Wheat septoria: SDHI sensitivity shifts seen, once again

When it comes to the UK’s top wheat disease, septoria tritici, all eyes are on signs of efficacy shifts in two key fungicide groups – azoles and SDHIs. With possible further shifts in SDHI sensitivity detected in 2020, the industry needs to continue its collective stewardship efforts to protect chemistry.

Today (1 December 2020) saw trends in fungicide performance outlined, as part of AHDB’s Agronomy Week. This article explores the key developments for septoria and provides links to related resources.

To measure (quantify) changes in fungicide efficacy (especially, before loss of control is clear in the field), the best approach is to test pathogen populations under controlled conditions – and this involves swapping muddy boots for a lab coat.

A team of plant pathologists at Rothamsted Research and NIAB conducts fungicide-sensitivity screens on septoria isolates taken from crops early in the season (before fungicides are applied). The screens test the efficacy of azoles (represented by prothioconazole) and SDHIs (represented by bixafen) in vitro. The aim is to determine a population’s EC₅₀ value for specific fungicides.

Changes over time show up best in the long-term data set from Rothamsted populations (2003–20), although regional results show a mirroring of trends up and down the country.

An EC₅₀ – half-maximal effective concentration – is the fungicide dose that results in a 50% growth inhibition of the fungal isolate (after a specified exposure time). As pathogens become less susceptible to a fungicide, the EC₅₀ value increases. If pathogen isolates with increased EC₅₀ values remain at low levels, they may have no impact on the field performance of the fungicide, when used at commercial doses. However, the spread of insensitive isolates may eventually lead to a loss of control.

Septoria populations have become gradually less sensitive to azoles. However, this shift appears to have stabilised in the last three years (2018–20). The loss of sensitivity is mainly due to various mutations in the azole target-site protein. The presence of these mutations is highly variable across the UK. Additionally, some isolates also feature alterations that affect the expression of the target-site protein and/or the ability of the pathogen to process fungicides. This complex situation has led to a relatively large variation in performance of active ingredients in this group.

Although loss of field performance is clearer at reduced doses, prothioconazole continues to provide good control at full-label doses in protectant situations in fungicide performance trials.

Following a period of relative stability (2008–15), monitoring started to detect significant shifts in sensitivity to SDHIs in 2017. Although this shift slowed in 2019, it appears to have gathered pace again, following an analysis of 2020 data.

Of the isolates tested, 85% carried a target-site mutation that affects SDHI sensitivity. As SDHIs show cross-resistance with other SDHIs, the bixafen data provides a warning for all users of SDHI chemistry. In fact, field performance, particularly of older SDHIs, is being affected. On a more positive note, the frequency of highly SDHI-resistant mutations remains low (based on samples collected early in 2020) and it is possible to achieve good control with a well-designed fungicide programme.

In the fungicide performance project, there is a preference to test single active ingredients, rather than mixtures. This reveals the potential power of each element in a programme. Obviously, the use of solo active ingredients may not be appropriate/authorised in commercial situations. The key is to mix and alternate fungicides across the programme.

• Use mixtures of different modes of action, effective against the target pathogen, to slow fungicide resistance
• Mixtures should be balanced, such that mixing partners give comparable efficacy, where possible
• In tank mixes, all components should have an effective mixture partner for the diseases present
• Do not assume that a pre-formulated mixture is balanced for resistance purposes. Although many are, not all are – it depends on the disease target (check product labels)

• Alternating fungicides reduces the period of exposure of pathogens to any one mode of action
• In many circumstances, mixtures may provide a more practical and effective strategy than alternation, and may be a legal requirement (check product labels)
• Both mixing and alternating can be practised – it is not an ‘either/or’

The following products (active ingredients) were used in fungicide performance trials targeted at septoria:

• Arizona* (folpet)
• Proline (prothioconazole)
• Imtrex (fluxapyroxad)
• Ascra Xpro (bixafen + fluopyram + prothioconazole)
• Revystar XE (mefentrifluconazole + fluxapyroxad)
• Elatus Era** (benzovindiflupyr + prothioconazole)
• Bravo** (chlorothalonil***)

*Applied at 100% dose only. **Not in trials in 2020. ***No longer approved.

The main messages for 2020 are as follows:

• SDHI + azole mixtures continue to give the highest levels of control
• Revystar XE offers highly effective control, substantially better than Imtrex alone
• Full-dose Arizona gave good septoria protection in 2020, which was reflected in yield
• Possible further shift in SDHI sensitivity, but overall efficacy similar to 2018/19

NIAB's Stuart Knight presented the annual fungicide performance update in 2020.