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Keep an eye out for tar spotKeep an eye out for tar spot

Corn Source: New corn disease is a growing concern, especially in a wet year.

Brian Lang

July 8, 2019

5 Min Read
close-up view of tar spot on a corn leaf
SYMPTOMS: Tar spot, a relatively new corn leaf disease in the U.S., is most recognizable as black spots covering the leaf surface.

A relatively new corn leaf disease, tar spot has continued to move into more Midwest fields since it was first identified in the U.S. in 2015. Plant pathologists believe a storm system blew tar spot north from Mexico, as it first showed up in fields in Illinois and Indiana. In 2016 and 2017, it spread into parts of Michigan, southern Wisconsin, Ohio, eastern Iowa and Florida.

In 2018, tar spot was detected in 12 counties in eastern Iowa. The disease was observed late in the growing season and didn’t cause a lot of damage. In Wisconsin, Illinois, Michigan and Indiana, however, the disease was observed earlier during the grain fill stage of corn plants and caused significant yield loss. From 30 to 40 bushels per acre was lost in some fields.

When it first appeared in the U.S., agronomists didn’t know if tar spot would overwinter here. Since then, the disease has been reported each growing season, which suggests the fungus is overwintering in the Midwest. It likely remains in the soil and crop residue. Like other fungus diseases that produce spores, tar spot can be carried by wind and spread from field to field.

Watch for these symptoms

Tar spot in corn is recognized as small, raised, black, irregular-shaped spots scattered across the leaf surface. These spots are fruiting structures of the fungus, ascomata P. maydis. It prefers cool, wet weather; similar to environmental conditions favorable for development of northern corn leaf blight.

As with most diseases, tar spot does have look-a-likes — namely common rust and southern rust. At the end of the growing season, both of these rust fungi switch from producing orange-red spores to black teliospores. Rust pustules filled with teliospores can be mistaken for tar spot. To tell the difference, remember that rust spores burst through the epidermis on the corn leaf, and the spores can be scraped away from the pustules with a fingernail. Tar spots cannot be scraped off the leaf tissue.

If you observe corn with tar spot symptoms, notify an Iowa State University Extension plant pathologist (Alison Robertson or Ed Zaworski) or tweet at @isu_ipm, with a photo (if possible). Give them the name of the county where the disease was found, so they track of the disease in Iowa.

Tar spot risk predictions for Iowa

The University of Wisconsin has developed a tar spot prediction model called Tarspotter. It’s being validated this growing season by university Extension specialists and industry colleagues throughout the Midwest.

On June 23, the prediction model showed a high risk for tar spot for Dubuque County and other parts of northeast Iowa. Obviously, that means spores must be present. But the elevated risk identified by this model for the ISU Northeast Research Farm at Nashua might not mean much since the disease hasn’t been found there yet.

The disease was present in Dubuque County last year. Keep in mind that the high-risk assessment does not mean you should spray the corn with a fungicide based on the prediction, but rather you should perform timely scouting and treat only if the disease is identified in your field.

Managing disease

Here are answers to some commonly asked questions I receive from farmers about this new corn disease:

Does tar spot infect first-year corn or just corn-on-corn acres? Corn on corn is probably higher risk. But tar spot has shown up on corn following soybeans, too. Spores that develop on an infected corn plant can disperse up to about 250 feet. Talk to your neighbors. Has tar spot shown up on any of their fields? Since the spores can be blown from field to field, if your neighbor has it, you will likely get it.

In a wet growing season, if it’s humid and cool with a lot of leaf wetness, wouldn’t it be better to apply a foliar fungicide early as a preventive treatment? No, this is not good management. Your fields may still be in an area where the disease is not yet present. Let’s scout for signs of disease lesions of tar spot or other diseases like northern corn leaf blight and gray leaf spot. If found, then consider treatment. If not found, scout again in a week. If still not found, why pay for more inputs?

What about using tillage to bury crop residue? Wouldn’t that decrease the risk of tar spot showing up in a field? Maybe, but tillage isn’t an option in a no-till or conservation tillage system. Crop rotation offers a similar benefit. While not fully controlling the disease, it will help reduce the inoculum in the field prior to the next season the field is planted to corn.

Are there any corn hybrids that show resistance or tolerance to tar spot? Is choice of corn hybrid a tool to help manage this corn disease? Currently, there are many corn hybrids available with good resistance to gray leaf spot and northern corn leaf blight. However, while tar spot fungicide trials in 2018 showed that some corn hybrids offered more resistance than others, strong resistance to tar spot was not common, and there was no immunity either. It will take at least a few years for plant breeders to develop a stronger line of resistant hybrids. Meantime, we need to be more vigilant with scouting.

Keep in mind that tar spot is just one disease. There are also other foliar corn diseases you should also be scouting and watching for in your fields. For example, gray leaf spot and northern corn leaf blight occurrence is fairly common in Iowa, especially in a wetter-than-normal cropping season.

A helpful publication “Tar Spot” is available at cropprotectionnetwork.org. ISU plant pathologists Alison Robertson and Daren Mueller, along with colleagues from other universities, wrote the publication. It shows diseases with similar symptoms to tar spot and explains how to distinguish corn rusts, Physoderma brown spot and other corn leaf diseases from tar spot.

Lang is the ISU Extension field agronomist in northeast Iowa. Contact him at [email protected].




About the Author(s)

Brian Lang

Brian Lang conducts Iowa State University Extension programs in crop production and protection in northeast Iowa. Frequent clients include farmers, ag chemical and fertilizer dealers, seed dealers, crop consultants, and farm managers.

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