A lichenicolous fungus (from Latin cola 'inhabitant'; akin to Latin colere 'to inhabit') is a parasitic fungus that only lives on lichen as the host.^[1] A lichenicolous fungus is not the same as the fungus that is the component of the lichen, which is known as a lichenized fungus.^[2] They are most commonly specific to a given fungus as the host, but they also include a wide range of pathogens, saprotrophs, and commensals.^[1]

Sometimes a lichenicolous fungus can steal the symbiont of another lichen (kleptosymbiosis) into their own thallus structure, becoming lichens too (trans-lichenisation). In this case, the fungus is known as a lichenicolous lichen. Lichenicolous lichens are generally very selective for their hosts.^[3] By stealing another lichen's photobiont, a lichenicolous lichen saves energy for itself. Diploschistes muscorum is a typical example.^[4]

As of 2018, there are 2,319 accepted species of lichenicolous fungi, distributed across 10 classes of Fungi (including Ascomycota and Basidiomycota), 55 orders, 115 families, and 397 genera. This group comprises 2,000 obligately lichenicolous species, 257 lichenicolous lichens, and 62 facultatively lichenicolous taxa. The majority, 96% (2,219 taxa), belong to the ascomycetes, and the remaining 4% (100 taxa) to the basidiomycetes. About half of the genera (198) are entirely lichenicolous. Additionally, six families (Abrothallaceae, Polycoccaceae, Adelococcaceae, Sarcopyreniaceae, Obryzaceae, and Cyphobasidiaceae, and two orders (Abrothallales and Cyphobasidiales) are completely dedicated to lichenicolous life. DNA sequence information is available for species in 128 genera.^[5] In 2021, the genus Crittendenia was proposed to contain a new lichenicolous lineage in the Pucciniomycotina – the first known example in this subdivision of fungi.^[6]

• List of lichenicolous fungi of Iceland
• Amerosporiopsis phaeographidis