The term "biodiversity" is nowadays one of the most often used words, both, in biological and in social sciences. However, a generally accepted definition of this term is lacking. In biological sciences the concept becomes a catchall and is mostly understood in the categories of taxonomy and systematics. The understanding is obvious and necessary and yet, very unsatisfactory. Apart from the occurrence of various taxons on an area, the important elements are also: their frequency, distribution, interactions, etc., which are most often omitted or unmeasurable. When they are all considered - in the concept at least- it seems obvious that phytopathology, dealing with diseased plants, is involved in the objects reflecting the biodiversity in a very distinct way.