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Avoid the Cosmic Freakout Over Tar Spot
Tar spot is a new corn disease that’s attracted attention the last few years. So far, though, it’s not close to the widespread severity seen last year, says Nathan Kleczewski, University of Illinois field crop plant pathologist. Below are some factors Kleczewski gives regarding tar spot for this summer.
Last Saturday, colleagues in Indiana reported tar spot presence in some of their research plots located in northwest Indiana. 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 5, 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 had 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 (tasseling) around this time. This season we are at V6 and V7, respectively. There is not much canopy to retain moisture, especially when conditions three 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. 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 are 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 those that 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 soybeans 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!
So What Disease Should Really Worry Farmers?
I’d be more concerned about the recent report or Southern rust from southeastern Missouri, especially for corn growers in areas like southern Illinois. 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-planted cornfields.