Elsevier

Crop Protection

Volume 23, Issue 5, May 2004, Pages 393-406
Crop Protection

The effect of fungicide programmes based on epoxiconazole on the control and DMI sensitivity of Rhynchosporium secalis in winter barley

https://doi.org/10.1016/j.cropro.2003.09.009Get rights and content

Abstract

To prolong the effective life of the newer, more active DeMethylation Inhibitor (DMI) fungicides, two-spray programmes were evaluated for their effectiveness in combining good control of rhynchosporium on winter barley with prevention of the build-up of DMI-resistant pathogen strains. Programmes were based on the DMI epoxiconazole alone or in combination or alternation with three partner fungicides with different modes of action, fenpropimorph (a morpholine), cyprodinil (an anilinopyrimidine), azoxystrobin (a QoI/strobilurin fungicide). In the majority of six field trials in Northern Ireland and South-west England (1998–2000), DMI sensitivity of Rhynchosporium secalis isolates was lower after fungicide treatment than before and the least sensitive isolates came from the plots treated with two half-rate applications of epoxiconazole alone. All fungicide programmes reduced disease compared with the untreated control. Epoxiconazole used alone gave the poorest disease control, but did increase the yield. The three partner fungicides all improved disease control and yield; two applications of half-rate epoxiconazole with half-rate azoxystrobin or cyprodinil performed best overall. With the range of pathogen sensitivities found in the present study, a DMI such as epoxiconazole remains a useful component of a control programme for R. secalis, but should be supplemented by a partner fungicide with a different mode of action such as a strobilurin or anilinopyrimidine.

Introduction

Rhynchosporium disease of barley (also known as leaf blotch or scald) is a major problem in the UK. The barley disease surveys of 1997–2001 have estimated the losses in England and Wales due to the residual levels of rhynchosporium after fungicide treatments as (£m/annum) 3.23, 6.65, 4.07, 5.43 and 2.59, a total of nearly £22 m in 5 year period (N.V. Hardwick, personal communication). This is with most farmers applying relatively effective control measures. Disease pressure from rhynchosporium remains high, indicating that fungicides remain the principal method of control over cultivar resistance. This is in contrast to mildew and net blotch which have both dropped in importance in recent years at least in part due to the availability of barley cultivars with effective resistance. This makes rhynchosporium currently the most economically important disease of barley.

During the 1970s and 1980s, rhynchosporium was effectively controlled by fungicides from two groups, the benzimidazoles and the DeMethylation Inhibitors (DMIs). Benzimidazole fungicides were first recommended for control of a range of barley diseases in the UK in 1975, but although highly benzimidazole-resistant strains of some fungal pathogens were selected (e.g. King and Griffin, 1985), this did not occur in Rhynchosporium secalis until the 1990s. In the late 1970s, the introduction of the first DMI fungicides, often referred to as ‘azoles’ or ‘triazoles’, was a major advance in cereal disease management, providing broad spectrum control of all major fungal pathogens (Martin and Morris, 1979). Since then a very extensive range of DMIs has been developed, which differs in the spectrum of diseases controlled, the degree of systemicity and their inherent activity; the more recently introduced, such as epoxiconazole (Saur et al., 1990) are effective at much lower doses than their predecessors.

Resistance to DMI fungicides impaired their performance in controlling barley and wheat powdery mildews during the 1980s (e.g. Wolfe, 1984; Heaney et al., 1986; Clark, 1992) and led to the adoption of strategies in which DMIs were used in mixtures with fungicides with different modes of action. The sensitivity to DMIs of the UK R. secalis population also declined in the period 1986–1990 and this was paralleled by reduced disease control (Kendall et al., 1993). For control of R. secalis, combinations of DMIs and benzimidazoles appeared the most effective option and also provided an anti-resistance strategy. Unfortunately, at the beginning of the 1990s, benzimidazole-resistant R. secalis was detected for the first time (Taggart et al., 1994; Locke and Phillips, 1995) and rapidly became both common and widespread (Taggart et al., 1998; Taggart et al., 1999). Thus continued reliance on either DMIs alone or DMI+carbendazim mixtures was concluded to pose risks in terms of selection for resistance and reduced control.

The use of DMI+morpholine mixtures provided one possible alternative, because even though morpholines alone are relatively ineffective against R. secalis, they improve control when used in combination with DMIs (Taggart et al., 1998). However, the introduction in the 1990s of the QoI fungicides (strobilurins), e.g. azoxystrobin (Godwin et al., 1992), and the anilinopyrimidines, e.g. cyprodinil (Heye et al., 1994) provided fungicides with novel modes of action and activity against R. secalis, which could be used as components of control programmes.

At the time this study started in early 1998, the possibility that strains of pathogens resistant to these two fungicide groups would develop was recognised. In the case of the QoI fungicides, laboratory generation of a resistant strain of Septoria tritici had been demonstrated, but although 10 times less sensitive to azoxystrobin than the wild-type in vitro, it was controlled better than the wild-type in vivo (Ziogas et al., 1997). With the anilinopyrimidine group, highly resistant strains of Botrytis cinerea had already been obtained from the fields in France and Switzerland (Leroux and Gredt, 1995; Forster and Staub, 1996; Hilber and Schuepp, 1996). Cross resistance was shown to occur between the three available anilinopyrimidines (cyprodinil, mepanipyrim and pyrimethanil) and in some cases decreased sensitivity was associated with reduced disease control.

Shortly after the project began, during summer 1998, resistance to the QoI fungicides was reported in wheat powdery mildew in northern Germany and was subsequently detected in the UK (Napier et al., 2000). Since then strobilurin resistant strains have been identified in several other plant pathogens including barley powdery mildew and, most recently, S. tritici; in almost all cases resistance has been associated with a single major gene change (Heaney et al., 2000; Gisi et al., 2002). With respect to the anilinopyrimidine fungicides, isolates of the cereal eyespot pathogens, Tapesia yallundae and T. acuformis, with reduced sensitivity to cyprodinil have been isolated from UK trial sites monitored over several years and, although the reduction in sensitivity was not clearly associated with a decline in disease control, it was concluded that there is clearly a resistance risk in eyespot to cyprodinil (Babij et al., 2000).

This study was designed to explore the use of morpholine, QoI and anilinopyrimidine fungicides as mixture partners for DMIs with the dual objectives of managing resistance to DMI fungicides and achieving sustainable disease control. In addition, the use of DMIs with QoIs and anilinopyrimidines should, theoretically, help to reduce the risk of R. secalis strains resistant to the latter fungicides.

Section snippets

Location of field trial sites

Six field trials were carried out in commercially grown crops of winter barley in areas prone to heavy rhynchosporium infection. In each of the years 1998–2000, there was one trial in South-west England and one in Northern Ireland. In 1998, the trials were near West Bagborough, Somerset (cv. Epic) and Temple, Co. Down (cv. Fighter). In 1999, they were near Exeter, Devon (cv. Fighter) and Gransha, Co. Down (cv. Pastoral). In 2000, they were near Wembury, Devon (cv. Fighter) and Killinchy, Co.

Effect of fungicide treatments on disease development and green leaf area

The levels of infection varied greatly between seasons and sites, being low in Northern Ireland in 1999 and particularly in 2000 (Fig. 1, Fig. 2). The untreated control did not invariably have the highest levels of R. secalis infection on leaves 1 and 2; in some assessments more infection occurred on the epoxiconazole alone treatments (either Tr. 2 full-rate epoxiconazole at T1 only or Tr. 3 half-rate epoxiconazole at T1 and T2).

In order to evaluate which programmes were the most effective

Discussion

To achieve consistent and sustainable control of a pathogen such as R. secalis, where the key fungicides are single-site inhibitors and the pathogen is known to adapt to fungicide use by the selection of resistant strains, both effective disease control and resistance management must be considered.

The DMI fungicides have been the key element of control programmes for R. secalis since the 1980s, but their effectiveness has gradually been eroded by selection of less sensitive strains (Kendall et

Acknowledgements

We thank the Home-Grown Cereals Authority for funding this work (Project no. 1181) and the farmers in Northern Ireland and South-west England who kindly permitted the use of their fields for trials. We are grateful to A. Ruddock and F. Andrews for field and laboratory assistance in Northern Ireland and to S.J. Kendall (IACR-Long Ashton) and J.-B. Speakman (BASF) for assistance with the assay technique.

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