How far has tar spot spread in Iowa in 2019?

October 11, 2019 7:16 AM
Blog Post

Tar spot is the latest new disease of corn to be observed in the U.S. The disease was first observed in Indiana in 2015. In 2016, it was found in eastern Iowa, Illinois, Michigan, Wisconsin and Florida. Last growing season, the disease caused significant damage to corn in northern Illinois, Southern Wisconsin, Michigan and Indiana. In Iowa in 2018, the disease was observed again along the eastern boarder counties and no significant damage was reported.

Throughout the 2019 growing season, pathologists at Iowa State University have been mapping the distribution of tar spot in Iowa. To our surprise, the disease has been observed in 75 counties (Figure 1). While the disease has not been severe enough to cause losses, it is a concern to see it widespread throughout the state. Firstly, it has spread west across the state more quickly than we expected. Secondly, the pathogen is now present throughout the state and consequently tar spot will be another disease for us to keep an eye on in 2020 and beyond.

Figure 1. Distribution of tar spot in Iowa as of 10 October 2019


Tar spot produces small, raised, round to irregular-shaped black spots (stroma (singular); stromata (plural)) on the upper and lower leaf surface. These can occur singularly or in clusters with more severe infections. Sometimes tan or brown lesions develop around the tar spots, called fisheye lesions. The key to identifying tar spot is that the black spots cannot be scraped or wiped off the leaf.

We are finding tar spot in fields that still have green leaves and other diseases (northern corn leaf blight, southern rust, grey leaf spot) present in the canopy. Look for tar spots on the ear leaf and above. Be careful not to confuse southern or common rust with tar spot. At this stage of the season, rusts start to produce black teliospores and consequently the pustules are black (Figure 2), so it is easy to confuse rusts for tar spot. If you are not sure, send a sample to the ISU Plant Disease and Insect Clinic.

If you have observed tar spot in a county where we have not documented the disease yet, we would like to know. Diseased leaves can placed in an envelope and sent to the ISU Plant Disease and Insect Clinic, 2445 ATRB, 2213 Pammel Dr., Ames, IA 50011. There is no charge associated with samples submitted to the clinic for tar spot diagnosis.

Photo of two corn leaves, one with tar spot and one with rust

Figure 2. Comparison between tar spot stromata (A) and rust pustules producing black teliospores (B).  The orange arrow in A points to a rust pustule among tar spot stromata; the white arrow in B points to a tar spot stroma among rust pustules

The tar spot triangle

Observations at hybrid demonstration plots and trials over the past two growing seasons suggest that while all hybrids are susceptible, some are more susceptible than others. Epidemiological studies suggest the disease is favored by cool (60-70 F), wet conditions. The presence of the disease in the U.S over the past 4 years plus research studies done at Universities across the Midwest indicate that the pathogen that causes tar spot (Phyllachora maydis) survives the winter in infested crop residue.

Managing tar spot in 2020

We have a lot to learn about tar spot. Pathologists across the Midwest have been collaborating on various research projects throughout 2019. Data will be shared at winter meetings. For the moment, talk with your seed dealer regarding what hybrids to plant in 2020. If you have observed tar spot on your farm, rotating affected fields out of corn may help reduce the inoculum in the field by allowing time for the infected corn residue to decompose. Stay up-to-date with tar spot reporting at Scout corn fields. If tar spot is observed, a fungicide may help reduce the disease depending upon when the disease is observed and the timing of the fungicide application.

Additional resources

The Crop Protection Network has a publication describing what we currently know about tar spot:


Thanks to Daren Mueller and the IPM team for their scouting efforts; to Ed Zaworski for confirming tar spot samples, and Meaghan Anderson and Rebecca Vittetoe for reviewing an earlier version of this blog.



Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...