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Southern rust crowded out tar spot in 2025, pathologist says

Corn farmers who saw heavy southern rust pressure in 2025 probably encountered less tar spot.

Dr. Alison Robertson is an extension plant pathologist with Iowa State University.

“I think what was most interesting to me last year was that we probably didn’t have as much tar spot as we could have because there was no real estate on the leaf. There was so much southern rust that it completely took over that leaf (and) there was nowhere for the tar spot to infect.”

But she tells Brownfield growers should be more concerned about tar spot going into this year.

“We did see tar spot, and tar spot is definitely here to stay. Tar spot will also survive our winters and will be in residue, so it doesn’t have to blow up here from the south (like southern rust). We already have the inoculum here.”

Robertson says farmers concerned about tar spot or southern rust should consider the disease triangle, which consists of hybrid susceptibility, the presence of the pathogen, and the environment.