Depending on the extent of the damage, a fungicide may or may not be warranted. Professional foresters, like medical doctors, don't like giving blanket "treatment" recommendations over the Internet without knowing more information.
Therefore, might I suggest you begin by contacting your local Univ. of Illinois Extension office and ask to speak with a horticulturist or Master Gardener. You will likely need to provide more information such as the exact species, high-resolution photographs, or maybe even a sample.
UI Extension Office Locator: https://extension.illinois.edu/global/where-we-serve
Best of luck!
