More and more reports of tar spot on corn

August 9, 2019 3:57 PM
Blog Post

This past week there have been numerous new reports of tar spot within the Midwest, including Iowa (Figure 1). For updates maps, please visit

Map of IA, IL, WI, MI and IN with counties colored red or yellow to indicate where tar spot has been observed

Figure 1.  Map of the Midwest showing where tar spot has been confirmed (red) or is pending confirmation (yellow) as of 9 August 2019

Although tar spot is reasonably easy to identify - small, raised black spots on the lower leaves of corn that do not rub off - we are requesting that suspected samples of a few leaves get sent to the ISU Plant Disease and Insect Clinic. Please include the date, county (or nearest township) and information on how much disease is present in the field.  This may include:

  • Percent of plants in the field with tar spot?
  • Which leaves on the plant have tar spot?
  • Approximately how many tar spot occur on each leaf?
  • Does the field have a history of tar spot?  If yes, in what previous years was tar spot observed?

These data will be shared with colleagues in the Midwest who are researching tar spot with a goal of understanding how the disease is spreading. 


Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...

Ed Zaworski Plant Pathology Diagnostician

Edward R. Zaworski is a plant diagnostician in the Iowa State University Plant and Insect Diagnostic Clinic. He earned his master's degree in plant pathology in 2010, with a focus on field crop diseases.