Arum-type refers to the morphology of fungal hyphae living in, or around plant root cells.
This is a type of mycorrhizal infection whereby the fungus in question invaginates the cell membrane of a plant cell, and branches in arbuscular manner. Arum-type growth of hyphae is used in endomycorrhizal symbiosis with a plant.
References
Arum type Wikipedia(Text) CC BY-SA