Tar spot has been confirmed in all 99 of Iowa’s counties after continuing its march to the state’s western border this year, according to a pest mapping network coordinated by Iowa State University and other land-grant institutions.

“For the most part, the disease remains at pretty low levels, but there have been some fields along the eastern border of Iowa where the disease has become a concern,” says Alison Robertson, Iowa State University plant pathology professor.

Late summer rains and high humidity levels were favorable for the disease to spread more aggressively than in the...