Depicted: aspergillus on corn, in human brain, turning bread to fuzzy heap of fungus.
Aspergillus is a fungus, and a pathogen of animals and plants. It just wants the reduced carbon. Once it is established it will take the whole thing, turning the plant or animal into a fuzzy heap.
Aspergillus is everywhere, waiting. Live plants and animals have defenses. A bioengineered aspergillus with the ability to bypass these defenses would be a formidable weapon. As it turned the plants to spores any animals in the area would be affected too. The landscape would turn to gray fuzz with heaps of fuzz where larger things once were.
I like engineered aspergillus for this because it would make for terrifying apocalyptic fiction, and it is fiction. Malicious persons might find some chemical to throw but are unlikely to be able to engineer their own bioweapons.