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Today there were two first observations for some important corn diseases in surrounding states including tar spot and southern rust.  Click here to access my article posted on The Bulletin regarding these issues.

So far it is looking like we avoided major Fusarium head blight issues in those counties that have already harvested in the Southern and central parts of the state.  Aside from a couple of rejected loads, quality appears to be decent.  Parts of N. Illinois appear to have elevated FHB, but again, not the widespread epidemic that we were anticipating.    Towards the end of the growing season I heard a lot about severe FHB in some parts of S. Illinois, but upon visiting sites, it was apparent that fields were far past the point where FHB damage can accurately be assessed.  In general I tell people to make assessments 18-24 days after 10.5.1 (flowering) as this assessment point gives the fungus enough time to colonize and produce easily rateable and diagnostic symptoms.  Symptoms can start earlier or later, but that 18-24 day window in my experience is the best in terms of assessing FHB damage.  Later than that and you end up with confounding effects of senescence and secondary fungi that can make field severity look worse than it actually is, whereas assessing too early may underestimate severity.

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Crops are finally starting to pick up growth throughout the state.  In parts of the south we have tasseling corn, while we will hit VT soon in some of the earlier planted corn fields in central and Northern parts of the state.  Soybeans are starting to put on growth as well, with some starting to flower in areas.

In corn, field scouting for should begin at least 2 weeks before tasseling and continue until at least 2 to 3 weeks past tasseling.  If you haven’t checked your fields and have yet to hit VT, now is a good time to get out there.

There are a few diseases that are just starting to pick up in parts of the state.  In the south, we have pockets of Diplodia  leaf streak  starting up.  For an article and images of Diplodia click here.  This disease seems to be increasing in incidence in parts of Illinois and other surrounding states.  If you have any suspect samples, feel free to sent it to our lab or the University of Illinois Plant Diagnostic Clinic.

Common rust can be found at extremely low levels and as I mentioned in a previous article I wrote for The Bulletin, this isn’t something to be too concerned about in the majority of cases.

Grey leaf spot is starting in many fields, although I have not seen or heard of reports of severe infections.  Last year the environment was extremely conducive for this disease, and it was easily the most impactful issue in Illinois last year.  As corn approaches VT, keep scouting for this disease.  Lesions are rectangular, limited by the veins, and can range from brown to grey depending on age and other factors.  Grey leaf spot does best in warm (75-85F), humid conditions and frequent rainfall. This pathogen has a long latent period after infection when symptoms are not visible.   This period may last from 2 to 4 weeks.

Northern corn leaf blight is hard to find, but it is out there at low levels.  Remember that this one produces larger, cigar-shaped lesions.  Northern corn leaf blight does better under more moderate temperatures (64-81F) compared to grey leaf spot, but similar to that disease, needs persistent, high humidity to infect.

There have not been any tar spot confirmations in Illinois or any surrounding states.  However, there have been an abundance of tar spot lookalikes.  The most frequent culprit is insect frass/feces, meaning that our insect populations are eating well and are regular.  Most often, individuals will see small, round, shiny to matte, black dots at very low amounts on a small number of plants in the field.  Sometimes these dots can be 1/8 inch or more if an extremely productive insect was involved.  The easiest way to check suspect tar spot leaves it to take it back to your vehicle and wet the leaves with some water, then gently rub the spots between your fingers.  Insect crud will wash off, tar spot stroma will not.  Alternatively, you can often scrape the feces off with your fingernail.  Still not sure? Send off a sample to the clinic.  Some other tar spot lookalikes have been what may be early infections by other diseases and dried up or old common rust pustules.  Again, if you aren’t sure, send it off for examination.  Pictures can be deceiving.

Soybean diseases have been few and far between.  I thought we may see more Phytophthora this season given the warm wet conditions during planting, but that prediction thankfully has not come true.  I guess I need to purchase a new Magic 8 Ball.  There have been some disease lookalikes, such as herbicide damage, as well as Phyllosticta in a few areas.  Phyllosticta can look like frogeye leaf spot, but instead of a silvery or grey mass on the underside of lesions, black specks are visible inside the lesions. Phyllosticta tends to like cool, wet conditions and is not considered to be of significant economic importance.

In parts of Northern Illinois soybeans will be flowering soon.  This means that white mold management decisions should be on your radar.  The Sporecaster tool can help you with in season management decisions.  To download this app for your phone, click here.

Continue to scout your fields and let us know if anything odd pops up!  You can always email me images or send images to me on twitter @ILplantdoc.  Phone calls are always welcome.

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The UIUC applied field crop pathology lab will be monitoring Tarspot risk this season though testing the Tarspotter forecast application under development by the University of Wisconsin Madison.  The forecast uses weather data to predict the likelihood of favorable conditions for tar spot infection.  Fields of highest risk for tar spot will be those that had severe tar spot infections in 2018, and are in no-till, corn after- corn production systems.  Please review my article on understanding these maps by clicking here .   Remember that this disease has a latent period of at least 14 days, meaning that the pathogen infects and you will not see symptoms for at least 2 weeks after the infection event.    With increasing temperatures and scant rain forecast until the middle of next week, I expect to see these values go down over the weekend and into next week.

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We are working with our colleagues at the University of Wisconsin-Madison to test and improve their weather based forecasting tool for tar spot on corn.  This tool is currently called Tarspotter and is not publicly available.  However, I will be posting forecasting maps weekly over the growing season.  In this article, I will discuss how to interpret the maps and determine your specific risk level.

First, we need to remember that tar spot is a residue-borne disease.  To our knowledge, the tar spot pathogen only overwinters on infected corn residue, and can overwinter at least one year under conditions in Northern Illinois.   Under cool, wet conditions, particularly at night, spores are extruded from black stromata on infested residue spread locally and at least nearby (yards).  If spores land on susceptible corn and the environment is conducive, infection can occur.  After infection, symptoms develop within 2-3 weeks.  We still have a lot to figure out regarding the biology of the pathogen, and we are working on aspects of this in our lab.  Since 2015, we have observed this disease every year in Northern Illinois to some extent, but typically not until late in the season, when things cool down and yield has been made in the crop.  However, as we saw last season, if tar spot starts early in crop development (Before R2-3) we can see significant yield losses.  Our delayed planting this season might mean that tar spot could be an issue again this year.  Therefore it is important to stay ontop of scouting your fields for this disease.

Based on what I describe above, it makes sense that corn fields that are no till, corn-after-corn, and that had tar spot last year will be at the highest risk for seeing tar spot earlier this season if conditions are conducive.  Where was tar spot detected last year? Below is a link to the distribution of tar spot in 2018.

Tar Spot established in the United States-2018

Remember that plant disease develops only when the pathogen is present on a susceptible host and the environment is conducive.   The amount of disease is related to the duration of time that is spent within optimal conditions.  The amount of initial inoculum also can influence disease severity and disease progress.

The disease pyramid. Plant diseases only occur when the right pathogen is present with a susceptible plant and the environment is conducive for disease. The amount of time spent in disease favorable conditions results in the overall amount of disease. Image N Kleczewski

The Tarspotter maps I will post will tell you the overall risk for seeing tar spot given that you have a susceptible host and the pathogen is present.  It reads similar to a weather forecast in that it provides a likelihood of seeing tar spot based on historical weather data (temperature, relative humidity, etc).  Thus, if you have a field of corn planted into corn residue that was infected with the tar spot pathogen last season and your risk level today says 90%, the model is telling you that you have favorable conditions for infection and you might see symptoms start on existing foliage in 2-3 weeks.

When risk is over 30%, and the corn is between V8 and R2-3, then an in season application of a mixed mode of action fungicide might be justified for suppressing tar spot. Recent studies indicate that fungicide applications made at VT/R1 are the most likely to pay (click here for more information).

Below is the risk map for today, 6/24/2019.  As you can see, conditions in Northern Illinois has been the most conducive for tarspot.  Conversely, even though it has been wet at our site in Champaign, other factors, likely temperature, have kept our overall risk for disease relatively low.

Tar spot risk map for 6/24/19

In sum, when you view the tar spot risk map, take into account your crop history with tar spot, severity of the disease last season, and corn growth stage at the point in time that the map was generated, to determine if in season management might be needed for this disease.

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With tight profit margins, and other issues affecting farm profitability, some producers that use iLeVO for helping manage sudden death syndrome (SDS) of #soybean have decided to skip including base fungicide treatments in an effort to save money.  Another reason for not including a base fungicide seed treatment is because iLeVO contains fluopyram, an SDHI fungicide.  Therefore, it should work on our other seedling diseases, such as Rhizoctonia, Pythium, and Phytophthora, right?

The answer to that question is unfortunately no.  Fluopyram, like many fungicide active ingredients, has a spectrum of activity, meaning that it is effective on some fungal pathogens and others.  Fluopyram is great against Fusarium virguliforme, the causal agent of SDS; however, it does not fair so well on other soilborne pathogens.  For example, fluopyram will not have activity against Pythium nor Phytophthora, which are common soilborne diseases under wet conditions.  It also will not protect you against Rhizoctonia, which is problematic under hot, somewhat wet conditions.

As an example, we planted a seed treatment trial last year to determine the impact of using iLeVO alone vs iLeVO plus base treatment (vs non-treated seed) on germination, root rot, SDS, plant growth, and yield.  The experiment was planted at the UIUC Field Lab located in Monmouth, IL.  The study was designed as a random complete block design with 4 replications per treatment.  Treatments included iLeVO (0.15 mg a.i. / seed), Base (Evergol energy 0.019 mg / a.i. per seed + Allegiance FL 0.02 mg a.i. / seed),  iLeVO + Base, and a non-treated control.  Plots consisted of 4, 30 inch rows 20 ft in length, and soybean planted at 152,700 plants / A.  At planting (5/18/2018), sorghum infested with F. virgliforme and Rhizoctonia was placed in furrow.   Stand counts were taken on 5/31 (V1) and 7/3 (R1).  For each plot, six seedlings were dug at V6, washed, and roots rated for root rot severity on a 1-5 scale where 1 = no rot and 5 = 80-100% no roots (based off of Dorrance ,2003).  Plants were dried to constant moisture and weights recorded.  All plots were harvested on 10/2/2018 using a Massey 8xP small plot combine, and test weights and yields determined. Data were statistically analyzed.  The results of that study are found below.

Although skipping your base fungicide treatment may save some money, it may expose you to other issues that can impact your bottom line.  Remember that different fungicide active ingredients control different fungal (and oomycete) pathogens differently- try saying that three times fast!

For information on soybean seed treatments and their efficacy against soilborne pathogens click the link below:

2019 Soybean Seed Treatment Fungicide Efficacy Table_final

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This week, growers and ag professionals have noticed round tan colored lesions with red margins and yellow halos on Illinois corn. These symptoms likely were a minor disease called Holcus spot. Holcus spot is a bacterial disease of corn that can occur on young corn and is rarely damaging.

Lesions start off as dark green, water-soaked spots that turn white to tan to grey with time. Lesions are often surrounded by a reddish boarder and a cream to white colored halo. Lesions are range from ⅛ and ½ inch in diameter. Holcus spot typically occurs early in the season after windy, warm, and wet weather (75-85°F). The pathogen resides on residue, where it can be splashed onto the lower parts of the plant. Therefore you may see more of these symptoms on no-till corn. The pathogen enters corn leaves through wounds or natural openings, but does not appear to spread from leaf to leaf.  Holcus is not the only source of these symptoms on corn.  Herbicide drift (e.g. gramoxone) can also result in similar symptoms.

Potential Holcus spot on a young corn plant.

Separating Herbicide drift and Holcus Spot
You can ask yourself the following questions to help you determine if the spots are a result of herbicide drift or Holcus spot:

1) Is the symptomatic corn near a field that recently received a burndown herbicide application? If the answer is yes, then you might suspect herbicide drift.

2) Are the symptoms more severe at the field edge or uniformly distributed throughout the field? If the symptoms are more severe on the outside of the field and gradually decrease as you move towards the center of the field, this may indicate potential herbicide drift.

3) Are plants around the field (ditches, driveways) showing symptoms? If yes, then again, the potential culprit is herbicide drift.

Regardless of the source, corn showing potential herbicide drift injury or Holcus spot are not at risk. Plants will grow out of herbicide drift and Holcus spot is not known to be yield-limiting. Although the lesions may cause concern to some, within-season management is not available nor is it required.

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Persistent humid weather and rains over the last 14 days has significantly elevated the risk of Fusarium head blight (FHB) in the south central portion of Illinois.  Symptoms of FHB will start to develop around 18-24 days from the start of flowering, depending on temperatures and the variety (see figures below for examples).  Understanding the field-level severity of FHB can be very useful in making harvest based management decisions.

Here are some general guidelines for assessing FHB in your wheat fields: 

1) Try and assess the crop 18-24 days after the start of flowering (when 50% of main tillers start to flower) before plants have started to senesce.  It is much easier to estimate FHB when heads are still green.

2) Take a paper bag and run a transect approximately 50 yards long through the field.  Run one transect per 10 acres of field.  Ensure that the transects are randomly selected but are representative of the field.

3) As you walk each transect, every so often pick a wheat head without looking down and place into your bag.  Pick 10-20 heads per transect.

4) Once you have collected all the heads, take them back to your vehicle.  Record the percent of the head with visual symptoms of FHB (bleaching, pink growth on spikelet).

5) After you have recorded all of your heads, take all the data from your heads and determine the percent FHB severity (sum of ratings of FHB for all heads including 0’s/total number of all heads collected).

Let’s illustrate on a small scale.  You rate 10 wheat heads.  The percentages of FHB for each of the 10 heads are: 0, 0, 1, 30, 20, 10, 30, 0, 0, 5, and 10.  Add these up =  106

106 / 10 heads = 10.6% FHB severity

The greater the amount of FHB severity the more likely you may have elevated DON and issues with yields and test weights.  A field level severity of 10% or more is where we typically start to see significant losses and problems related to FHB.

After rating your fields, you may have some that have elevated FHB and some that do not.  This is where harvest-based management can help.

1) Research has shown that in cases where an FHB epidemic occur, changing your combine settings can result in an economic benefit to producers (click here for summery) .  For example, in a study conducted by our colleagues at The Ohio State University, showed that, although some loss in yields occur when increasing combine fan speeds and increasing shutter openings to remove FHB-damaged grain, the improvement in grain quality and reduction in financial losses from price discounts (vomitoxin-DON, test weight, etc) can be great enough to compensate for the reduction in overall yields.   The exact settings needed to optimize removal of FHB infected grains will differ from combine to combine, but in general, when harvesting fields with severe FHB infections, you should consider increasing fan speeds and shutter openings.

2) Harvest FHB affected fields as early as possible.  The longer infected wheat stays in the field, the more potential DON can be produced in grain.

3) Ensure that harvested grain is dried and stored below 18% moisture.  Dry conditions stop the growth of the fungus and potential DON that can be produced in storage.  It will not reduce DON, but will prevent additional development of DON.

4) Separate fields severely affected by FHB from fields where FHB levels were low/absent.  This is easier said than done, but should be considered in cases where only selected fields are affected.

A spikelet showing signs and symptoms of Fusarium head blight infection. Note the orange growth on the spikelet. Wheat heads showing bleaching above the point of infection of the rachis
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Wheat in the southern part of Illinois is approaching heading or just headed, which means that flowering will start within 4-5 days, depending on temperature (heads produce small yellow anthers-(Figure 1)) Vomitoxin (DON) can accumulate in the absence of significant visual symptoms, and thus that is often the focus of my Fusarium head blight (FHB) recommendations.  Why?  Because pink or lightweight, FHB-infected kernels can be cleaned at the elevator, but plump, healthy kernels containing DON cannot.  This reduces the overall quality of the grain and ultimately, grower profits in the long run.

In order to understand how to manage DON, you need to realize that FHB is a fungus that overwinters not only on wheat residue, but also corn (and to a lesser extent soybean) residue.  When conditions are wet and warm during the flowering period, FHB infections are most severe.  Experience indicates that if sufficient rain falls and cool temperatures occur, FHB outbreaks are still a possibility.  The wet conditions “wake up” the fungus, and it starts to produce spores and spore bearing structures on residue.  These spores can be rain dispersed locally or forcibly ejected.  The latter spores can travel several miles and settle onto heads via atmospheric deposition during calm nighttime periods.  When spores land on heads that are flowering, they germinate and infect wheat heads.  The fungus can can infect individual spikelets (Figure 2) and or the rachis (the stem where the spikelets attach).  Colonization of the rachis can impede movement of water and nutrients and result in dry or bleached heads above the point of infection (Figure 3).  You may see an orange to pink growth on infected spikelets (Figure 2).  DON can be produced by the fungus after infection, typically following alternating wet and dry periods before harvest.

During the season FHB can be SUPPRESSED by making an application of either Caramba, Miravis, Prosaro, or Proline from the start of flowering (Feekes FGS 10.5.1, when approx 50% of MAIN tillers have started to flower) through six days after this point in time.  Replicated university trials still confirm that this is the best application window to target for FHB and DON suppression, regardless of product.  The Fusarium head blight prediction center  can be used to help you decide if a fungicide application for suppression of FHB is warranted.

Between seasons you should be selecting a high yielding, moderately FHB resistant variety  .  The combination of a moderately resistant variety and a well timed FHB fungicide application can reduce FHB by over 70% on average.

Other things you can consider for further suppressing FHB:  Plant wheat after soybeans because the pathogen does not grow as well on soybean residue or tillage to bury residue.  Both practices can reduce local inoculum but will not impact deposition of spores from distal sources.

It has been wet, but cool and most of the crop has yet to reach the critical time for FHB infection.  Continue to monitor the FHB prediction center as your crop approaches flowering to determine if fungicide applications are needed in your fields this season.

Figure 1. A wheat head in the process of flowering. Note yellow anthers with pollen attached. Figure 2. A spikelet showing signs and symptoms of Fusarium head blight infection. Note the orange growth on the spikelet. Figure 3.  Wheat heads showing bleaching above the point of infection of the rachis
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Currently, most small grains are approaching jointing or just past jointing in many parts of the state.  Now is the time that you most likely will start to see early season viral diseases in some fields, specifically Wheat Soilborne Mosaic Virus and Wheat Spindle Streak Virus. These diseases are transmitted by soil borne microbes that thrive in cool, wet conditions. Infected plants typically are chlorotic and may be stunted. Often affected plants occur in low lying areas of the field or areas suffering from compaction. However, on some occasions entire fields can become symptomatic. Symptoms of spindle streak include necrotic dashes that run along the venation, giving the appearance of a spindle (Figure 1). Additional symptoms of soilborne mosaic virus are less conspicuous, but include mottling of lower foliage (Figure 2). Symptoms, including chlorosis and even stunting can look very similar to nutirent deficiencies.  Symptoms cease once temperatures are above 65°F and may be reduced after fertilization.  Even though symptoms may be reduced, it does not mean the viruses and their effects are removed from the crop.  I consider these diseases hidden yield robbers due to the fact that they often go misdiagnosed as nutrient issues and their effects often go unnoticed.    Confirmation can only be made through specialized testing methods such as ELISA and PCR.

How should you send a virus sample?  The key for getting good virus results is to keep the sample cool and avoid excess moisture.  If the samples get warm or are kept too wet microbes can degrade the viruses, resulting in false negative results.    Place 15-20 leaves in a plastic bag with a dry paper towel and place immediately in a cooler on ice.  Make sure to ship samples overnight on ice early in the week so that they are processed immediately and not allowed to set at room temperature.  Our lab is conducting a wheat virus survey in 2019 and we are seeking people to help us sample fields.  If you are interested in helping, send me an email .

What should you keep in mind? 1) Keep track of fields with these viruses. Once the viruses are established they will be present in those fields from here on out.  2) Try to harvest or work in these fields last to prevent spread to other fields. 3) Avoid compaction. 4) Plant tolerant varieties in fields with a history of these viruses. Unfortunately many varieties are screened for these viruses as a complex, so it may be difficult to determine if you are planting a spindle streak or soilborne mosaic tolerant variety in these cases.  However, the University of Illinois is one of the few places that does conduct screening for these viruses.  More information on wheat varieties for Illinois can be found by clicking here.

Figure 1. Symptoms of wheat spindle streak in wheat. Figure 2. Symptoms of wheat soilborne mosaic virus. Note stunting and chlorosis that can be misdiagnosed at nitrogen deficiency.
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We are offering a second free webinar on tar spot of corn for those of you who were unable to attend today’s meeting.  To access the registration click here

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