Terrible Means
by B. Mure
The art is the most extraordinary part of this; the story is competent but a bit compressed. It also doesn't 100% work on a biology and geology level - how do they know that the plant is afflicted vs regular causes of plant disease and death? How is the upstream of the river black but not the downstream? I guess "magic" but that's unsatisfying. (Hi, I'm a nerd.)