and is not derived from actual data.


In addition to rotating sources of SCN resistance, other management practices for this pest may include the use of certain cover crop species grown during the fall and winter months.  While data on the efficacy of certain cover crop species for suppressing SCN populations in the field is often inconsistent, there have been some studies exploring this subject.  One such study was conducted by Nathan Johanning and Marc Lamczyk at the University of Illinois Ewing Demonstration Center in 2014-2017.  This research investigated the effect of three grass cover crops on yield and SCN egg counts (performed by the Illinois Plant Clinic) taken at harvest using full-season soybean under a no-till system.  Treatments included cereal rye (70 lbs/A), triticale (70 lbs/A), annual ryegrass (15 lbs/A), and no cover crop.  Each treatment was replicated four times in a given year using strip plots measuring approximately 10’ x 270’.  The experimental plot was rotated each year so that plots could remain in their corn, soy, wheat rotation. Results show no significant effect of cover crop on yield in the first three years of the experiment, but a significant yield decrease was seen in annual ryegrass plots in 2017.  This result is also seen when yield data from all years are combined, as well as when data from 2014-2016 are combined, indicating this effect is persistent although not statistically apparent in every growing season alone (Table 2).  The negative yield effect associated with this cover crop in 2017 was likely due to increased vole activity (possibly because of mild winter) and subsequent stand losses (observational, no data available), as this grass appears to be a preferred habitat for this rodent.  This may also explain the significant yield reduction following annual ryegrass seen in the 2014-2016 combined analysis.  Although the stand losses were not as noticeable in those years, localized stand loss in the center of plots would not be as apparent but would likely still affect plot yield.

Table 2.

Mean Separation of Yields (bu/A)
Treatment 2014 2015 2016 2017 All Years 2014-2016
No Cover 52.52 A 48.06 A 51.42 A 47.90 A 49.97 A 50.66 A
Triticale 51.76 A 46.06 A 51.43 A 49.26 A 49.63 A 49.75 A
Annual Ryegrass 50.18 A 42.91 A 44.20 A 29.88 B 41.79 B 45.76 B
Cereal Rye 53.17 A 48.85 A 49.90 A 49.39 A 50.33 A 50.64 A
Conducted using SAS University Edition: PROC MIXED; Type 3 SS; Year, Year(Rep), and Year*Treatment = RANDOM; Treatment = FIXED.  Main effects and interactions containing ‘Year’ were not included in individual year analyses.  Different letters within a column indicate significant differences (α=0.1) based on a Tukey’s multiple comparison test.

As mentioned above, SCN egg counts were also performed on soil samples taken at soybean harvest.  These data were highly variable, making it difficult to draw any conclusions in which we have high confidence pertaining to the effect of these cover crops on SCN populations.  A significant effect was only observed in 2014, wherein plots that had contained any of the three grass cover crops the previous fall showed significantly lower SCN egg counts at soybean harvest (Table 3).

Table 3.

Mean Separation of SCN_Harvest (SCN eggs/100 cc soil)
Treatment 2014 2015 2016 2017 All Years
No Cover 6150 A 50 A 160 A 1210 A 1892.5 A
Triticale 1410 B 40 A 280 A 1760 A 872.5 A
Annual Ryegrass 370 B 130 A 260 A 1230 A 497.5 A
Cereal Rye 720 B 60 A 10 A 200 A 247.5 A
Conducted using SAS University Edition: PROC MIXED; Type 3 SS; Year, Year(Rep), and Year*Treatment = RANDOM; Treatment = FIXED.  Main effects and interactions containing ‘Year’ were not included in individual year analyses.  Different letters within a column indicate significant differences (α=0.1) based on a Tukey’s multiple comparison test.

It is worth noticing that this is also the only year where the ‘No Cover’ treatment showed substantial SCN egg counts, although that did not translate into a significant decrease in yield (Table 2).  In years where SCN pressure was relatively low, such as 2015 and 2016, egg counts were more or less unaffected by the grass cover crops.  Egg counts in 2017 were in the “low to moderate” range, according to the University of Illinois Report on Plant Disease titled “The Soybean Cyst Nematode Problem” (RPD No. 501).  And in this year, cereal rye plots showed a relatively low egg count compared to the other treatments, but this was not statistically significant due to the large variation in egg counts between replicates.  This variation also contributed to the lack of statistically different means across all years of the study.  Despite this, comparison of the four year average egg counts show triticale, annual ryegrass, and cereal rye plots to have had 46.1%, 26.3%, and 13.1% SCN eggs/100 cc soil, respectively, compared to the no cover (Figure 3).  The results of this study suggest that cereal rye would be a preferred cover crop to control SCN before soybean, with repeatedly low egg count averages and no negative yield effect.

Figure 3.

There is no standardized threshold for SCN egg counts after which damage is likely to occur, but 2,000 eggs per 100 cc of soil is considered “moderate to high”, according to RPD No. 501, and counts above this have been called “cause for concern” by the Illinois Soybean Association (ISA).  This statement came in a press release announcing an ISA checkoff-funded study, led by Dr. Jason Bond of Southern Illinois University, that will further explore the relationship between grass cover crops, including wheat, and SCN.  Larger studies like these are necessary for better understanding how cover crops may interact with SCN under a wide range of soil types, weather conditions, etc., particularly when dealing with a highly variable indicator of potential damage, such as SCN egg counts.  Hopefully, from continued research into alternative control methods of SCN, such as cover crops, we will be able to add more tools for combating this pest to our proverbial toolbox, thereby reducing the farmers’ dependence on non-host rotation and host plant resistance.


If you would like to test your field(s) for SCN egg counts or HG Type, samples can be submitted to the University of Illinois Plant Clinic.  Samples should be taken from a field following harvest, the season before soybeans are to be planted in that field to allow time for the development of a control strategy.  More information on sampling methods and service fees can be found on the Plant Clinic’s website.


University of Illinois personnel will also be conducting an SCN survey this summer in concert with several other pest surveys.  If you are a landowner and are interested in allowing traps to be placed on your land and/or soil samples to be taken from your fields, please contact Kelly Estes at



  1. Allen, T. W. et al. Soybean Yield Loss Estimates Due to Diseases in the United States and Ontario, Canada, from 2010 to 2014. Plant Health Prog. (2017). doi:10.1094/PHP-RS-16-0066
  2. Tylka, G. L. & Marett, C. C. Known Distribution of the Soybean Cyst Nematode, Heterodera glycines, in the United States and Canada, 1954 to 2017. Plant Health Prog. (2017). doi:10.1094/PHP-05-17-0031-BR
  3. Golden, A. M. & Al, E. Terminology and identity of infraspecific forms of the soybean cyst nematode (Heterodera glyecines’). Plant Dis. Report. 54, 544–546 (1970).
  4. Riggs, R. D. & Schmitt, D. P. Complete Characterization of the Race Scheme for Heterodera glycines. J. Nematol. 20, 392–395 (1988).
  5. Niblack, T. L. et al. A Revised Classification Scheme for Genetically Diverse Populations of Heterodera glycines. J. Nematol. 34, 279–288 (2002).


Diagnosing disease related issues in the field

Well, it is that time of year where we start to see issues developing in the field.  Questions such as, “What happened?”  and  “Why me?” will become more common.  The key to managing diseases is proper diagnosis, and this starts in the field.  In my recent post on the Field Crop Disease Blog, I provide several tips for diagnosing issues in the field, and distinguishing disease related problems from abiotic issues.  Check out the post, and sign up for updates!