Understanding fungicide mixtures to control Rhynchosporium in barley and minimise resistance shifts

Summary

Sector:
Cereals & Oilseeds
Project code:
PR436
Date:
01 July 2004 - 31 March 2008
Funders:
AHDB Cereals & Oilseeds.
Total project value:
£164,214 From HGCA (Project No. 3028)
Project leader:
S J P Oxley1, F Burnett1, E A Hunter2, B A Fraaije3, L R Cooke4, P C Mercer4 and A Gilchrist5 1SAC Commercial Ltd., West Mains Road, Edinburgh EH9 3JG 2Biomathematics and Statistics Scotland (BioSS), The University of Edinburgh, JCMB, The King's Buildings, Edinburgh, EH9 3JZ 3Rothamsted Research, West Common, Harpenden, Herts. AL5 2JQ 4Applied Plant Science & Biometrics Division, Agri-Food & Biosciences Institute (AFBI), Newforge Lane, Belfast, Northern Ireland, BT9 5PX 5Scottish Agronomy Ltd, Arlary Farm, Milnathort, KINROSS, KY13 9SJ

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About this project

Abstract

The objective was to investigate the impact of fungicides used in mixtures and sequences of barley in terms of eradicant and protectant activity against rhynchosporium leaf scald (rhynchosporium) caused by Rhynchosporium secalis. Since resistance to several fungicide groups is becoming a major problem, the research also tested for potential resistance shifts to ensure that the most effective fungicide mixtures did not increase resistance. New molecular techniques were used to identify the presence of R secalis prior to symptom development to determine their practical use as a guide to disease risk.

Treatment with a single fungicide did not achieve the best disease control, yield or margin. Prothioconazole (Proline) was the key fungicide component in a fungicide mixture for both disease control and yield in winter and spring barley. Cyprodinil (Unix) was also key for yield in winter barley, but less important in spring barley. Pyraclostrobin (Vivid) was an important component of a mixture where rhynchosporium eradication was required. Chlorothalonil (Bravo) was a useful mixing partner, but in two-way mixtures, rhynchosporium eradication was reduced where the dose ratio was 1:1. This effect was not seen in a three-way mixture where the dose ratio of chlorothalonil to other fungicides was 0.5:1.

Using prothioconazole alone shifted the rhynchosporium population towards greater resistance compared to using the fungicide in mixture with a second active ingredient. Mixtures therefore will limit the increase in resistance occurring. Prothioconazole provided good control of rhynchosporium in these situations, but pyraclostrobin and cyprodinil also gave favourable disease control.

At grain prices of £175/tonne, two-way fungicide mixtures were the most cost effective approach for spring barley and three-way mixtures for winter barley. At lower grain prices of £75/tonne, two-way mixtures were the most cost effective for both.

Fungicide diagnostics were a useful tool to determine disease levels in high pressure crops by testing leaves and shoots before treatment. Visual assessments were effective, but a diagnostic test was more sensitive where disease symptoms had yet to appear. By testing rhynchosporium levels late in the season, it can be concluded that a yield response to fungicide occurs both in crops where visual symptoms are present and also where rhynchosporium DNA levels were high in the absence of symptoms. The lowest yield responses occurred where DNA levels and symptoms were low in the upper leaves. Plant breeders will need to redefine a resistant variety as one where visual symptoms are not present and where the fungus cannot be detected inside the plant. These results suggest varieties can respond to fungicide in the absence of visual disease symptoms but where the fungus is detectable at 10-40 pg DNA inside symptom free plants.

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