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Tar Spot: A New Corn Disease for Pennsylvania

Tar Spot: A New Corn Disease for Pennsylvania

By Alyssa Collins and Paul D. Esker

Tar spot, a fungal leaf disease, was positively identified in a cornfield in Lancaster County this week. First discovered in Pennsylvania in September 2020, this potentially yield-limiting disease arrived initially in the US in 2015 and made headlines during the 2018 growing season when there was widespread economic impact in the Midwestern states. See a map of the most current reports.

The symptoms of tar spot are primarily the presence of glossy black, raised lesions on leaves, which may be surrounded by dead tissue as the disease progresses (Figure 1). These signs are easiest to see on green tissue but can also be found on dried leaves and fodder (Figure 2). 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, 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.

Since the disease is developing late in the season, there is not a great deal of risk to the 2021 crop. However, identifying areas where tar spot is developing now 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 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.

Please help us find tar spot across the state!

If you suspect tar spot in any fields in Pennsylvania or neighboring states, report it to us. If you are not sure about a diagnosis, contact your nearest Extension educator to obtain a positive identification and alert state specialists Alyssa Collins and Paul Esker. More details about this disease and its causal agent can be found in this publication from Crop Protection Network.


Figure 1. Raised black lesions indicative of tar spot (Alyssa Collins, Penn State)


Figure 2. Tar spot lesions as they appear on dried corn residue


Figure 3. An upper canopy leaf affected by tar spot, backlit by the sun (Alyssa Collins, Penn State)

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