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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
This news item is this week’s Agronomy Highlight, scheduled for Thursday October 8 at noon. The Agronomy Highlight discussion is an opportunity to ask the author questions about the highlighted article, get updates from Penn State Extension Agronomy Educators around the commonwealth, share observations from your part of the state, and request content for the next issue of Field Crop News. Learn more about the weekly Agronomy Highlight discussion .
Tar spot, a fungal leaf disease of corn, was discovered last week for the first time in Pennsylvania in Lancaster County. This is a potentially yield-limiting disease that 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.
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 help 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 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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