Cut resistance risk with your wheat fungicide programme

There is no denying that the recent detection of a new mutation in septoria populations which has the potential to make them significantly less sensitive to SDHI fungicides is not what cereal growers wanted to hear.

With AHDB-funded trials showing triazole performance continuing to slide – particularly where septoria is concerned – the need for product efficacy and robust performance from the remaining chemistry has never been greater.

See also: First sign of resistance to SDHI fungicides found in the UK

So the news that the C-H152R mutation has been found in two septoria isolates at a field site in Hampshire is a timely reminder of what could happen on a wider scale.

It also serves as a warning to anyone planning to ignore industry stewardship advice and use unsupported fungicide mixtures.

Jonathan Blake in a field of wheat

Jonathan Blake

UK discovery

The latest discovery, which comes just ahead of the new spraying season and follows on from one of the mildest winters on record, is relevant because it is the same mutation as that found in Ireland towards the end of last year.

Cereals 2016

Cereals logoVarious agchem and agronomy firms will be on hand at the Cereals event in June to give more detailed fungicide resistance advice. There will also be spray application advice in the Syngenta Sprays and Sprayer arena. 

This year’s Cereals event is on the 15 and 16 June at Chrishall Grange in Cambridgeshire. For more information, visit the Cereals event website

The simple explanation behind this find is that it allows isolates to survive up to 100 times the concentration of SDHIs – giving them medium to high resistance to this chemistry and the potential to spread in field populations.

Certainly, the new isolates are a step change, acknowledges Jonathan Blake, senior research scientist with Adas, especially when it is compared with the isolates that have been seen before.

“But we simply don’t know what size that step is,” he admits.

There are other unknowns too, he adds. “Whether this particular mutation has a fitness penalty or not is one of those. Another is whether it will be cross resistant to other SDHIs.”

In the meantime, it’s important to recognise that the stewardship advice and recommended fungicide strategies are correct, and will continue to be so, he adds (see “Reminder: The Frag recommendations”, below).

“There’s no getting away from the fact that all effective fungicides exert a selection pressure on pathogens,” he points out.

“So if you can identify situations where you can minimise the use of SDHIs, it will help.”

Fiona Burnett

Fiona Burnett

Fiona Burnett of Scotland’s Rural College and chairman of the cross-industry body Fungicide Resistance Action Group (Frag) stresses that the need for careful stewardship has never been greater.

Balanced mixtures

“Following on from this discovery, the main message to come from the latest guidelines and advice is that fungicide mixtures must be balanced,” she says.

In practice, that means that triazole rates must be high enough to support the SDHI properly, she explains. “So if the risk dictates that the SDHI rate is raised, then the azole rate must go up too.”

Multi-sites are also important, she adds. “They are low risk in terms of resistance and can help to relieve the pressure on other chemistry, especially early in the season.”

Bill Clark

Bill Clark. © Jonathan Page

Bill Clark of Niab Tag says that the C-H152R mutation has been known about for a while and that there are now 13 different mutations in septoria to the SDHIs.

“There is always some variation within a population anyway and this mutation was found at a very low frequency. So there’s no need to panic.

“What’s important for growers to understand is that the field performance of these fungicides is still very good,” he comments.

Reminder: The Frag recommendations

The following guidelines, which were issued in December 2015, apply to both spring and winter cereals

Scenario 1 – SDHI foliar applications

  1. Limit the number of applications to two SDHI-fungicide containing sprays
  2. Always use SDHI fungicides in mixtures with at least one fungicide from an alternative mode of action group which has comparable efficacy against the target pathogen.
  3. Tank mixing two SDHI fungicides is not an anti-resistance strategy. In any tank-mix, the SDHI should be applied in a balanced mixture with at least one fungicide with comparable efficacy against the target pathogens from an alternative mode of action group.

Scenario 2 – SDHI seed treatments with no efficacy against foliar pathogens

  1. These do not count towards the limit of two foliar applications, so apply any subsequent foliar SDHIs as described above.
  2. The SDHI seed treatment should be co-formulated with a fungicide with an alternative mode of action to reduce selection pressure on seed-borne pathogens.

Scenario 3 – SDHI seed treatments with efficacy against foliar pathogens

  1. There are no SDHI seed treatments with efficacy against foliar pathogens approved in the UK
  2. Should any such treatments be approved, the Frag advice will be that these should count as one of the limit of two SDHI applications to a crop.

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