Your Field Guide For Identifying and Managing Tar Spot
Tar spot is a recent but concerning foliar disease in corn for much of the Corn Belt. Our understanding of tar spot is quickly evolving and much remains to be learned about the long-term economic importance of this disease and best management practices.
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The Origin
How to Identify
Tar Spot Test
Management
The Origin and Spread of Tar Spot
Tar spot is caused by the fungus Phyllachora maydis. Historically, it was not associated with yield loss unless a second pathogen, Monographella maydis, was also present. In the U.S., tar spot has caused significant yield loss without an additional pathogen and led researchers to reconsider what they knew about tar spot.
50%
Up to 50% yield loss in the U.S. under extreme conditions in 2018 & 2021
Explore the spread of Tar Spot
Select a date on the timeline to begin
1900s
1930s
1940s
2010s
2020s
Mexico
Tar spot was first identified in Mexico in 1904.
1900s
South America
1930s
Discovered in South America in 1931.
Central America & the Caribbean
1940s
Discovered in Central America & the Caribbean by 1944.
The pattern of spread was slow with some Central & South American countries not discovering the disease until the mid-90s.
North America
2010s
Tar spot was first confirmed in the United States in 2015 in Illinois and Indiana and has now spread throughout central and eastern United States.
Early tar spot appearances in the U.S. led to hope that the disease would only cause minor cosmetic damage with little economic impact. That changed in 2018 and again in 2021.
North America
2020s
2019 and 2020 raised hopes that the disease would be largely mild, but 2021 proved that 2018 was not a fluke. Impacting much of the Corn Belt, tar spot proved capable of quickly proliferating through a rapid reinfection cycle and causing total plant death.
Identification
When trying to identify tar spot, look for:
Fungal properties
Temperature
Spread rate
You can identify tar spot by the fungal fruiting bodies, the ascomata, that develop on the leaf.
Tar spot favors cool temperatures (60-70 F), high relative humidity (>75%), frequent cloudy days, and 7 or more hours of dew at night.
Tar spot can rapidly spread through the corn canopy under favorable conditions, causing premature leaf senescence.
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Scouting Resources For Your Region
How Tar Spot Spreads
Tips for Scounting & Managing Tar Spot
Test your tar spot identification skills!
Which one of these images is the tar spot?
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Hybrid Selection
Fungicides
Agronomic Practices
Management
Tiny black spots
Oval or circular shape
Bumpy, uneven texture
Spreads from the lowest leaves to the upper leaves
Can spread to developing ear husk
Can't be scraped off
Checklist
Several foliar fungicides are labeled for control of tar spot in corn. While research is limited and still developing, fungicides can reduce tar spot symptoms and help protect yield.
Fungicides
Two applications may be needed due to tar spot’s rapid reinfection cycle, particularly with irrigated corn.
A 2019 Purdue University study compared single-pass and two-pass treatments for tar spot control using Aproach® and Aproach® Prima fungicides under moderate to high tar spot severity.
Fungicide treatment effects on corn yield under moderate to high tar spot severity in a 2019 Purdue University study.
Yield
(bu/acre)
Corn Yield
200
180
160
140
120
Nontrt
Aproach VT
Aproach Prima VT
Aproach VT fb Aproach Prima R2
220
Aproach Prima VT fb Aproach R2
186.7
199.3
200.7
207.8
213.9
C
BC
B
AB
A
Matthew Vandehaar
Pioneer Field Agronomist
"In most years, a single,
well-timed fungicide with a tar spot resistant hybrid should be adequate for control. However, we must scout fields regularly and plan to adjust our fungicide plans if disease pressure
gets aggressive."
John Kultgen
Pioneer Field Agronomist
"The potential for tar spot infection is here to stay. Mitigation starts with hybrid selection. At Pioneer, our hybrid selection process and local observations continue to support excellent plant health and tar spot tolerance."
Mike Swoish
Pioneer Field Agronomist
"Scouting for tar spot should begin in early July and continue through R4. Extended periods of leaf wetness in July and August strongly influence tar spot severity and likelihood of loss."
Our Leading Agronomists
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Prioritize a hybrid with genetic resistance to tar spot, as it appears to have a greater impact on symptoms and yield loss than either cultural or chemical management practices.
Longer maturity hybrids may be at greater risk of yield loss.
Hybrid Selection
Residue management – tar spot overwinters in corn residue so tillage may reduce severity, but observations so far point to little effect overall.
Night irrigation – duration of leaf surface wetness appears to be a key factor in development and spread. Farmers with irrigated corn in areas affected by tar spot have experimented with irrigating at night to reduce the duration of leaf wetness.
Agronomic Practices
University hybrid trials estimated yield losses of up to 39 bu/A under severe infestation while anecdotal reports from growers in severely impacted areas reported yield reductions of 30% to 50% compared to 2016 and 2017 yield levels.
These yield losses were hard to attribute solely to tar spot, as other foliar disease and stalk rots were prevalent.
1
2
1
2
Minnesota (2019)
Wisconsin (2016)
Michigan (2016)
Ohio (2018)
Iowa (2016)
Missouri (2019)
Nebraska (2021)
Kentucky (2021)
New York (2021)
Pennsylvania (2020)
This is common rust, but it's almost indistinguishable at a glance from tar spot late in the season. To check for tar spot, try scraping the pustules off with your fingernail. Rust spores can be scraped off the leaf surface, while tar spot cannot.
NOPE
This is tar spot. The ascomata are oval or circular without jagged edges.
YES
This is southern rust. Note the jagged edges around the pustules.
NOPE
In Mexico, tar spot was only associated with yield loss if it was present with a second pathogen.
Together these are known as tar spot complex and caused yield losses of up to 30%.
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Want to learn more about identifying different diseases?
Download Corteva’s Corn & Soybean Disease ID Guide
Wind-driven rain and storms can spread spores to new locations.
Wind-driven rain and storms can spread spores to new locations.