Fungicide resistance: Septoria status and spray strategies

Septoria tritici is developing resistance to the fungicides used to control it in winter wheat. We explain the current resistance status and outline strategies to slow the severity and spread of resistance.

Septoria tritici (also called septoria leaf blotch) is a key disease target for winter wheat in the spring, but the pathogen has developed reduced sensitivity or resistance to several important mode of action (MoA) fungicide groups.

Resistance will continue to get worse and collective action across the UK is needed to protect the efficacy of fungicides.

A quarter of a century ago (in the early 2000s), a single mutation in Zymoseptoria tritici (the pathogen that causes the disease) led to a rapid and large jump in resistance to strobilurins in the UK.

QoI fungicides still have a place in wheat fungicide programmes, such as for controlling rusts, but they are now ineffective against septoria tritici.

In contrast to Qols, small year-on-year shifts for DMI/azoles have seen resistance to this MoA group in septoria tritici develop gradually, but not fully.

Azoles can still provide useful septoria tritici control but may require higher doses (up to the full label rate), earlier timing (to maximise protectant activity) or newer azole products (which have better efficacy) to keep disease at acceptable levels.

The latter is only an option because azoles only suffer from partial cross-resistance.

Cross-resistance can affect fungicides within the same MoA group.

When a pathogen strain becomes more resistant to one fungicide, it also becomes more resistant to others with the same MoA.

This may or may not (partial cross-resistance) be the same level of resistance.

Cross-resistance is one reason why it's important to use a range of MoA groups across the spray programme: overreliance on any single group is bad news for resistance.

Since 2018, septoria tritici strains with moderate resistance to SDHIs have increased.

These strains, which now dominate the UK population, have cross-resistance to many SDHIs, including bixafen and fluxapyroxad.

However, newer SDHIs, such as isoflucypram (Iblon active) and pydiflumetofen (Adepidyn active), currently have better efficacy against them.

Although AHDB fungicide performance trials show that all SDHIs can still effectively control septoria tritici in the field, we've seen a limited (but increasing) number of septoria tritici strains with even higher levels of resistance (since around 2023).

Fungicide performance trials treated with either isoflucypram or pydiflumetofen in 2025 selected for these higher-resistance strains, which are also cross-resistant to multiple SDHIs.

In the fungicide performance trials, we applied these active ingredients as straights (not mixed with another product) to reveal their efficacy.

Of course, this is not commercial practice, but it shows why resistance management is essential to slow the selection for higher-resistance strains.

The Fungicide Resistance Action Group (FRAG) produces guidance to protect crops and the long-term efficacy of fungicides.

Good resistance management limits exposure of the target pathogen to specific fungicides.

Integrated pest management (IPM) can reduce the need to spray and the dose required, both of which reduce resistance risks.

• Use integrated control measures to reduce disease pressures (for septoria tritici, later drilling helps to reduce disease)
• Grow varieties with relatively high disease resistance ratings for the common diseases
• Only use fungicides where the disease risk or presence warrants treatment
• Monitor crops regularly and treat before infection becomes well established
• Use a dose to deliver effective disease control (appropriate for the variety and disease pressure)
• Make full use of effective fungicides with different MoA groups across the spray programme (in mixtures or as alternate sprays)
• Use mixing partners at doses that give similar efficacy and persistence
• Avoid repeat applications of the same product or MoA
• Never exceed the maximum recommended dose or number of applications

Information on the fungicide resistance status of septoria tritici in this article was provided by Niab’s Dr Nichola Hawkins.

Nichola leads the current phase of our fungicide resistance project that monitors how septoria tritici samples (isolates) from UK fields respond to various doses in the laboratory.