Athelia arachnoidea is a corticioid fungus in the family Atheliaceae. The species forms thin, white, cobwebby basidiocarps (fruit bodies) and typically occurs saprotrophically on leaf litter and fallen wood. It can, however, also be a facultative parasite of lichens and can additionally be a plant pathogen (typically found in its asexual Fibulorhizoctonia carotae state), causing "crater rot" of stored carrots.