Tar spot risk in Illinois 6/21/19

As many of you are aware, our tar spot coalition is working to refine a tar spot model developed by colleagues at the University of Wisconsin Madison this season.  This model, currently named Tarspotter, uses weather data to predict risk of tar spot disease on corn.  For more information on the model and fungicide applications click here

Tar spot risk map for 6/21/19

Much of the corn in Illinois is just emerging, but some is past the V6-V8 stage.  This is the point in time where paying attention to the weather, and future forecasts can help you determine if a fungicide application for disease management is needed.  Remember that there is an approximate 14 day latent period with tar spot, meaning that if your plants are infected today, you likely will not see the black tar spots on  for another 2 weeks.  Tar spot does not like the warm weather moving through much of the state, so I do not expect the values in the middle part of the state to change much, even with the rains forecasted over the next five days.

I have heard reports (unconfirmed) of common rust in parts of the state, and Southern rust appears to be starting up in Texas.   It is very important to be in your fields scouting for diseases prior to the VT stage to help you know if you need to pull the trigger on a fungicide this season.

New mapping system to monitor corn rusts

This season we will be using a new system to monitor and  track southern rust in Illinois #corn.  We will be operating and managing a site that clientele can access and view in real time.  I will provide information pertaining to the site once it is operational in a few days.

Why do we need to monitor rusts?  Because rusts are obligate fungal pathogens.  This means they need a living, green host to grow and reproduce.  Rusts do not survive our Illinois winters and instead overwinter in warmer southern locations.  Once temperatures increase, spores from the south are blown north.  Corn planted in the United states can then be infected as the pathogen infects new corn, produces more spores, and moves on air currents to new areas and fields.  Therefore, by tracking rusts we can determine if and when fields may be at risk.  This information can help make timely fungicide applications or avoid unneeded applications.   This process takes time and often rusts do not reach our corn until later in the season, once yield has been made.  However, due to our late planting throughout the state we may see these rusts infect plants at earlier stages in growth and development, which may result in greater potential yield impact if disease occurs.  This is why scouting your corn this year will be especially important.  


Example lifecycle of Southern rust. Note that it blows in from warmer, southern regions.

In the meantime, any growers, consultants, ag professionals in Illinois that observe rust on corn should send a picture to myself at nathank@illinois.edu OR tweet a picture to me @ILplantdoc and @corndisease with #southernrust hashtag.  Southern rust, common rust, and other diseases may not be easily identified by images, and may require expert analyses.  Samples of corn rust can be sent to the UIUC Plant Diagnostic clinic– instructions found HERE.

Common vs Southern rust in corn. Due to late planting we may see both earlier than normal during the season, but tracking the diseases and their progress is critical for rusts.

Fungicide efficacy for foliar corn and soybean diseases

Each year our team of plant pathologists collaborate to develop efficacy tables for fungicides that can be used to help manage fungal diseases in field crops.   Our most recent sheets can be found by clicking the following links:

CDWG Fungicide efficacy table_2019_final

Soybean Fungicide efficacy table_2019_final

These sheets will eventually be housed for download on the Illinois field Crop Disease Page along with factsheets, applied research data and guides,  scouting videos, and other updates in the near future.  Consider bookmarking this page, and signing up for email alerts when new articles or posts are available.

It is great to see crops finally coming out of the ground.  Let’s hope plant diseases are not major issues for you this year!


Fusarium head blight-assessing FHB in your field and late season considerations

A few weeks ago we wrote an article on how to assess severity of Fusarium head blight (FHB) in small grains as well as some practices to consider that can help improve potential profitability in cases where outbreaks are severe.  Now that symptoms of FHB are starting to develop in the earliest flowering wheat, it is a great time to assess your fields and determine if any considerations for harvest need to be made.  To access the article click here.

What impact will late planting have on crop diseases?

Most Illinois producers are behind in getting corn and soybeans into the ground this year as a result of persistent rains and cool temperatures.  Some people are wondering what this might mean for some of the diseases we encounter in our field crops.

Keep in mind, disease occurs when you have the correct host, plant pathogen, and environment together.  The longer those three factors are together, the more disease will occur.  Although we cannot speculate much on the long term environmental conditions we will face this year and how that will impact diseases, we can make some educated guesses on how late planting could potentially impact some diseases.  Let’s take a look at our magical plant disease crystal ball, shall we?

  1. Soybean cyst nematodes in soybean and other nematodes in soybean and corn.  These organisms grow and reproduce in/on crop roots, and will continue to reproduce and damage plants over time.  One potential impact of late planting is that these organisms will have less time to damage plants prior to harvest, and therefore their overall impacts may be reduced compared to other seasons.
  2. Sudden death syndrome (SDS).  The fungal pathogen that causes this disease is favored by cool, wet soils.  These conditions also reduce soybean germination, growth and development early in plant development.  Planting later in the season hedges your bets of encountering warmer soils, which are not as favorable for the SDS pathogen.
  3. Pythium species on corn and soybean.  Similar to SDS, Pythium diseases tend to be favored by cool wet conditions.  However, unlike SDS, Pythium seedling diseases are caused by a complex of fungi.  We now know that a single field can host many different pathogenic species of Pythium, which may differ in terms of their optimal temperatures to cause disease.  Consequently, if wet weather is encountered soon after planting, regardless of temperature, issues with these pathogens may still occur.
  4. Residue borne foliar diseases and stem diseases.  There are numerous foliar diseases that can impact soybean and corn, and many stem diseases that may cause issues in soybeans.  Unlike some of the other diseases I mentioned earlier, many of these diseases will be affected by the environmental conditions during the growing season and the when favorable conditions occur relative to plant growth and development.
  5. Rusts.  The rusts that can impact corn and soybean blow in from warmer regions to our south.  Many rusts arrive later in the season, typically after yield has been made, or after in season management decisions have been made/fungicides applied.  A good example of this is common rust in corn.  However, as many of you experienced with southern rust a few years ago, when these diseases arrive earlier in plant development, and conducive conditions persist, they are more likely to result in yield losses.  Consequently, late planting could potentially result in plants being exposed to rusts earlier in growth and development. There has not been much activity with rusts in the south this season, which hopefully translates to less potential issues with these pathogens during the season.  Of course, similar to the residue borne foliar diseases,within season weather is essential for disease development during the growing season.


Cool season viruses in wheat

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.  The University of Illinois Clinic and Agdia are two options for having samples tested.


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 .

Figure 1. Symptoms of wheat spindle streak virus in wheat.

Figure 2. Symptoms of wheat soilborne mosiac virus.

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.