Qol resistance in Mycosphaerella graminicola in the UK: implications for future Qol fungicides

摘要

Strains of Mycosphaerella graminicola (anamorph Septoria tritici) resistant to Qol fungicides were first found in the UK in 2002. Sequence analysis of the cytochrome b gene by Rothamsted Research showed that all resistant isolates carried a mutation resulting in the replacement of glycine by an alanine residue at codon 143 (G143A). Retrospective testing revealed that G143A was also present at a low frequency in UK field populations in 2001. The G143A mutation was reported in Blumeria graminis f. sp.tritici and Sphaerotheca fuliginea to result in disruptive selection, resulting in a rapid increase in the frequency of the resistant isolates, which have been shown not to be controlled at full label doses of Qol fungicides. The G143A mutation has sincebeen linked with several disease control problems. This can be partly explained by the rapid rise in the frequency of resistant isolates in populations and the apparent absence of a fitness penalty associated with the mutation. Because of the importanceof G143A as a predictive marker for Qol resistance, many PCR-based diagnostics have been developed to monitor this mutation in pathogen populations. However, in 2004 and 2005, it emerged that the presence of high frequencies of G143A in populations of M. graminicola did not always lead to a complete loss of activity of Qol fungicides as predicted. This paper questions the role of PCR-based diagnostics as an indicator of field performance of Qol fungicides and discusses possible explanations for the continuing activity of Qol fungicides in the presence of high frequencies of the G143A mutation in populations of M. graminicola.