Plant pathologists at Penn State head investigation into tar spot disease in Pa.

Tar spot of corn, a fungal leaf disease, was discovered in a cornfield in Lancaster County in late summer 2020. Credit: Alyssa CollinsAll Rights Reserved.

UNIVERSITY PARK, Pa. — Plant pathologists in Penn State’s College of Agricultural Sciences are keeping a watchful eye on a corn disease new to Pennsylvania that has the potential to cause significant yield loss and reduce grain quality.

The threat — tar spot — is a fungal leaf disease that traditionally has been a problem in Mexico and Central America, explained Alyssa Collins, director of Penn State’s Southeast Agricultural Research and Extension Center in Manheim.

However, the disease made its way to the United States in 2015, when it was reported in Indiana. In the years since, it has spread to surrounding states, including northwestern Ohio. The disease breached Pennsylvania borders in late summer 2020, having been discovered in a cornfield in Lancaster County during a routine inspection.

“It just so happened that the agricultural industry professional involved had worked in the Midwest, so when he saw it, he knew what it was,” said Collins, who was called to take samples and confirm the diagnosis. “We weren’t sure how it got here or if it was an isolated incident, but we alerted the state’s growers right away.”

She said the disease spreads when spores are transported by wind, rain or contaminated seed, and it gets its name from its appearance — glossy black, raised lesions on leaves. These signs are easiest to see on green tissue but also can be found on dried leaves and fodder.

“Tar spot is easily identified when severity is high, but at very low levels, it is easy to overlook and mistake for spots caused by other fungi or by aphids, insect frass or other debris,” she said. “In severe cases, tar spot may cause yield loss of up to 50% due to low test weight, reduced kernel fill, poor silage quality and other issues."

Collins said identifying areas where tar spot is developing now will be necessary for monitoring and managing this disease. “While it was limited to Lancaster County in 2020, we have received and confirmed credible reports from surrounding counties and from counties in northwestern Pennsylvania,” she said.

Penn State scientists are working with colleagues in the Midwest to investigate how to prevent disease spread, management techniques and disease-resistant corn varieties. Researchers plan to conduct studies that focus on the disease’s adaption to Pennsylvania’s climate.

“It is not yet known if tar spot will be a persistent threat to the state,” Collins said. “Information regarding the management of tar spot is still limited, but crop rotation, residue management, hybrid selection and appropriate use of fungicides may help limit its impact.”

Contributing to the investigation at Penn State, the results of which were published recently in Plant Disease, were Ananda Y. Bandara and Dilooshi Weerasooriya, postdoctoral scholars, Sara May, Plant Disease Clinic coordinator, and Paul Esker, associate professor of epidemiology and field crop pathology, all in the Department of Plant Pathology and Environmental Microbiology.

Growers who suspect tar spot in their fields are asked to file a report online or email Collins at or Esker at A current map of where tar spot of corn has been reported in Pennsylvania can be found at ipmPIPE.

Disease updates will be provided through Penn State Extension channels, including the Field and Forage Crops Team newsletter. Visit this link to subscribe.

Last Updated October 04, 2021