Introduction to the Plasmodiophorids
On this web site we are using the informal term "plasmodiophorids" for the group that was the subject of Karling's 1968 monograph, The Plasmodiophorales, and for a number of years was included in the fungi (Sparrow 1960, Waterhouse 1972). Donald Barr recognized that ultrastructural evidence suggested that plasmodiophorids should be considered as protozoa (Barr 1992), and relatively recent studies (Adl et al. 2005, Bass et al. 2009, Cavalier-Smith & Chao 2003) classified plasmodiophorids as Phytomyxea within the Cercozoa, which is in a supergroup known as the Rhizaria in what many people traditionally would consider as protozoa. Burki et al. (2010), however, grouped the Phytomyxea with Gromia Dujardin along with a clade of Acantherea and Foraminifera outside the core Cercozoa when some assumptions were made using large numbers of phytomyxid gene sequences. It does not matter if you put the plasmodiophorids in the Cercozoa vs. the clade with Acantherea and Foraminifera close to the core Cercozoa, they are still in the Rhizaria, not in the fungi. So mycologists and plant pathologists it is time to get over it and stop referring to the plasmodiophorids as "parasitic fungi." Stay tuned to see what the final outcome of the molecular phylogenetic studies will tell us. I'm looking forward to it.
Molecular studies have led to recognition of two orders within Phytomyxea (Bass et al. 2009, Cavalier-Smith & Chao 2003): Plasmodiophorida (the group we are referring to here as "plasmodiophorids"); and Phagomyxida, which includes the genera Maullinia I. Maier, E. R. Parodi, R. Westermeier et D. G. Müller, and Phagomyxa Karling. For the time being this web site will be focused on the plasmodiophorids sensu stricto, leaving the Phagomyxida to others.
Major reviews for plasmodiophorids include Cook (1933), Karling (1968), Dylewski (1989), Dick (2001), and Neuhauser et al. (2010). Braselton (1995) reviewed problems in the plasmodiophorids associated with terminology and several genera, and summarized the group for protozoan (Braselton 2000) and mycological (Braselton 2001) collected reviews.
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, the causative agent of powdery scab of potato; S. nasturtii M. W. Dick, 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, S. subterranea and S. nasturtii 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).
