Chemical Interactions Between Two Lichen-degrading Fungi

Many obligate fungal pathogens of lichens, the so-called lichenicolous fungi, seem to be tolerant of antibiotic lichen secondary metabolites. However, certain lichenicolous fungi are known to be strongly inhibited by chemical defenses of lichens, even those on which they are frequently found in nature. One of these fungi is Nectria parmeliae, an obligate lichenicolous fungus that is observed on many species of lichens. Field observations of this fungus in northern Virginia indicate a marked preference for the lichen Punctelia rudecta. Laboratory studies established that N. parmeliae is incapable of growing on tissues of this lichen unless they are first washed with acetone to remove phenolic defense compounds. It was determined that N. parmeliae can grow on P. rudecta in nature after another lichen inhabitant, a species of Fusarium, enzymatically degrades lecanoric acid (LEC), the dominant lichen compound of P. rudecta. Field studies in northern Virginia demonstrate that observed lichens harboring N. parmeliae generally also harbor the Fusarium sp. Furthermore, high performance liquid chromatography (HPLC) shows that LEC is completely degraded by Fusarium in 90 days, which permits growth of N. parmeliae. Taken together, these results suggest that some obligate lichenicolous fungi cannot colonize certain lichens unless there has been prior chemical processing of these lichens.