Why is kladonia written with a c?
heard as kladonia
spelled cladonia
Because Latin had no use for k and wrote Greek kappa as c. Klados became clado-, and the genus is Cladonia.
What the ear did
English inherited the same habit and applies it without exception before a, o, u and consonants: clematis, chlorophyll, cladistics, class, cotyledon. A c in that position can only be a k sound, which is why nobody misreads Cladonia once they have seen it written. The trouble runs the other way, from ear to page, and it is worse for this genus than for most because the word is short. There is nothing else in kladonia to search on: no second syllable that survives the mistake, no distinctive ending. The one letter carries the whole name. Cladonia covers the reindeer lichens that turn heathland grey-white, the cup lichens that grow like tiny golf tees on rotting stumps, and the ones with scarlet tips that children call matchsticks.
C-L-A-D-O-N-I-A
The full entry for cladonia covers the name itself.