Resistance management

Transcription

1 Resistance management Fungicide resistance management Modelling studies Chemicals Regulation Directorate (CRD) Review of experimental literature

2 The emergence, selection and adjustment Emergence Selection Emergence time FDR time Effective life time Time in selection Introduction of fungicide Loss of effective control

3 Mixtures

4 Model structure, selection leaf growth Healthy Infection* Sensitive/ Resistant strain Sporulation * Latent * senescence Infectious (dead/alive) Dead nonsporulating

5 Development of resistance in time

6 Low-risk + High-risk selection Low-risk (% label dose) Fungicide A Max. effective life Constant dose Fungicide B A B

7 Summary: High-risk + Low-risk Conclusions: 1.Mixing does reduce selection. 2.By using an as large as acceptable low-risk dose: - Time till emergence is maximised - Time in selection phase is maximised. 3. The effective life of a fungicide is maximised by using an as large as possible fraction of the low risk fungicide, and lowering the dose of the high-risk if this does not compromise control.

8 Two high-risk fungicides Mixing two at-risk fungicides: (i) Reduces the selection for resistance of the to-be-protected at-risk fungicide. (ii) Introduces a second risk, selection for resistance to the mixing component. Very few papers: Brent et al. (1989) tridemorph and ethirimol (Erysiphe graminis f. sp. hordei) mixture had a lower selection rate for tridemorph and ethirimol resistance. Hobbelen et al. (2012) The mixture gave the longest effective life for the two fungicides.

9 Dose of B applied per season (% of label dose) Mixtures effective life Dose of A applied per season (% of label dose)

10 Application timing

11 Timing and dose FRAC has recommended that eradicant use of phenylamides should be avoided. This is because they are now always applied for control of foliage diseases as a mixture with a multi-site companion fungicide. The latter does not work as an eradicant, so that the phenylamide is acting alone when the mixture is applied to existing infections. Avoidance of eradicant use could possibly delay resistance for another, more widely applicable reason. To wait until a threshold population of the pathogen appears, usually means that many sporulating lesions (occupying up to 5% of the foliar area) are exposed to the fungicide. Opportunity for selection could be much greater than if the fungicide had been applied prophylactically to keep populations permanently low. Presumably it is with this risk in mind that FRAC discourages the eradicant use of DMIs in some fruit crops. To the authors knowledge there is no experimental evidence comparing the resistance risks of prophylactic versus threshold-based schedules, and research on this would be useful. Brent & Hollomon (2007) FRAC Monograph No. 1

12 Timing and dose Single step (QoI) resistance in M. graminicola van den Berg et al, Phytopathology in press

13 Timing and dose HAD loss (%) Selection ratio Effective life (years)

14 Summary: Timing and dose For two sprays: Effective life maximised by applying the appropriate total dose at optimum times Spraying earlier reduces efficacy Applying a higher dose shortens effective life (by approx. one year) The split of total dose between timings, to obtain max. effective life, is flexible