Late season tar spot- what is on the horizon?

There has been some chatter about tar spot starting up a bit in Northern Illinois corn fields.

Tar spot risk map as of 9.3.19. Most fields are near or past R5 in many of the areas in question.

However, crops are almost at R5 in many places.  What options do we have and what can you expect?  Check out our new post on the Illinois Field Crop Disease Hub by clicking here.  Remember to sign up at the website for updates and look forward to new materials such as factsheets, images, and guides by Spring, 2020!


Diagrams to help you rate foliar disease on corn

Many in the agricultural community, as well as researchers annually rate corn for disease as a means to assess hybrid response, hybrid effectiveness, or potential disease level on field productivity.  It can be difficult to rain the eye to accurately measure disease on foliage, and differences in the type and size of the structure or lesion associated with the pathogen varies significantly.  The four links below will direct you to disease area diagrams we developed in order to help you obtain accurate disease severity estimates in your fields. The method you use to assess disease severity may differ depending on the overall objective. The diagrams below are cor grey leaf spot, northern corn leaf blight, common rust, and southern rust.  These can be printed, laminated, and taken to the field with you to assist your ratings.

SAD_CommonRust SAD_GreayLeafSpot SAD_NorthernLeafBlight SAD_SouthernRust


Southern rust in Illinois- it’s complicated

This week we started picking up Southern rust in the southern Illinois.  Thusfar, reports indicate that disease severity is low.  However, the recent hurricane remnant and warm forecasts may mean that we may see the disease progress somewhat in the coming days and weeks.

When people in Illinois hear the words southern rust, it brings back memories of a few years ago when the disease moved in and environmental conditions favored disease development for a prolonged period of time.  Many fields suffered losses as a result of the disease.  This year the situation is complicated and different from a few years ago.  First, we are dealing with extreme differences in planting dates throughout the state.  One field may have tasseled a week or two ago and the field across the road might just now be reaching V10.  Second, yield potential in late planted fields is likely to be substantially lower than typical, meaning that there is less yield to protect and less money to cover potential application costs.  Third, commodity prices, although they may have increased slightly in recent days, are low, making it hard to justify the cost of applying a fungicide unless necessary and the potential to recover costs is high.

 

Southern rust.  Image A. Sisson.

 

Common rust vs Southern rust on corn. Image C. Bradley.

Let’s take a minute and first go over southern rust, then move back into what factors you should consider before making a fungicide application to manage this disease in 2019.

Southern rust is caused by the obligate fungal pathogen Puccinia polysoraPuccinia polysora produces fuzzy, raised structures called pustules on leaves and stalks of corn.  Pustules contain thousands of small orange spores.  When you rub these pustules between your fingers, the spores may leave a dusty orange coat on your fingers, hence the reason it is called a “rust.”  Pustules of Southern rust are orange to light tan, and often small and circular.  Pustules are mostly found on the upper leaf surface, which can help distinguish it from the less damaging common rust.

Spores from pustules can be dispersed miles on air currents, allowing the disease to spread rapidly.  Under hot humid conditions, spores of the fungus can infect suceptible corn, and symptoms can be observed within 3-4 days.  Within 7-10 days, spores are produced and can be dispersed.  The cycle of spore-infect-spore can continue as long as conditions are conducive and corn plants are green.  Conditions that favor disease development include hot temperatures (morning low of 75°F and daytime high of 93°F) and at least 4 hr of consecutive leaf wetness.  Outside of these conditions disease progress can occur, but at a slower rate.  Our colleagues to the South state that Southern rust can continue chugging along at 110 degrees.  That’s pretty impressive.

 

Southern rust does not overwinter in Illinois and blows into the region from warmer regions.  In years where it develops to a significant degree early in southern regions, it can move into Illinois during critical stages in crop growth.  In general, we see the disease move in most years in late July or early August.  This means that in years when plantings are delayed, the disease can arrive on time but plants may be at greater risk for yield loss because the earlier  infections occur the more yield can be impacted.  Experience from our Southern colleagues indicates that stalk integrity isn’t likely to be affected unless you see significant infections during the vegetative stages of crop development.

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

Now that we are on the same page about this disease, what about management?  As I mentioned previously things this season are complicated.  Let’s start by considering management decisions.  Below is a table modeled after one produced by fellow plant pathologist and Jason Statham look-alike Travis Faske at the University of Arkansas, depicting the likelihood that a fungicide application for Southern rust will provide a benefit:

Growth Stage Southern rust present Forecast favors S rust (75-93F) min 4 hrs leaf wetness Benefit of fungicide?
Vegetative yes yes yes
VT/R1 yes yes yes
R3 yes yes yes
R4 yes yes Unlikely
R5 yes yes No
Maturity yes yes No

 

Experience from the South indicates that trying to hold off an application until VT/R1 if possible is going to give you the highest likelihood of coming out even or ahead of this disease.  If you apply during the vegetative stages, realize that that means that you might need to come back again and make a second application.  You now have likely doubled your application costs.

Now let’s consider some other aspects of controlling this disease in our late planted crops.  If you were unable to switch to a shorter day hybrid, and decide apply a fungicide for managing S. rust at say, R3, your plant will be protected from disease, and retain greenness later into the season.  Depending on your location in the state, this means you might need to consider frost, and what that means for your yield potential and crop harvestability.

Before making an application, consider these points and also run the numbers.  Remember that fungicides DO NOT INCREASE YIELD POTENTIAL.  They do not increase yield.  They protect potential yield by mitigating losses due to fungal disease.

That being said, you can calculate the amount of protected yield required to pay for a fungicide application by using the formula Yield protected (bu/A) = application cost ($/A) crop price ($/bu).  After calculating your required protected yield, you can then determine if potential yield, frost, and other factors will make it worthwhile to spray.

 

Continue to scout.  For updates on Southern rust and it’s presence in Illinois and surrounding states click here

 


New maps for tracking Southern rust in corn (repaired link)

Southern rust is caused by a fungal pathogen that does not overwinter in Illinois.  Instead, it blows in from warmer regions during the growing season.  When conditions favor spread and development of this disease significant damage can occur, especially if it arrives before tasseling (VT).  For more information on Southern rust, check out the Crop Protection Network publication by clicking here.  Yesterday our colleagues detected this disease in Southeast Missouri.  With the hurricane/tropical storm remnant pushing northward, and warm weather forecast, there is a good chance we could see some movement into the Southern / Western  part of the state within the next 7-10 days, and there may be some in Southwest Illinois already.  Scouts should be keeping their eyes out for this disease, as things can escalate quickly.  Suspect samples should be sent to the University of Illinois Plant Diagnostic Clinic for confirmation.  Images can also be sent to me via twitter @ILplantdoc, or email at  nathank@illinois.edu.

We have a new tracking system for rusts in corn that we are using this year.  To access the maps click here.  These maps are very useful for tracking a disease that does not overwinter in the area as it needs to spread, establish, and produce more spores to move, unlike diseases that overwinter on residue, alternate hosts, etc.  That means you can see the disease spread and know if you are at risk for it moving into your area.  If your fields are at a critical point in growth, and the disease is detected nearby, in season management can be considered, and unneeded treatments avoided.  We also have a tar spot map.  This disease overwinters in the region, and little is known about disease movement and spread within a season.  Data indicate it can move at least from field to field; however, after last season there is reason to believe it may move longer distances, but we simply are speculating at this point.  This map therefore can tell you when the disease is starting to be detected in certain regions, and also if the disease has been detected in new counties.  For more on tar spot click here.

Locations where tar spot was confirmed in 2018. This map shows incidence not severity at county level. Severity in general was highest in Northern Illinois and extremely low in central Illinois

 

 


Avoid the cosmic freakout

Today colleagues in Indiana reported tar spot presence in some of their research plots located in North West IN.  They found an extremely low number of stroma (less than 10) when assessing approximately 500 feet of plots.  When you see or read about the report keep a few things to keep in mind:

1) The amount detected was exceptionally low, and not close to the widespread severity we saw early last year.  For example, on  July 5th, 2018, we detected tar spot in DeKalb at 100% incidence (every plant had some) with an average of 6% severity at the ear leaf at VT.  Those were wet fields, closed canopies, and a history of moderate tar spot.

2) It will be hot and dry for the foreseeable future.  Tar spot likes moderate temperatures and persistent humid conditions.  In corn that is still in the early vegetative stages, the persistent levels of humidity the pathogen likely needs to sporulate, then transfer those spores to plants, germinate, and infect, might not be met.  Last year at this time our fields in DeKalb and Monmouth were at or approaching VT around this time.  This season we are at V6 and V7, respectfully.  There is not much canopy to retain moisture, especially when conditions 3 weeks ago were favorable for disease onset.

3) Continue to scout, but be aware that the majority of the chatter out there about tar spot being detected in the Midwest is based on misdiagnoses of insect frass.  Spraying poop with fungicide is not going to benefit your crop.  Click here for more information on that particular issue.  If you have any suspect samples, send them to the UI plant diagnostic clinic, send me images, and let us know the approximate location of the putative detection.  We are collecting samples as we did last season.

4) We have observed tar spot in Illinois every year since it was first detected.  This disease overwinters in the region, just like grey leaf spot, white mold in soybeans, and Fusarium head blight in small grains.  Last year was the first time that the disease was severe enough to cause yield loss.  Detecting it is not uncommon.  When it arrives and the amount of symptoms expressed during critical periods of grain fill is what is most important.  Last year was the perfect storm of susceptible crop, environment conducive to disease for a prolonged period of time, and infection during a period critical for yield.  We will observe it this season, the question is when, and how severe and widespread it will be.

5) Fields at highest risk for tar spot will be no till, corn after corn fields experiencing moderate temperatures and persistent humid conditions, and had tar spot last season.  Our collaborative research team has preliminary data indicating that any infested residue on the surface of fields can produce viable spores.  Tillage may potentially reduce the overall number of spores available for local infection of a particular field by reducing the amount of surface residue on the field, but there is no reason to expect the act of tillage alone to impact survival and viability of spores produced on the residue remaining on the field surface.  Planting into fields that were soybean last year may reduce initial disease onset.  This disease isn’t a rust.  Keep in mind, until we have hard data these are simply assumptions based on experience and similar pathosystems.

6) It is evident that there is a lot that is not understood about this pathosystem and in particular, pathogen biology and ecology.  Our tar spot coalition, which consists of a group of pathologists and breeders from the Midwest and Florida, is working on coordinated trials and collaborative projects to learn as much as possible about this disease in an effective, efficient manner.  We are working hard to help our producers minimize potential losses due to this disease.

In sum, keep scouting, don’t freak out, and stay hydrated- it’s going to get hot out there!

On a side note, I’d be more concerned about the recent report or Southern rust from Southeast Missouri, especially for our #corn growers in the southern portion of the state.  That disease blows around, and with hot temperatures and a predicted hurricane remnant moving in, it could move a bit, especially in some of these late plated corn fields.


Common rust on corn- a concern?

Over the past week there has not been much activity as far as corn disease is concerned.  The only exception has been the detection of very light common rust infections on some corn in the north central and southern parts of the state.  Although the occurrence of common rust on corn this early in development may be a concern to some, there are a few things that you should keep in mind.

1) Common rust, caused by Puccinia sorghi, is an obligate pathogen and requires living hosts to grow and reproduce.  It blows into Illinois from other regions each season.  It does not overwinter in Illinois.

2) Common rust is a disease that can be problematic during exceptionally cool and wet summers.  Disease development occurs between 60-74F.  When temperatures exceed 74 F disease progress slows and pustules may cease being active.

3) The amount of pustules needed to cause yield loss by common rust is high.  Consider that it takes over 160 pustules on a leaf to equal 1% severity.

A lone common rust pustule on a corn leaf located in Monmouth, IL on 6/28/19. Common rust likes it cool and humid. Image: N Kleczewski

 

With temperatures increasing, common rust likely should not be on the top of your diseases to be concerned about list.  That doesn’t mean you should not be scouting and paying attention to the situation in your fields.  Scout often, and be thorough.  One thing we do not want to miss is the occurance of southern rust, which is starting up in Georgia and Louisiana.

One disease I would like people to keep an eye out is bacterial leaf streak on corn.  We have had this disease in Northern Illinois for a few years now, and it likely is much more widespread than we think.  This disease is most likely to occur in no-till, corn on corn production systems and wet conditions (just like what we have had the past several weeks).  We have had several putative images over the past 10 days.  If you see anything that might be bacterial leaf streak please send to the UIUC Plant Diagnostic Clinic for assessment.  Feel free to text me images or email images as well.

For more information on bacterial leaf streak, click here.


Calculating fungicide net returns

Corn producers in parts of the state are nearing the point in time where they are thinking about fungicide applications to their fields.  In a recent post on my blog I discussed tar spot and also mentioned a recent publication that shows that a single fungicide application at the VT/R1 growth stage has the greatest chance of providing the producer with a return on their investment.  Click here for access to this article.

Nobody knows your farm history and yields better than you do.  That is why running the numbers yourself and thinking about your past experiences can help you determine how likely you are to break even or make a profit using various programs under your specific situation.

To calculate how much yield needs to be protected to break even at a given application cost (fungicide cost plus application costs)  and commodity price:

yield protected (bu/A) = application cost ($/A) / corn price ($/bu). 

This formula can be used to help you determine the amount of protected yield and commodity price needed to break even and see a return on your investment.

 

For example, to see how much yield would need to be protected by a fungicide to pay for the cost of a  $26/ A total application cost at a $4.50 per bu grain price:

yield protected = ($26.00 per A )/ ($4.50  per bu) = 5.8 bu/ A

 

the same situation but a program that costs $30 per A =

($30.00 per A)/($4.50 per bu) = 6.7 bu per A. 

 

Below is a table of the potential protected yields needed to break even at a few different commodity prices and total application costs.

$Price
Application cost $3.50 $4.00 $4.50 $5.00 $5.50
$20/A 5.1 bu 5 4.4 4 3.6
$25/A 7.1 6.3 5.6 5 4.5
$30/A 8.6 7.5 6.7 6 5.5
$35/A 10 8.8 7.8 7 6.4
$40/A 11.4 10 8.9 8 7.3

 

Knowing your application costs for 1 or 2 trips and product, and estimating the commodity price, what sort of yield response will you need?  Have you seen this sort of yield response on your field before or not?  Has this response been fairly consistent? Knowing this information can be very useful in selecting fungicide programs for your specific fields.


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.