Practical measures to combat fungicide resistance in pathogens of wheat

Low levels of resistance in some popular varieties, as well as the unpredictability of disease, mean fungicide use is relatively high in wheat crops. However, such use puts selection pressure on pathogens, making fungicide resistance more likely to occur. These steps will help you maintain sustainable levels of control.

In theory, any pathogen can develop resistance to fungicides. However, the risk is not the same in all pathogens. Septoria tritici blotch is currently of greatest concern in wheat, with significant shifts in sensitivity to strobilurins and azoles in UK populations. Isolates with mutations that confer reduced sensitivity to succinate dehydrogenase inhibitors (SDHIs) are also present across the UK, with increasing frequency each year.

For top tips on the general management of fungicide resistance in cereals, visit our FRAG page.

For wheat, septoria tritici blotch, yellow rust and brown rust are the main drivers of fungicide use, which require additional measures:

• Avoid very early sowing, as it can increase septoria pressure early in the season
• Control volunteers, as these are an important inoculum source for rusts

• Each application increases the period of exposure of pathogens to fungicides. This can select for resistant strains, even if the pathogen is at a very low level in the crop
• Only make applications in response to the disease risk (on a crop-by-crop basis)
• At some timings, the most appropriate action may be to avoid making an application
• On susceptible varieties, the consequences of applying too few fungicide treatments can be severe. On more resistant varieties or in lower-risk situations, the economic consequences of overtreatment or undertreatment are in closer balance
• Avoid pre-T0, T0, T1.5 and T4 spray timings, unless there is a clear economic benefit. If a spray is applied at these timings, the use of a multisite will reduce resistance risk (note: take account of any timing and total dose restrictions)
• Follow statutory limits and never exceed the maximum number of applications for a product or mode of action

• Because spray timings have a significant impact on fungicide efficacy, a well-timed spray can avoid the need for higher doses or extra sprays
• Accurate timing is about applying in a protectant situation, rather than a curative/eradicant situation
• The efficacy of previous treatments should be monitored and subsequent inputs adjusted accordingly

• SDHI resistance in septoria evolves more quickly at higher doses, according to current evidence. Azole resistance is less affected by dose
• Use the minimum dose of SDHI required for effective disease control, for the variety and disease pressure, to help slow the spread of resistance
• When tank-mixing SDHI and azoles, use a high rate of azole relative to the SDHI. This will offer greater protection to the SDHI, without significantly increasing the risk to the azole

• 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 mixing 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’

Fungicides that have multisite modes of action are at lower risk of resistance and have no recorded instances of resistance in cereal foliar diseases.

Do not

• Apply more than twice to any cereal crop
• Apply without a mixing partner
• Apply only with a strobilurin as a mixing partner

Avoid

• Applying when disease risk does not merit it
• Applying only with a multi-site as a mixing partner

Do

• Mix with at least an equivalent label rate of an azole
• Include a multi-site, but be aware these can antagonise the curative activity of partner products

FRAC Code 7

SDHI fungicides include boscalid, benzovindiflupyr, bixafen, fluopyram, fluxapyroxad, isopyrazam and penthiopyrad.

They have strong activity against septoria in wheat, moderate-to-strong activity against yellow and brown rust, and low-to-moderate activity against powdery mildew.

They are classed as medium-to-high risk for the development of resistance and there is cross-resistance between different SDHIs.

Septoria isolates with mutations conferring reduced sensitivity to SDHIs have been found across the UK. Field activity remains good but robust anti-resistance measures must be used to slow the further spread of resistance.

Do not

• Overexpose azoles (use alternative chemistry where possible)
• Use azoles alone for mildew control

Avoid

• Applying when disease risk does not merit it
• Applying azoles alone without a mixing partner

Do

• Alternate azoles through the season
• Mix with a multi-site, except in highly eradicant situations

FRAC Code 3

Azole fungicides include difenoconazole, flutriafol, metconazole, prochloraz, propiconazole, prothioconazole and tebuconazole.

They have low-to-moderate activity against septoria, moderate-to-high activity against yellow and brown rust and low-to-moderate activity against powdery mildew and eyespot.

They are classed as medium risk for the development of resistance and there is partial cross-resistance between different azoles.

Septoria isolates with multiple mutations conferring reduced sensitivity to azoles are found throughout the UK and there continues to be selection for even less sensitive isolates.

Reduced sensitivity in powdery mildew to azoles is also common in the UK. Robust anti-resistance measures must be used to slow the further development of resistance.