Parasitic and pathogenic fungi obtain their nutrition from other living organisms, and have a negative effect upon the individuals – hosts – they are parasites on. As the diversity of potential hosts declines as you move further north towards the Arctic, so does the diversity of parasitic fungi.  Complicated life cycles coupled with short growing periods hampers the fungi’s ability to fulfil their life cycle. For example, rust fungi and some species of Ascomycota alternate between anamorphic (mitosporic) and teleomorphic (sexual) phases.The rust fungi (Uredinales, Basidiomycota) have up to five different spore types and are often dependent on two hosts to fulfil their life cycle. In the Arctic, rust fungi with a full set of spores and two hosts are rare.

In the field, parasitic fungi can be recognised by the damage they cause to their hosts, e.g. brown, necrotic spots on green leaves or the plants reproductive structures. Widespread and conspicuous parasitising fungi in the Arctic are Rhytisma salicina (Ascomycota) and the genus Exobasidium (Basidiomycota). Rhytisma salicina produce black (often swollen and glossy) spots on leaves of Salix. Exobasidium, attack plants from the heather family (Ericaceae) and deform and discolour the leaves (pale yellow to red).  The smut like Basidiomycota, Microbotryum, produces spores inside the anthers of the carnation family (Caryophyllaceae), e.g. Silene acaulis and Stellaria longipes.  Pollen production is inhibited and pollinating insects are powdered with spores instead of pollen, infecting new plants they visit.