Tar spot reported in Central Iowa

June 30, 2022 1:38 PM
Blog Post

In pathology, we joke that to control a disease, just put a research plot out, and you’re guaranteed no disease. A couple of days ago I blogged about tar spot, hoping for a similar effect.  Alas, yesterday evening, I received a report of that dreaded black spot in Marshall County, central Iowa (Figure 1).

Map of U.S. showing reports of tar spot in 2022

Figure 1.  Tar spot was confirmed in Marshall County, IA on V11 corn on 30 June 2022 (corn.ipmpipe).

The report came from a seed company agronomist who was out scouting some competitive hybrid plots.  Incidence was low (approximately 1% plants with signs). Growth stage of the corn was V11.

Tarspotter forecasts Marshall County at high risk for tar spot. In fact, most of Iowa has been at high risk for June.  It is VERY important to get out and scout fields. Look for tar spot signs, small black circular-to-irregular shaped spots that cannot be scratched off the leaf surface in the lower canopy (Figure 2).

Early signs of tar spot on the upper (A) and lower (B) surfaces of a corn leaf

Figure 2. Early signs of tar spot are small, black circular to irregular raised spots visibile on both the upper (A) and lower (B) leaf surfaces

Iowa State University still recommends that a fungicide be applied no earlier than V12.  This is based on data from WI, IL, IN, MI and Ontario, Canada from field trials done over the past five years.  Moreover, tar spot should be present in the field, increasing in incidence (number of plants with signs of tar spot), and environmental conditions favorable for tar spot development (wet leaves for several hours). Fields that are sprayed prior to tasseling, may require a second application of fungicide three to four weeks later, if conditions remain favorable for tar spot development.

If you think you have found tar spot, please send a photo with the county name to alisonr@iastate.edu, dsmuelle@iastate.edu, pidc@iastate.edu or DM @alisonrISU or @dsmuelle on Twitter.  Samples may also be submitted to ISU Plant Disease and Insect Clinic.

Useful resources

  1. All you ever wanted to know about tar spot: https://cropprotectionnetwork.org/publications/an-overview-of-tar-spot
  2. Thoughts for 2022 from Dr Darcy Telenko, Field crops pathologist at Purdue University
  3. 2022 Fungicide data from Dr Damon Smith, Field crops pathologist at University of Wisconsin: https://badgercropdoc.com/wp-content/uploads/sites/5/2021/12/2021_FungicideSummary_final.pdf
  4. Will a Second Fungicide Application be Worth the Cost for Tar Spot? https://cropprotectionnetwork.org/publications/will-a-second-fungicide-be-worth-the-cost-for-tar-spot-management



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...