Eastern Iowa corn growers are dealing with a new invasion: tar spot.

“Tar spot was first observed in Iowa in 2016 in a couple of eastern counties,” said Alison Robertson, Iowa State University (ISU) associate professor of plant pathology and microbiology. “In 2018, it was reported from 15 counties in eastern Iowa.”

Tar spot was first reported in the United States in 2015 in Indiana and was also confirmed later that season in Illinois.

Robertson said the disease — identified by...