Chapter 3: Protozoa: Plasmodiophoromycota

Molecular phylogenetic analysis has revealed that these pathogens are evolutionarily distant from both Eumycota and Oomycota, positioning them more closely with slime moulds within their own phylum. The group comprises the class Plasmodiophoromycetes, organized into two orders: Plasmodiophorales and Haptoglossales. The chapter traces the complex life cycles characteristic of these parasites, featuring biflagellate anisokont zoospores that initiate infection, amoeboid plasmodial stages within host cells, and the distinctive cruciform nuclear divisions unique to this taxon. Primary plasmodia produce secondary zoospores, which subsequently develop into secondary plasmodia that undergo meiosis to generate durable chitinous resting spores capable of persisting in soil for multiple decades. Plasmodiophora brassicae, the causative agent of club root disease, demonstrates the mechanisms of pathogenic success: zoospores penetrate root hair cells through specialized structures including adhesoria and stylets, establishing plasmodia that trigger massive cell hypertrophy and hyperplasia, ultimately producing the characteristic club-shaped root deformities that severely restrict plant growth, nutrient uptake, and crop productivity. Spongospora subterranea causes powdery scab in potatoes while simultaneously serving as a vector for potato mop-top virus, complicating disease management strategies. Polymyxa betae similarly transmits beet necrotic yellow vein virus. The chapter addresses the formidable challenges in disease control, as resting spores remain viable for up to twenty years, conventional fungicide treatments prove largely ineffective or have been prohibited, and pathogenic races continually evolve to overcome host resistance. Practical mitigation strategies include soil pH adjustment through liming, enhancement of soil drainage, nutritional amendments such as boron application, and cultivation of resistant plant varieties, though comprehensive long-term solutions remain limited. The chapter also explores Haptoglossa, a genus parasitizing nematodes and rotifers, featuring remarkable gun cells that explosively discharge sporidia into hosts through needle-like injection mechanisms comparable to stylet structures in Plasmodiophorales, reinforcing their phylogenetic placement within the phylum.