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Title: Recent evolution of Rhynchosporium secalis populations in response to selection by triazoles
Author: Hawkins, Nichola
ISNI:       0000 0004 2726 6511
Awarding Body: University of Reading
Current Institution: University of Reading
Date of Award: 2012
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Rhynchosporium secalis is a major fungal pathogen of barley. Fungicides, including triazoles and QoIs, play an important part in R. secalis control programmes, but can select for resistance. Reduced triazole sensitivity had been reported in R. secalis, but the mechanism was not known. QoI resistance had not been reported in R. secalis until 2008, when the G 143A substitution in cytochrome b was reported in two R. secalis isolates from France. A high-throughput fungicide sensitivity assay was developed for R. secalis, and isolates were screened for Quinone outside Inhibitor (QoI) and triazole sensitivity. QoI sensitivity was reduced by over lOO-fold in the isolates with G143A. This was not found in any UK isolates, but smaller sensitivity shifts were detected. These sensitivity shifts were mostly reversed by alternative oxidase (AOX) inhibitors, and there is preliminary evidence of AOXupregulation following exposure to azoxystrobin. Shifts in triazole sensitivity were not correlated with point mutations or constitutive over- expression of the target site encoding gene, CYP 51, or reversed by putative efflux inhibitors. However, a second CYP51 paralogue, CYP51A, was sequenced from less-sensitive isolates but absent from sensitive isolates. CYP51A was upregulated more than CYP51B following exposure to tebuconazole. Pyrosequencing analysis of the Hoosfield archive showed that levels of CYP 51 A were low until 1998, then rapidly increased. Phylogenetic analysis suggests a CYP 51 gene duplication event basal to the filamentous ascomycetes, followed by multiple losses of CYP51A. Therefore it appears that CYP51A was almost lost from the R. secalis population, but re-emerged due to selection by triazoles. This project has identified the mechanism responsible for an initial shift in R. secalis sensitivity that compromised the effectiveness of some older triazoles, and reported further variation in sensitivity to newer triazoles that currently provide control in the field. Furthermore, the G143A cytochrome b substitution can confer QoI resistance in R. secalis. Therefore resistance management is important for sustainable R. secalis control.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available