Remember to check your hybrids for tar spot ratings, scout your fields

It is that time of year again.  Soon corn will be in the ground, and the 2019 field season will be taking off.  It is no surprise that I spent the majority of my time on the speaker circuit discussing tar spot in corn.  We have learned a fair amount since then, but there are many more things that need to be researched and learned before we have excellent tar spot IPM management programs.  However, there are a few points you should keep in mind this season that can help you determine your risk for tar spot and management practices that can help your bottom line.

The incidence of tar spot was fairly widespread last year.  Incidence is simply asking the question, “Do I have any tar spot in my field?”  Incidence does not incorporate the severity of infection.  One could have a field with a high incidence of tar spot, yet the severity (number of lesions on leaves of plants) could be low.  This link shows the tar spot incidence in 2018 :Tar Spot established in the United States-2018

If we were to estimate where the greatest severity of disease was last year, it likely tracked with the late season storms that pushed through the region in August and September.  In Illinois, severity was greatest in the region North of I-90, and most severe in the north central part of the state.  Increased severity likely means increased local inoculum for this season.  If you are planting corn in a region that was hit hard by tar spot last season, your risk for disease is elevated compared to areas where disease was sparse or absent.

The fungus that causes tar spot overwinters in residue, and spores are released from the stromata (raised black spots on foliage, stalks, husks) at night during periods of moderate, humid weather.  These spores spread locally and also can move at a minimum to nearby fields on rain and wind.  If you are planting into a field of corn residue from plants that were severely affected by tar spot, you may be at increased risk for disease compared to if you are following a field that was in soybean last year or is tilled.  That does not mean tar spot will not occur, as it can spread from nearby fields; however, planting after soybeans or tilled fields may reduce local inoculum levels, reducing disease onset and potentially severity.  The later the disease starts, the less impact it is likely to have on your crop.

All commercially available hybrids are susceptible to tar spot, but some hybrids are more tolerant than others.  No particular brand is better than another.  Ask your seed dealer or check out Dr. Smith’s website    for information pertaining to specific hybrids and tar spot response.

Tar spot severity by brand. Numbers indicate individual hybrids within a brand. Data from DeKalb OVT, 2018. Rated at R5/6

 

Scouting is critical for this disease.  CCA’s and producers should ensure that fields are being scouted frequently and often, especially in the days/weeks approaching tasseling.  If you notice tar spot showing up prior to VT, a fungicide may help.  There are several products with a label or 2ee for tar spot suppression.  Like rusts, this is an obligate fungus, and you want to ensure that the ear leaf and leaves above are protected during the critical periods of grain fill.  You do not want to chase this disease-revenge sprays will not work.

Lastly, although tar spot is the hot topic, our most severe and widespread disease last year was, without a doubt, grey leaf spot.  Do not lose sight of this disease and other diseases that are observed and encountered more frequently and consistently in Illinois.  Tar spot is likely to be episodic, much like Fusarium head blight in wheat and white mold in soybeans.  It may be a while before we see significant disease as we did in 2018 (I hope this is the case).


Register now for Tar spot Webinar, March 1

3/1/19 at 9:00 am CST

 

 

Join Dr. Nathan Kleczewski from the University of Illinois Extension  for an update on Tar spot in corn.  This disease was first observed in the United States in 2015 in Northern Illinois and Indiana.  In 2018, the disease significantly affected corn production in the Midwest and Florida.  What is tar spot of corn?  How does it work?  What is our current understanding of this disease and its management?  These and other questions will be addressed through this free webinar.

 

Registration is free, but capped at 100 participants.

 

To register for this meeting, click the following link: https://web.extension.illinois.edu/registration/?RegistrationID=19924

 

We look forward to seeing you March first!


What effect will cold temperatures have on pests and pathogens?

Nathan Kleczewski Research assistant Professor and Extension Field Crop Pathologist

Nick Seiter- Research Assistant Professor and Extension Field Crop Entomologist

 

Many in the Illinois agricultural community are wondering what effects the recent extreme cold might have on pests and pathogens. While it would be nice if the cold temperatures we are experiencing could help to reduce our potential for pest damage, past experience tells us that the most serious pests we deal with are unlikely to be impacted much by these conditions.

Many of the pathogens and insect pests that commonly affect field crops in Illinois are well adapted to survive our winter conditions.  In many cases, pathogens produce recalcitrant survival structures (e.g. cysts in soybean cyst nematode, oospores in Phytophthora, sclerotia in white mold).  These structures allow the pathogen to survive extreme conditions including cold, drought, and flooding. Different species of insects overwinter in different life stages, including eggs (for example, western corn rootworm), larvae (Japanese beetles), pupae (corn earworm, though they do not survive the winter in most of Illinois), or adults (stink bugs). The overwintering stage has characteristics that help these insects to survive the winter, either by adjusting its physiology to better survive the cold, seeking out an overwintering site that protects it (such as soil, tree bark, or leaf litter), or both. The overwintering sites that insects find mean that they are not experiencing the same temperatures that we are when we venture outside. Wind chill has little effect for this reason (even though it has a major, unpleasant effect on us).

Extreme cold temperatures can impact some insects and plant pathogens, particularly those that may not overwinter as well (e.g. powdery mildew).  When cold weather pushes into the Southern regions of the country it can push certain diseases, such as rusts, further south, delaying disease onset in Illinois and other regions further north. The same is true of migratory insects, such as black cutworm and fall armyworm, which do not usually overwinter in Illinois; colder temperatures during winter often delay the arrival of these insects, and may ultimately lead to lower numbers. The opposite is also true – warmer than normal temperatures during the winter can allow these migratory insects to become a problem earlier in the season.

Although cold temperatures may not impact most of the diseases we encounter in Illinois field crops, fluctuation between conditions of cold and warm may have a negative impact on some diseases.  Dormancy by fungi can be broken by environmental conditions such as higher temperatures.  This is similar to what occurs in plants, where warm weather may result in trees flushing out buds and flowers.  Consequently, the wide swings in temperature that we have experienced during the 2018/19 winter may negatively impact some diseases. While some insects (such as stink bugs) can also break dormancy during brief warm periods, many of our most serious pests will stay “hunkered down” until the spring and avoid these fluctuations. Unfortunately, insects and plant diseases are unlikely to suffer as much from the recent cold as we have. The best way to reduce the impact of insects and pathogens on those cold days is to stay inside, grab a hot cup of coffee, and curl up to the latest UI Extension recommendations or UI applied research results guide.


New Tar Spot Publication Available

A new publication on Tar spot of corn is available through the Crop Protection Network.  In this publication we have summarized our current knowledge of the disease, it’s impacts, as well as presented management suggestions.  You can access the link to the library by clicking here.

In addition, I will be presenting a webinar on Tar spot at the end of February.  Pay attention to the Bulletin and my Field Crop Disease blog for additional details in the near future.

If you are not familiar with the Crop Protection Network, it is a great resource for disease and pest management information, plus it is free for use by the agricultural community.


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: https://web.extension.illinois.edu/plantclinic/  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 nathank@illinois.edu.  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.