Tar spot – an emerging fungal disease that damages corn leaves – terrified farmers when it first came into fields a few years ago, said Faist, a member of the Michigan Corn Growers Association board of directors representing St. Joseph, Calhoun, Branch, Jackson and Hillsdale counties. Tar spot in silage corn – used to feed dairy cattle – causes significant yield loss, hurts nutritional value and lowers milk production, according to a new study. Aside from reducing yield, tar spot decreases the nutritional value of corn for dairy cattle feed, the study said.