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.

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

Physoderma reports on the increase in the region

Physoderma brown spot and node rot is a disease that has been increasing in incidence in the Midwest over the past 5-10 years.  We observed this disease frequently during scouting trips throughout the state conducted in late July.  Recently, we have been receiving more reports of this disease and questions pertaining to it’s impact on the crop and management.  I recently wrote an article on this disease and management recommendations on the Illinois Field Crop Disease Blog.  Click HERE to access the article.


Scout those fields!

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.

Corn disease updates

This week there has been a slight uptick in the amount of foliar disease reports in corn, likely as many people are actively scouting prior to making a fungicide application decision.  The most common disease, as you may expect is Grey leaf spot.  This disease is present to varying degrees in most fields.  No big surprise there.

We have seen and received several reports and samples of Diplodia leaf streak.  This disease can be caused by two different species of Diplodia, D. macrospora and D. maydis.  We have thusfar identified those samples where we were able to acquire spores as D. macrospora, but we have many more samples to assess.  Diplodia leaf streak can easily be misdiagnosed as Grey leaf spot, Northern corn leaf blight, or other foliar diseases and disorders.  Characteristic foliar symptoms include oblong, irregular lesions with green/yellow edges.  Occasionally, blocky lesions are observed.  Often “targets” can be seen in the lesions, where the initial infections occurred (Figure 1).   Older lesions contain black pycnidia, which resemble tiny pinheads or dots.  You will not see these in Grey leaf spot or Northern corn leaf blight lesions.

Diplodia infections can appear blocky. Sometimes targets can be observed within the lesions. Photo E. Alinger.


The other disease that we have started to look for more intensely is Southern rust.  Everyone should be familiar with this one, as it hammered parts of Illinois in 2016.  Remember, that year the corn was not as far along, and there was a period of persistent heavy rains throughout many parts of the state.  The disease likes it wet and warm, typically 80 F or more, and the earlier it arrives on a plant the more damage is can cause.  Thus, the need for spraying also is related to when it arrives in a given field (Table 1).  Remember, Southern rust does not overwinter here.  It needs to blow in every year from the south.  When stripe rust arrives early, (VT or earlier), and warm, wet weather occurs, severe epidemics can result.  If the disease arrives late (R3 or later) the amount of potential damage and yield impacts are reduced.  Under the correct conditions the Southern rust fungus reproduces rapidly, producing  spores that can infect new tissues or plants (Figure 2).

Table 1.  The amount of damage and yield impact due to stripe rust is related to the time of arrival on the crop. From Crop Protection Network

Through the Southern rust iPiPe map,  we can see where Southern rust is located, preventing it from sneaking up on us without notice.   The only item that is not included in the map is the incidence and severity of the Southern rust reports, which provide additional information for determining the risk of disease.  For example, a single leaf with a few Southern rust pustules  OR a field with severe Southern rust infestations result in the the county being turned red on the iPiPe map (Figure 4).  For this reason, it is important to stay on top of reports and scout when you notice Southern rust is nearby.

Figure 2.  The Southern Rust Life cycle. Photo Crop Protection Network

This year conditions have not been very conducive for Southern rust in the United States.  Until a few days ago, no reports of the disease were present.  Over the last week, more reports have occurred, and extremely low levels of the disease were detected yesterday and today in Franklin and Bond Counties in Illinois.  Most corn in those areas was well past R1 and close to R3 in many areas.  In both cases, the disease was only detected in a single field in each county and was extremely difficult to locate.

Figure 4 The Southern Rust iPiPe map as of 7/19. Red=detected. Green = scouted none detected. Yellow = suspected, not confirmed


Signs of Southern rust include pustules, typically somewhat bunched on the foliage, what often have a light brown to orange color.  Pustules are circular, and tend to be found mostly on the upper side of the leaf blade, and may be surrounded by chlorotic halos.  Occasionally you can find the fungus infecting stalks.  When you touch a pustule of Southern rust with your hand, you will notice it having a dusty, rusty appearance (hence the name rust).  Common rust, which is rarely impactful to corn production in Illinois, produces scattered, red to brown pustules.  Pustules are often somewhat elongated, and can be located on both sides of the leaf surface.  It is important to be able to distinguish Southern rust from Common rust due to their different potential impacts on crop yield.


Southern rust pustules are often circular, orange, and located on the upperside of the leaf. Note the halos surrounding the lesion. Photo N Kleczewski


Although Southern rust currently is not considered to be a significant threat to the corn crop, it is important to continue to scout, as changes in the environment could cause the situation to change.  If you think you may have found Southern rust, email me at or tag me on the picture on twitter @ILplantdoc with an indication of the severity (amount of pustules on leaves) and incidence (approximate number of plants with symptoms).  Additional information on this disease can be found on the Crop Protection Network.

Keep an eye out for Tar Spot on Corn

Today we received reports of Tar spot appearing in corn grown in Illinois.  We are currently studying tar spot of corn in the United States, and are requesting that any CCA, producer, or ag professional that encounters this disease, even if outside of Illinois, please send samples to Diane Plewa at the University of Illinois Plant Diagnostic Clinic.

S-417 Turner Hall, 1102 S.Goodwin Ave.
Urbana, IL 61801
Phone: 217-333-0519

Please note  “Tar spot study” on the sample.


Update on wheat in Illinois

This past week we spent a few days surveying wheat fields throughout the state in order to see how the crop is progressing as well as better understand what disease related issues we may be experiencing.  Most of the crop was near flag leaf emergence (Feekes growth stage 8/9) with a few fields near boot in locations further south.  The good news is that of the 26 fields we looked at, none had any stripe rust, nor have I received any additional reports of this disease in the state.  In general, diseases were minimal.  In southwest portions of the state Septoria leaf blotch (aka speckled leaf blotch) was fairly common.

Septoria (speckled) leaf blotch is often found in the lower canopy when conditions are cool and humid. Photo N Kleczewski


This is a residue-borne disease that is favored by cool, wet conditions and can grow and persist on small grain residues.  The disease is often located deep within the lower canopy, and causes irregular brown lesions on the foliage.  At the center of the lesions you will often see black structures that may resemble tiny peppercorns.  These structures are why the disease has the extremely creative common name speckled leaf blotch.  The disease spreads upwards predominantly via rain splash, and seldom causes significant yield impacts.  This typically is due to increased temperatures that do not favor disease development as the crop develops and the flag leaf is produced.  Remember, the flag leaf and green tissues above contribute the majority of carbohydrates for grain fill (over 70% from the flag leaf alone).  Foliar diseases that do not reach these tissues are typically not a major concern.

Similarly, I came across a few fields with light powdery mildew.  Unlike Septoria leaf blotch, powdery mildew is an obligate pathogen and requires a living host to grow and reproduce.  Cool, humid (not wet) conditions favor powdery mildew development.  In general, production practices that favor rapid plant growth and lush, full canopies early in the season favor this disease.  For example, high nitrogen rates or manure use can result in rank growth early in the season.  Powdery mildew can reproduce more quickly than Septoria, and therefore can occasionally impact early season growth or tillering in some instances.  Although I did not see anything that would be of concerns and have not had any reports of severe powdery mildew, management is best achieved through selection of a resistant variety and avoiding excessive nitrogen application.   Early season fungicide applications with nitrogen applications can have some benefit when a field is at high risk for disease (i.e. susceptible variety, heavy N use, disease present early, cool weather forecast for several days/weeks) but are not recommended if disease is low.  Anything in the triazole (FRAC group 3), SDHI (FRAC group 7) or Strobilurin (FRAC group 11) fungicide classes will help control powdery mildew in high risk situations.

As we approach boot and heading you should keep an eye on the Fusarium Head Blight Prediction Center for updates on disease risk.  I will follow up with a post on how to best use this tool on my blog in the next few days. Forecasts are calling for e moderate and potentially rainy conditions over the next 7-10 days  depending on your location.    In the meantime, keep an eye on your fields, and enjoy the weather!


Nathan Kleczewski Extension Field Crop Plant Pathologist University of Illinois   email:

Stripe Rust in S. Illinois

I received notice of stripe rust in S. Illinois today.  Stripe rust is an important disease affecting wheat.  Please find an article on this disease and management by clicking here.  

If you locate stripe rust in your field please tweet a picture to me (@ILplantdoc) or email ( with the wheat variety, growth stage, and approximate percent of field infected.  This information will be useful to IL wheat producers this year and in upcoming seasons.

An example of a stripe rust in wheat. Photo N. Kleczewski 2016