Sabikui Bisco, Volumes 1 and 2, by Shinji Cobkubo

Light novel authored by Shinji Cobkubo, illustrated by K Akagishi, with world concept art by mocha.

The back of the first book reads: "In the days that followed the end of the world, few could have predicted the rust wind, a phenomenon that swept across Japan like a plague, choking for the life from the land and its people and corroding  flesh into brittle, iron sand. No one knows how the Rust began, but one mushroom wielding renegade known as Bisco Akaboshi, the Man-Eating Redcap, is determined to bring about its end. He's the latest in a long line of Mushroom Keepers, and together with thr dashing young doctor Milo Nekoyanagi, the two will brave the horrors and wonders of the Iron Desert in search of the Rust Eater: a mythical mushroom rumored to have the abaility to cure thr ailment once and for all."