Fungicide resistance in cereal pathogens 1991-96

Abstract

1. Summary

This report explores resistance questions surrounding fungicides used to control three of the most important cereal diseases in the UK: Rhynchosporium secalis (leaf blotch) on barley; Erysiphe graminis (powdery mildew) on both wheat and barley; and Septoria tritici (wheat leaf blotch). Yellow rust (Puccinia striiformis) was also considered. The work focused on many field experiments across the UK during 1991-6, and which have kept abreast of fungicide use strategies as they have developed. Fungicide sensitivity has been monitored by methods appropriate to each disease/fungicide combination, as well as exploring new assay methods aimed at increasing accuracy and throughput of monitoring programmes.

The report identifies benzimidazole resistance in R secalis populations for the first time, and the extent to which its frequency impacts on field performance, especially in wetter regions of the UK. Resistance to some DMIs has also increased in R. secalis populations, but field trials clearly show a benefit from mixing existing DMIs with morpholine fungicides. It is suggested that this is one strategy that can be adopted in practice.

Although some decline in morpholine sensitivity in powdery mildew populations may have occurred in 1994 and 1995, changes have been small and unlikely to affect performance.

Selection for reduced sensitivity to DNA fungicides that occurred in S. tritici populations prior to 1990 has not continued, and control by a wide range of azole fungicides remains good. Reduced dose rates do not appear to encourage resistance in S. tritici, but this may be due to the rapid mobility of the azole (Flutriafol) used which may allow survival of a reservoir of sensitive individuals in the basal, unprotected, parts of leaves.

The appearance of many new fungicides with systemic protectant action may alter how fungicides are used. How to incorporate early applications of persistent products into effective anti-resistance strategies, and preserve the usefulness of existing, broad-spectrum products, should be a key element of any future independent research programme. Mixtures of fungicides with different modes of action are likely to remain the main anti-resistance strategy, and research resources need to be focused on optimising these strategies.

Novel monitoring methods can improve the accuracy and number of field isolates that can be tested. Lack of good quantification limits the value of dot blot methods, but developments in DNA diagnostic technologies should provide robust, micro-titre plate assays, which overcome these difficulties.

2. AICC Review

Introduction

Until recently the choice of fungicides was limited to four systemic groups MBC, DMI, Morpholines and one non systemic (chlorothalonil dithiocarbamates).
1980s: HGCA studies established baseline sensitivity for the most important cereal diseases.
UK wide monitoring of Rhynchosporium populations gave early warning of resistance to DMIs such as Tridimenol.
1991: detection of MBC resistant strains prompted some of the research described here.
Growers want to reduce fungicide rates but the consequences of this are unknown. The trials programme here examines this with mildew on wheat and barley and Septoria tritici on wheat.

Techniques used

 Results

R. secalis

Before 1991 no resistance to carbendazim detected.

1991 and 1992 the frequency of resistant strains increased but no further increase has been detected since. Cross resistance with DMI's - some resistance shown to DMI's particularly triadimenol and propiconazole. DMI/morpholine mixtures helped to counteract any resistance problems.

S. tritici

No real evidence of resistance to DMI's.

Mildew

Sensitivity of barley powdery mildew to fenpropimorph monitored since 1988 and no change in sensitivity has been found but a reduction in sensitivity to wheat mildew detected. No change in sensitivity to tridemorph between 1991 and 1993.

Fenpropidin monitoring began in 1991and a decline in sensitivity to barley mildew detected but no decline in sensitivity to wheat powdery mildew found.

Yellow rust

No evidence of a decline in sensitivity to morpholine fungicides since the introduction of fenpropimorph in 1983.

Agronomist Analysis

Unfortunately none of the data here gives a complete picture of what is happening. One of the main criteria for a good HGCA project is that it covers all of the main cereal growing areas of the UK and unfortunately this does not. All the mildew work reported was done in Scotland and nowhere else. Most of the Rhynchosporium work was done in SW England and Northern Ireland.

I would like to have seen the effects of combining a dithiocarbamate or chlorothalonil with the carbendazim or DMI's for Rhynchosporium or Septoria control as I believe that this enhances their effectiveness. Some useful conclusions but it is a pity the story was not more complete.