Practical measures to combat fungicide resistance in pathogens of barley

Low levels of resistance in some popular varieties, as well as the unpredictability of disease, mean fungicide use is relatively high in barley 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. Ramularia is currently of greatest concern, with significant shifts in sensitivity to strobilurins, azoles and succinate dehydrogenase inhibitors (SDHIs) in UK populations. Though good disease control can be achieved in barley with robust fungicide programmes, it is essential to use comprehensive anti-resistance strategies to slow resistance development and preserve the efficacy of both existing and new chemistry.

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

Several modes of action are effective against most barley diseases, providing excellent scope for programmes with strong anti-resistance benefits.

When designing a fungicide programme for barley, consider the following:

• Varietal resistance to disease remains patchy, with some popular varieties having low resistance ratings. However, where markets permit and the wider agronomy package suits, it is important to select varieties with resistance to the diseases prevalent, where possible
• Early sowing of spring varieties (December to February) can increase rhynchosporium pressure
• Controlling volunteers and reducing crop debris can help reduce rhynchosporium, net blotch, brown rust and mildew pressure
• Crop stress, caused by some chemical applications (e.g. late-season-applied morpholines), waterlogging or under-fertilisation with nitrogen, can make ramularia symptoms worse
• As seed can be a disease source of rhynchosporium and net blotch, do not save seed for resowing from heavily infected crops

At some timings, the most appropriate action may be to avoid making an application.

• 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
• Applications should be made in response to the disease risk on a crop-by-crop basis, taking account of regional disease pressure and varietal resistance
• 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 over- or undertreatment are in closer balance
• Only use T0 sprays in winter barley if overwintering disease levels are high. To minimise selection for resistance, different modes of action to those planned for later in the season should be used
• T3 sprays seldom give an economic yield benefit in barley
• Follow statutory limits and never exceed the maximum levels

• 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
• In barley, T1 sprays help preserve maximum tiller numbers ahead of significant disease ingress. T2 sprays continue this protection and protect against ramularia, helping to retain green leaf and maximise grain fill
• The efficacy of previous treatments should be monitored and subsequent inputs adjusted accordingly

• Evidence from wheat suggests that SDHI resistance evolves faster at higher doses, while azole resistance is less strongly affected by dose and may be more heavily driven by application number
• This relationship between dose and application number may not hold true in barley, with different diseases and mutations driving resistance
• However, use the minimum dose of azole and SDHI required for effective disease control and include a robust dose of a multisite to protect other chemistry and help slow the spread of resistance

• Mixtures of different modes of action, which are effective against the target pathogen, should be used to slow fungicide resistance
• Mixtures should be balanced, such that mixing partners give comparable efficacy, where possible
• Compared with wheat, there are more actives available with efficacy against some of the key target barley pathogens. Strobilurins, morpholines and cyprodinil should be considered, as well as the azole, SDHI and multisite options
• In tank mixes, all components should have an effective mixing partner for the diseases present
• It should not be assumed 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
• Rely on for ramularia control

Avoid

• Applying when disease risk does not merit it
• Using in a two-way mix to control net blotch (add a third partner)

Do

• Make use of strobilurins to diversify fungicide programmes

FRAC Code 11

Strobilurin fungicides include azoxystrobin, fluoxastrobin, kresoxim-methyl, pyraclostrobin and trifloxystrobin.

They have low-to-moderate activity against rhynchosporium, moderate-to-high activity against brown rust, and low-to-moderate activity against net blotch.

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

Activity against powdery mildew and ramularia has been significantly reduced by resistance and strobilurins can no longer be relied upon for mildew and ramularia control.

Robust anti-resistance measures must be used to slow the further spread of resistance to strobilurins in other barley diseases.

Do not

• Apply more than twice to any cereal crop
• Apply without a mixing partner
• Rely on for ramularia control

Avoid

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

Do

• Mix with at least an equivalent label rate of an azole or a strobilurin
• 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, fluxapyroxad, isopyrazam and penthiopyrad.

They have moderate-to-strong activity against rhynchosporium, moderate activity against brown rust, and 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.

Although SDHI activity against net blotch has previously been strong, isolates with mutations conferring moderate resistance to SDHIs have been detected in the UK, resulting in variable performance in trials.

Ramularia isolates with strong resistance to SDHIs are now common in the UK and SDHIs can no longer be relied upon for ramularia control.

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
• Rely on for ramularia 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

FRAC Code 3

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

They have low-to-moderate activity against powdery mildew.

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

Ramularia is now commonly resistant to azoles in the UK.

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.