Protecting the efficacy of azoles and SDHIsagainst the development of resistance by Septoria should remain a key part ofspray programmes, according to guidelines from the UK Fungicide ResistanceAction Group (FRAG).
Over the last three years, HGCA-supportedresearch has looked at the best way to maintain azole and SDHI performance inthe years ahead. Researchers have done field experiments to test the best SDHI+ azole mixtures for anti-resistance strategies.
In general, spray programmes need to balancethe need for good control with appropriate practices to minimise resistancerisks to azoles and to SDHIs. In practice, this is challenging as both groupsof actives are at high risk of losing efficacy, while the UK climate isconducive to Septoria.
Selection for azole resistant strains isdriven mainly by the number of applications rather than the dose, so programmesshould aim to use the minimum number of applications necessary. At the coretreatment timings of T1 and T2, robust azole rates will maximise control andprotect any SDHI partners from resistance.
At other timings, alternatives to azolesshould be considered, depending on the situation in the field. For example,consider using multi-site inhibitors at T0 for Septoria protection orstrobilurins against rusts. Using azoles at additional timings T1 and T4 will increase selection for resistant strains of Septoria.
For SDHI resistance, selection is likely tobe increased both by dose and the number of treatments. As a result, it isimportant to use the minimum SDHI dose and number of treatments for effectivecontrol. Partnering with a robust dose of azole will reduce SDHI resistancerisk without markedly increasing selection for azole resistance.
Throughout Septoria control programmes, itis important to use a range of modes of action and maximise the use ofmulti-sites which have a low resistance risk, to protect azoles and SDHIs andprolong their useful lifetimes.
A longer summary of azole and SDHIstewardship from FRAG is available at hgca.com/disease