Fungicide resistance stewardship

The introduction of the SDHI group of fungicide chemistry has been very welcome over recent seasons and has brought extra disease control and yield increases in wheat and barley crops. It was always flagged as being at moderate or high risk of fungicide resistance and sadly over the last two seasons we have seen the first examples of cereal diseases starting to adapt to the chemistry.

Fungicide resistance development is very like the much publicised development of hospital superbugs where antibiotics are overused. The driving principle is very Darwinian and all about the survival of the fittest. Where fungicides are widely used there is a competitive advantage to any individuals in a disease population which carry mutations that make them better able to survive (fitter) compared to the original ‘wild’ population. These mutated individuals thrive and prosper and soon the sensitive population is overrun and the new, less sensitive part of the population becomes widespread. This can be rapid and complete – as was seen when we first picked up resistance in septoria to the strobilurins. Within a season this group was all but ineffective against that disease. Or resistance development can be slow, in the manner seen with the azole group of chemistry where we have seen a slow erosion but have been able to retain them in septoria fungicide programmes. SDHI resistance is likely to sit somewhere between these two examples and so strong stewardship to keep them for as long as we can in as effective a form as possible is really important. It is particularly topical for this season as just before Christmas there were confirmed examples of reduced resistance in septoria sampled from UK crops. Some of these isolates were 100-fold less sensitive compared to the original population which sounds dramatic. It clearly isn’t a good news story but to set it in context we are quite used to isolates of septoria that are 500 or even a 1000-fold less sensitive to azoles. And of course it comes on top of detections of mutants of reduced sensitivity in net blotch in barley.

Good stewardship is a mix of regulatory restrictions and more voluntary good practice in the field. We know that the SDHIs are at risk and that azole fungicides have been declining gradually in efficacy for years so the best strategy now is to use both groups in as balanced and as sensible a way as possible to try to delay any sudden declines in efficacy that would be hastened by high risk practices such as the use of straight products or very unbalanced mixtures. Multi-site fungicides have a very important role to play in protecting both groups – and ideally we would be growing varieties with better resistance too. The statement below is the guidance on SDHI stewardship prepared by the Fungicide Resistance Action Group–UK which SRUC contributes to through Scottish Government RESAS funding.

SDHI Anti-resistance strategies

Scenario 1 – SDHI Foliar applications

1. Follow the statutory requirement to limit the number of applications to two SDHI fungicide-containing sprays per crop.
2. Always use SDHI fungicides in mixture with at least one fungicide from an alternative mode of action group which has comparable efficacy against the target pathogen(s).
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 statutory limit of two foliar SDHI applications; advice would therefore be to apply any subsequent foliar SDHI applications as described in Scenario 1 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 currently no SDHI seed treatments with efficacy against foliar pathogens approved in the UK
2. Should any such seed treatments be approved, FRAG-UK would advise that these should count as one of the statutory limit of two SDHI applications to a crop. It would therefore become necessary that growers keep records of seed treatments applied.

IPM statement

In order to reduce disease risk and the selective pressure on fungicides, integrated crop protection measures should be used, such as the use of resistant varieties.
Fiona Burnett, Crop & Soil Systems, SRUC