Detection and characterisation of Rhizoctonia solani affecting UK Brassica crops

Rhizoctonia so/ani is the causative agent of wirestem on Brassica crops. R. so/ani is a species complex comprising genetically. distinct groups known as anastomosis groups (AGs). Knowledge of which AGs are responsible for disease is necessary to formulate appropriate management strategies. Important knowledge about the specific AGs responsible for disease in UK Brassica crops and where in the production chain the fungus became associated with plants was lacking. A survey of UK .f3rassica o/eracea crops was completed to. establish which anastomosis groups of Rhizoctonia so/ani were associated with module raised plants. No R. so/ani was recovered from plants collected directly from UK propagators. R. so/ani was identified from asymptomatic stem bases collected from field crops using classical mycology, suggesting field inoculum is important. Such data suggests UK propagation houses have high standards of hygiene and future research should concentrate on elucidating the field biology of R. so/ani. The pathogenicity of the recoveredi.isolates was demonstrated across a range of crops, including B. o/eracea. However, the isolates were not pathogenic to monocotyledonous species suggesting these may act as effective break crops. Sequence data were generated from ribosomal and ~-tubulin regions and the anastomosis groups of the R. so/ani isolates identified using parsimony and Bayesian based phylogenetic methods. Both methods suggested the majority of isolates recovered from the stem bases of UK B. o/eracea plants belonged to AG2-1 and all but one of the remaining isolates belonged to AG4 HGii. These data are consistent with research from the USA, however this is the first report for UK crops. AG determination using nucleotide sequence information proved more successful and less time consuming than classical mycological approaches. AG2-1 formed three distinct clusters in all analyses suggesting this' subgroup is genetically diverse, a conclusion supported by the problems encountered when investigating AGs using classical mycology. The monophyly of genera Ceratobasidium and Thanatephorus were investigated using constrained analyses, however no firm conclusions could be drawn to accept or reject this hypothesis. Protocols were devel~edto' deteet AG 'and ·spedfic sub~groups in plant and ·soil samples using real-time peR. Soil testing suggested AG2-1 was more frequently detected in samples collected from the upper 10 cm of fields used for B. oleracea production. Such information is consistent with other research and suggests the growth of R. solani may be limited by lack of air filled pores within soil matrixes. The molecular methods were used to investigate the spatial and temporal association of R. solani with field grown B. oleracea plants. The molecular protocols confirmed that R. solani AG2-1 became rapidly associated with a large number of B. oleracea plants. The principle arguments for such an association hinged on identifying the dominant behaviour expressed by the fungus. Rapid colonisation of the root system and stem base would benefit saprophytic behaviour of R. solani as the fu.ngus could capitalise when the crop lifecycle was complete. As a pathogen the benefits of early colonisation are clear and perhaps the reason for low disease levels can be explained by the absence of suitable environmental conditions for disease to progress. A third hypothesis could be that R. solani may form mycorrhizal associations with B. oleracea crops.EThOS - Electronic Theses Online ServiceGBUnited Kingdo