Development and field testing of fungicide anti-resistance strategies, with particular reference to strobilurin QoI group of fungicides

Abstract

The development of fungicide resistance in plant pathogen populations has become a major problem since the introduction and widespread use of single-site mode of action fungicides. The overall aim of this project was to provide a scientific basis for improved fungicide resistance management by testing hypotheses relating to dose and frequency in exerting selection pressure for fungicide resistance.

The focus of the project was on the widely used strobilurin (QoI) fungicides that were introduced in 1997 but quickly encountered resistance problems with some pathogens. The initial experimental system used was powdery mildew of barley (Blumeria graminis f.sp. hordei), in which QoI resistance had already been reported, and subsequently Septoria leaf blotch of wheat (Septoria tritici) where resistance emerged during the course of the project. Field trials designed to test the three factors most likely to influence selection for resistance, viz fungicide dose, number of applications, and mixture or alternation with fungicides with a different mode of action, were done at sites in England and Scotland. Samples were taken at different times in the season and tested using a sensitive real-time PCR assay that determines the proportion of the pathogen population carrying the strobilurin-resistant form(G143A) of the mitochondrial cytrochrome b  target protein.

A countrywide survey was also done to monitor the incidence and distribution of strobilurin resistance in S. tritici populations from commercial wheat crops. 

Results from the powdery mildew trials showed that both dose and the number of applications influenced resistance development, with higher doses and additional sprays selecting in a stepwise manner for an increased frequency of the cytochrome b G143A allele. Alternations and particularly mixtures delayed resistance development. 

The mutation responsible for the G143A allele was first detected in S. tritici isolates from plots at Rothamsted in 2002, and over the following two seasons (2003-4) the frequency of resistance rose from c. 30% to c. 80%, despite implementation of measures to limit strobilurin use to only two sprays per season, mixed with an alternative fungicide.  Results from the field trials confirmed this rapid selection for resistance in S. tritici in plots receiving strobilurin treatments, and selection was apparently independent of the dose used or number of applications tested. Alternation or mixture of strobilurins with alternative fungicides was ineffective in delaying this selection. Between 2002 and 2005 the performance of QoI fungicides against Septoria leaf blotch showed a dramatic reduction in efficacy.

By comparison, triazoles remain effective, although there are concerns about their long-term efficacy. An integrated approach to disease management, including use of less susceptible cultivars, is necessary to reduce the risk of fungicide resistance and to safeguard the efficacy of single-site fungicides.