Protecting the efficacy of azoles and SDHIs against the development of resistance by Septoria should remain a key part of spray programmes, according to guidelines from the UK Fungicide Resistance Action Group (FRAG).
Over the last three years, HGCA-supported research has looked at the best way to maintain azole and SDHI performance in the years ahead. Researchers have done field experiments to test the best SDHI + azole mixtures for anti-resistance strategies.
In general, spray programmes need to balance the need for good control with appropriate practices to minimise resistance risks to azoles and to SDHIs. In practice, this is challenging as both groups of actives are at high risk of losing efficacy, while the UK climate is conducive to Septoria.
Selection for azole resistant strains is driven mainly by the number of applications rather than the dose, so programmes should aim to use the minimum number of applications necessary. At the core treatment timings of T1 and T2, robust azole rates will maximise control and protect any SDHI partners from resistance.
At other timings, alternatives to azoles should be considered, depending on the situation in the field. For example, consider using multi-site inhibitors at T0 for Septoria protection or strobilurins against rusts. Using azoles at additional timings – T1½ and T4 – will increase selection for resistant strains of Septoria.
For SDHI resistance, selection is likely to be increased both by dose and the number of treatments. As a result, it is important to use the minimum SDHI dose and number of treatments for effective control. Partnering with a robust dose of azole will reduce SDHI resistance risk without markedly increasing selection for azole resistance.
Throughout Septoria control programmes, it is important to use a range of modes of action and maximise the use of multi-sites which have a low resistance risk, to protect azoles and SDHIs and prolong their useful lifetimes.
A longer summary of azole and SDHI stewardship from FRAG is available at hgca.com/disease
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