Porcini

Grows only on the Autumn farm, by itself: one springs up on free ground every 30s, and a batch when the farm loads. Each ripens in 8m and then waits to be picked; building over one destroys it.

Only Harvest Bots pick them. It earns no spare parts. In a Biofuel Converter each one is worth 2 biofuel. The Reaper never offers chips for it, and on other farms it doesn't appear among your crops at all.