Tar spot - a baffling new threat to Pennsylvania corn

By Blake Jackson

Tar spot, a fungal disease-causing black spot on corn leaves, is rapidly spreading across Pennsylvania and neighboring states. While the exact impact and management strategies remain unclear, researchers are working hard to find solutions.

First detected in Pennsylvania in 2020, tar spot is now established in most corn-growing areas of the state and has reached southern New York, northern Maryland, and New Jersey. Experts predict it will become a recurring challenge for farmers.

The disease survives winter in corn residue, suggesting practices like tilling and adding nitrogen in fall could help reduce its prevalence. However, wind dispersal from neighboring fields limits the effectiveness of these methods. Spores travel short distances, primarily causing early-season infections during wet weather.

Unlike most fungal diseases, tar spots seem less affected by later rains, which can even reduce their spread within a field. This unique behavior makes traditional control methods less reliable.

Researchers are encountering difficulties studying tar spot. In some controlled test fields, attempts to trigger infections failed despite ideal conditions. This unpredictable behavior poses a significant challenge.

The potential yield loss from tar spot is concerning, with reports of 50-80% losses in the Midwest in 2018. While Pennsylvania hasn't seen such drastic effects, any yield loss is problematic during low corn prices.

Fungicide application seems like a logical defense, but researchers haven't determined its effectiveness yet. The lack of understanding about what drives the disease makes it difficult to predict how well fungicides work.

Planting corn hybrids with some resistance is currently the best option, though no variety is completely immune.

The impact of tar spot on corn silage is another concern for farmers. The disease can dry and kill leaves, affecting decisions about silage chopping and moisture management. While the fungus isn't known to be harmful to livestock, and silage fermentation might eliminate it, further research is needed to confirm this.

Despite the unknowns, researchers are actively investigating tar spot. As more information becomes available, growers will have better tools to manage this new threat to their corn crops.

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