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Designing fungicide programmes to minimise the risk of resistance in wheat and barley mildew and wheat yellow rust

Project Report No. 243

Designing fungicide programmes to minimise the risk of resistance in wheat and barley mildew and wheat yellow rust

by

Dr Rosemary Bayles1, Dr Fiona Burnett2 and Dr Bruce Napier1

1NIAB, Huntingdon Road, Cambridge CB3 0LE
2SAC Edinburgh, West Mains Road, Edinburgh EH9 3JG

 

Abstract

 Concerns over the development of fungicide resistance to 'new chemistry' resulted in the need:

• to establish baseline data for the sensitivity of wheat and barley powdery mildew and wheat yellow rust to the new strobilurin group of fungicides in the UK
• to detect early indications of shifts towards resistance to strobilurins
• to gather evidence on the influence of contrasting fungicide programmes on pathogen sensitivity
• to provide insight into the value of different fungicide strategies as 'anti-resistance' measures

Wheat and barley mildew and wheat yellow rust were sampled from fungicide trials throughout the UK. Isolates were tested, on seedlings, to determine their sensitivity to morpholines, triazoles and strobilurins.

Tests confirmed that the sensitivity of wheat and barley mildew to the 'conventional' fungicides (DMIs and morpholines) has shifted towards resistance since their introduction some thirty years ago. The shift has been greater for DMIs than for morpholines and for wheat mildew than for barley mildew. There has also been a shift in the sensitivity of wheat yellow rust to DMI fungicides over the past ten years.

Resistance in wheat mildew to the new strobilurin fungicides was detected in 1999 (approx. 1% of isolates tested), following its detection the previous year in Germany. Resistance in barley mildew was not detected during the course of this project, although there have been recent reports of its appearance at a low level in the UK in 2000. There was no evidence of strobilurin resistance in wheat yellow rust. In contrast to the gradual shift towards resistance to DMIs and morpholines, strobilurin resistance developed in a single step. Resistant isolates show little or no reduction in growth on seedlings sprayed with a full dose of strobilurin.

Because of the extremely low frequency of strobilurin resistance in wheat mildew during 1998 and 1999 it was not possible to evaluate the effects of fungicide strategy on the frequency of fully resistant isolates. Despite this, there were indications that strobilurin application produced a slight quantitative shift in sensitivity.

With resistant wheat and barley mildew isolates now present in the UK, strobilurins will rapidly become less effective as mildew control agents and should not be relied upon for mildew control. There is also a risk that resistance may develop in other key pathogens. Growers should follow resistance management guidelines to reduce the risk of this occurring. More experimental evidence is needed on the effects of dose rate, number of strobilurin applications and mixture partners to ensure that resistance management strategies are effective.

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