Introduction to the Plasmodiophorids
The plasmodiophorids comprise one class, Plasmodiophoromycetes (Plasmodiophorea); which includes a single order, Plasmodiophorales (Plasmodiophorida); one family, Plasmodiophoracea (Plasmodiophoridae); and 10 genera. The group has been included in either the protoctists (Margulis et al. 1989, Olive 1975) or fungi (Sparrow 1960, Waterhouse 1972). The coordinator for this web site prefers to use the informal term "plasmodiophorids" because of the various formal names for phylum, class, order, and family, depending on whether the group is being considered in the protozoa (Cavalier-Smith 1993) or fungi. Major reviews for plasmodiophorids include Cook (1933), Karling (1968) and Dylewski (1989). Braselton (1995) reviewed problems in the group associated with terminology and several genera.
Regardless of where the plasmodiophorids are classified, they may be considered a monophyletic taxon because all members share the derived character state, cruciform nuclear division. Other features of plasmodiophorids include a) zoospores with two, anterior whiplash flagella; b) multinucleated protoplasts (plasmodia); c) obligate, intracellular parasitism; and d) environmentally-resistant resting spores (cysts).
Economically significant plasmodiophorids include Plasmodiophora brassicae Woronin, the causative agent of clubroot of cabbage and other brassicaceous crops; Spongospora subterranea (Wallroth) Lagerheim f. sp. subterranea Tomlinson, the causative agent of powdery scab of potato; S. subterranea (Wallroth) Lagerheim f. sp. nasturtii Tomlinson, the causative agent of crook root of watercress; and Polymyxa betae Keskin, which, along with a virus, is associated with rhizomania of sugar beet. Also, Polymyxa graminis Ledingham, P. betae, and S. subterranea serve as vectors for viruses of crops including barley, wheat, potatoes, and watercress.
Terminology for structures and life cycle stages is varied because of contributions from researchers from different disciplines. The terminology used on this web site follows recommendations by John Karling (1981).
