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Penn State Extension photo by Alyssa Collins Raised black lesions are indicative of tar spot.

Tar spot, a fungal leaf disease of corn, was discovered last week for the first time in Pennsylvania — in Lancaster County — reports Extension plant pathologist Alyssa Collins.

This is a potentially yield-limiting disease that arrived in the U.S. in 2015 and made headlines during the 2018 growing season when there was widespread economic impact in Midwestern states.

Symptoms of tar spot, the fungal leaf disease of corn

Primary symptoms are the presence of glossy black raised lesions on leaves, which may be surrounded by dead tissue as the disease progresses.

These signs are easiest to see on green tissue but can also 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 can be mistaken as spots caused by other fungi, aphids, insect frass or other debris.

In severe cases, tar spot may cause yield loss due to low test weight, reduced kernel fill, poor silage quality and other issues.

There is no threat from this disease to the 2020 crop since corn across the state is already in late reproductive stages and harvest is underway.

However, identifying areas where tar spot may have been present in 2020 will be important for monitoring and managing this disease moving forward. It is not yet known if tar spot will be a persistent threat to the region.

Information regarding management of tar spot is still limited, but crop rotation, residue management, hybrid selection and appropriate use of fungicides may limit impact.

Laboratory analyses to confirm this initial report of tar spot are underway.

If you suspect tar spot in any fields in Pennsylvania or neighboring states, please work with your nearest Extension educator to obtain a positive identification and alert state specialists Alyssa Collins at aac18@psu.edu and Paul Esker at pde6@psu.edu.

More details about this disease and its causal agent can be found here.

Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.

This article originally ran on lancasterfarming.com.