Tar spot observed in six counties in Central Iowa

June 23, 2023 1:44 PM
Blog Post

Well, I’ll be darned! Yesterday I tweeted the only disease being seen in Iowa corn was common smut on leaves. Thanks to an overzealous crop scout from a seed company, we can confirm that tar spot was observed in central Iowa (Poweshiek, Tama, Marshall, Jasper, Story and Polk counties) yesterday. Last year, this same scout also found tar spot for the first time in Iowa on 30 June. (Does this mean Iowa has the best scouts, I wonder?).


You are probably as gob smacked as I am.  We are in the middle of a flash drought and now we have tar spot.  Doesn’t tar spot require leaf wetness for spore release and infection? Yes.  In controlled environment conditions, Breunig et al (2023) reported leaf wetness was important for spore production from stroma (the black structures embedded in leaf tissues).  Moreover, spore germination only occurred when spores were emersed in free water.  Stroma were observed after approximately 15 days only when leaves were inoculated, kept wet for 5 days with misting and then kept in dry conditions with no misting in the controlled environment.


In the development of the model for the Tarspotter app, Webster et al (2023) found the daily minimum or the daily mean temperature over the past 30 days was the most strongly correlated with tar spot development and increased severity.  Moderately warm (below 70F) mean temperatures increased development and severity. If the mean temperatures were greater than 70.7F, the probability of tar spot developing further decreased. Tar spot development was also strongly associated with the daily minimum dew point over the past 21 days. Interestingly, however, moisture variables such as dew point, and relative humidity were negatively correlated. It’s wet, dry cycles that appear to drive tar spot development. During the first week of June, there were several small precipitation events.  Since then, we have been dry.


According to Tarspotter, most of Iowa is currently at moderate to high risk for tar spot development. This means we should be out scouting.  Remember hybrids vary in their susceptibility to tar spot. If you find tar spot, it’ll likely be one stroma on a leaf mid-canopy (Figure 1), monitor that field for further development.  In 2022, at the ISU southeast research farm, tar spot was observed prior to tassel at very low levels – one spot on a few plants. At R3, severity on the ear leaf of the control plots was 1%, and this had increased to 16% by R5.  An application of fungicide at R1 was most effective at reducing disease (Figure 2).


Fungicides are not recommended at this time.  It’ll only be a couple of weeks before we are at tassel.  Several years of data have shown applications of fungicides between VT to R3 manage tar spot effectively.


To keep an eye on where tar spot has been observed in Iowa and the U.S., visit https://cropprotectionnetwork.org/maps/tar-spot-of-corn.


Figure 1. Single tar spot observed on corn in Central Iowa on June 22, 2023 (Photo credit Matt Vandehaar).

Cartoons of tar spot severity and a bar graph showing the effect of fungicide timing on tar spot severity in SE Iowa

Figure 2.  The effect of fungicide timing on tar spot development at the Iowa State University Southeast Research Farm in 2022. Data are from Cody Schneider, Farm Manager. Cartoons depicting tar spot severity are from https://severity.cropprotectionnetwork.org/.




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...