Parasitic and pathogenic fungi get their nutrition from other living organisms like plants, animals or other fungi, and they have a negative effect upon the individuals – hosts – they are parasites on. The number of plant and animal species, i.e. hosts, declines as we move towards the Arctic, hence the diversity of fungal parasites also declines. Moreover, the short growing season hampers the parasites’ ability to fulfil their life cycle – especially for those with a complicated one, such as rust fungi or species of Ascomycota alternating 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, parasites can be recognised by the damage they cause to their hosts, e.g. brown, necrotic spots on green leaves or the hosts’ reproductive structures. Widespread and conspicuous parasitising fungi in the Arctic are Rhytisma salicina (Ascomycota) producing black (often swollen and glossy) spots with ascocarps in a common matrix (a stroma) on leaves of Salix, and the genus Exobasidium (Basidiomycota), which makes the leaves of plants in the heather family (Ericaceae) deformed and discoloured (pale yellow to red). An interesting parasite is the smut-like Microbotryum (Basidiomycota) which produces spores inside the anthers of species in the pink family (Caryophyllaceae), e.g. Silene acaulis and Stellaria longipes. The whole production of pollen is blocked, and pollinating insects will be powdered with spores instead of pollen, infecting new plants they are visiting.