The plasmodiophorid organism, Spongospora subterranea f. sp. nasturtii, is the causal agent of crook root disease of watercress (Rorippa nasturtium-aquaticumy. The only current control measure is zinc, used such that levels do not exceed 0.07S~g mrl in effluent water. Laboratory results indicated that zinc acts by actively lysing zoospores of S. subterranea, but does not cause 100% mortality at field concentrations. At 8ug ml", around 20 % of zoospores were seen to survive compared to controls. Of other cations investigated as potential control agents, none were as effective as zinc at equivalent levels. Only cobalt was capable of significant zoospore lysis. Attraction of zoospores to watercress roots and total root extracts appeared to be non-specific when compared to attraction to tobacco (Nicotiana rustica), tomato (Lycopersicon esculentum), and mustard (Brassica napus). Results implied that a general plant attractant was present, and that this did not mask a more specific attractant. A molecular, PeR-based diagnostic test was developed for S. subterranea f. sp. nasturtii. The internal transcribed spacer (ITS) and I8S ribosomal DNA were used to design specific peR primers, capable of detecting DNA directly from zoospores. As little as Sng genomic DNA or 100 zoospores were required as template. Efforts to develop a sampling technique for zoospores in watercress beds were successful based on washing root material followed by peR. DNA analysis also identified a Chytridiomycete-like organism as being consistently associated with crooked roots. However, preliminary studies showed that it did not have a direct role in crook root disease. The I8S ribosomal DNA sequence was' used to infer phylogeny of S. subterranea f. sp. nasturtii, and plasmodiophorids generally, by performing parsimonious and distance-based analyses. Within the plasmodiophorid grouping, S. suhterranea f. sp. nasturtii appeared to be closely related to S. suhterranea f. sp. suhterranea and Plasmodiophora brassicae, based on 270 bases at the 3' end of the gene, whilst Polymyxa species aligned on a separate branch. Based on a complete I8S ribosomal DNA analysis, plasmodiophorids seemed to form a discrete taxonomic grouping, not closely linked to either protists or fungi.