Tar Spot - 2024 Tar Spot in Corn

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.

Learn More

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. 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.

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.

Corn leaves grow in intense dark colors.

Select

Want to learn more about identifying different diseases?

Scouting Resources For Your Region

Handling Tar Spot

Tips for Scounting & Managing Tar Spot

Test your tar spot identification skills!

Which one of these images is the tar spot?

Want to learn more about identifying corn and soybean diseases and pests? Visit the Pioneer Scouting Calendar for your region.

Scouting CAlendar

Watch Video

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

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

Hover over to see blurb

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.

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%.

Hover over to learn more

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.

Last year, parts of eastern Nebraska and northern Missouri along the Missouri River experienced high levels of tar spot pressure resulting in yield loss and standability challenges. Tar spot was detected in two new states in 2023 – Virginia and Delaware – and continued its westward expansion, with detections in numerous counties in Nebraska, Kansas, and Missouri.

These yield losses were hard to attribute solely to tar spot, as other foliar disease and stalk rots were prevalent.

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.