Tar spot lesions with typical black raised bumps and fisheye lesions (center two leaves). (Photo by Lauren D. Quinn, University of Illinois.)
Urbana, IL (September 5, 2018) - Corn growers in northern Illinois and surrounding areas are noticing a marked increase in tar spot this year, with 30% infection rates in some fields. University of Illinois pathologist Nathan Kleczewski has been fielding daily calls and emails about the disease from concerned producers.
“Many people are wondering what we know and what they can do to manage the disease,” says Kleczewski, a research assistant professor in the Department of Crop Sciences at U of I. “The bottom line is that we do not really know, but we are working on it and need help from producers.”
The relatively new disease was first described in Illinois near DeKalb in 2015, with additional sightings in Indiana. Since that time, the leaf disease has been detected in northern Illinois each year, but, until now, infections have been sparse and have occured late in the season, minimizing yield loss.
“This season we have seen this disease take off in northern Illinois, as well as southern Wisconsin, Michigan, and parts of Indiana,” Kleczewski says. “Symptoms vary from the traditional black raised bumps, to bumps with necrotic fisheye lesions, to spots on leaves that lead to blight and drydown.”
According to Kleczewski, the disease is not new in parts of Latin America, where it is known as tar spot complex. There, severe yield losses can occur. “In the case of the complex, we know two pathogens are involved,” he says. “One fungus produces the black tar spots we typically see, and another produces toxins that can cause varying degrees of foliar blight and necrosis. Our colleagues at the International Maize and Wheat Improvement Center (CIMMYT) in Mexico are currently working on identifying the toxins involved and how they may relate to virulence.”
Kleczewski says very little is known about tar spot complex, including the epidemiology of the disease and how the pathogens interact with one another and their corn host. In addition, it is possible that the disease may act differently in Midwest production systems, as hybrid genetics, production practices, and environments differ from those in Latin America.
That’s why he needs help from corn growers.
“Early this summer, prior to this outbreak, we started working with colleagues in other states and CIMMYT to better understand the tar spot pathogens and improve our abilities to detect and manage this disease if needed,” he says. “One item that we need for this project are samples. And since hybrids may differ in susceptibility to tar spot, it will be important to get samples from as many hybrids as possible to avoid planting susceptible hybrids in future years.”
Kleczewski says growers with symptoms of tar spot, particularly those with necrosis associated with the lesions, should send them to Dianne Plewa at the University of Illinois Plant Disease Clinic. The website with address and contact information is located at https://web.extension.illinois.edu/plantclinic/.
Growers should include the corn hybrid, county of origin, and whether a fungicide was applied. Kleczewski says fungicides have had wildly variable effects on the disease this year, so gathering more information on fungicide application and rate will aid in management decisions in the future.
Growers who want to get more involved in variety testing and yield response efforts are encouraged to contact Kleczewski at 217-300-3253 or email@example.com. He also can be reached on Twitter @ILplantdoc. For the latest news on this disease and others, stay tuned to Kleczewski’s blog, at http://cropdisease.cropsciences.illinois.edu/.