What new blight strains mean for crop protection programmes

Over the course of several seasons, intense late blight pressure in continental western Europe resulted in repeated use of products containing fungicides belonging to both the CAA and OSBPI modes of action groups. This led to the development of two new strains with resistance to one or both fungicide modes of action. These strains have since become established and spread to other countries.

Why it matters

CAA fungicides such as mandipropamid (as in Revus®), benthiavalicarb (Versilus® and Zorvec®) and dimethomorph (Paraat®) typically make up more than 50% of a programme. OSBPI fungicides consist of oxathiapiprolin (Zorvec) which is relied on the during rapid canopy growth stage to protect new growth.

What are the implications

To counter the threat these resistant strains pose to crops, growers are encouraged to follow the latest Fungicide Resistance Action Committee (FRAC) guidelines on resistance management by:

• Incorporating an additional mode of action (in tank mixtures) that is recognised to give effective control for the application rate used and intended spray interval;

• Limiting the number of CAA and OSBPI fungicides applied in a season to 50% of the programme for CAA-containing products and 33% for oxathiapiprolin;

• Not to begin a programme with either a CAA fungicide or oxathiapiprolin-containing product; and

• Limit sequential application of CAA fungicides to two per year and avoid consecutive applications of oxathiapiprolin.

Adhering to these guidelines will be easier for growers in Great Britain where the use of mancozeb is still permitted, but experience from Denmark in 2023 shows that it is still possible to successfully protect crops even with a more limited suite of products and tighter restrictions on fungicide use than is permitted elsewhere in Europe.

Are other fungicides affected

Disease monitoring by Bayer, other industry partners and the Europe-wide Fight Against Blight (FAB) programme, has so far found only Phytophthora strains with resistance to CAA and OSBPI fungicides.

This means, Infinito®, which contains fluopicolide + propamocarb, is not affected. Fluopicolide is a pyridinylmethyl-benzamides and propamocarb is a carbamate. These complementary modes of action combine to deliver in-built resistance management so there is no need to add a mixing partner belonging to a third mode of action group.

Background

The emergence of isolates demonstrating multiple resistances has been described by Dr Juergen Derpmann, Bayer laboratory leader for resistance research, as “a serious development with the potential to do far-reaching damage”.

“The greatest concern is the spread of Phytophthora sub-population characterised according to EuroBlight as EU_43_A1-type and its subsequent evolution, possibly resulting in a sub-population called EU_46_A1,” he says.

The first detection of the sub-population EU_43_A1 was reported in Danish samples in 2018. It became widespread in the Netherlands and Germany in 2022.

By 2023 it had accumulated CAA as well as OSBPI resistance and dominated late blight populations in Belgium, north-west Germany and the Netherlands, whereas in Denmark the frequency of samples with the EU_43_A1-type decreased due to strict resistance management.

“Single cases of OSBPI-resistance were also reported in the UK, France, Poland, Latvia, Lithuania, and Norway,” Dr Derpmann says.

It has been shown that fungicide sensitivity has changed over time within the EU_43_A1 sub-population. It is more diverse and less of a clonal sub-population as we have seen with other Phytophthora sub-populations.

In 2021, CAA-sensitive isolates of the EU_43_A1-type were detected in the Netherlands, whereas in 2022 all Dutch isolates analysed at Bayer were resistant to CAAs. In 2023, all Dutch isolates of the EU_43_A1-type analysed at Bayer were resistant to CAAs and a large part was additionally resistant to OSBPIs. It is estimated, that 99% of the EU_43_A1 sub-population in the Netherlands was resistant to CAA fungicides in 2023. Furthermore, the sub-population EU_46_A1 arose in 2023, which is fully resistant to OSBPIs and shows a reduced sensitivity or up to full resistance to CAAs.

“This is a concerning development. We have double resistance within not only one but two sub-populations, which spread quickly from 2022 to 2023. This presents significant challenges for resistance management,” Dr Derpmann says.

What this means for crops

There are multiple scenarios where, unbeknown to the grower, crops could be left exposed. If CAA and OSBPI modes of action are applied in combination without a partner belonging to another mode of action group or sequentially without being separated by a fungicide belonging to another group. Or if either CAA or OSBPI fungicides are applied with a partner conferring less than seven-days protection, such as the three-days observed with cymoxanil, then these crops will be inadequately protected.

OSBPI resistance has so far been found in four Phytophthora sub-populations characterised according to EuroBlight. Beside the previously mentioned EU_43_A1 and EU_46_A1 sub-populations, also continental EU_36_A1-type isolates have a full resistance to OSBPI fungicides while there are by EuroBlight as-yet-unnamed strains also fully resistant to this mode of action group.

“We see that all isolates of as-yet-unnamed, EU_36_A1, EU_43_A1 and EU_46_A1 sub-populations with mutation(s) in the osbp-gene cannot be controlled with the full dose-rate of oxathiapiprolin in greenhouse experiments,” Dr Derpmann says.

Lessons of history

Speaking at a late blight webinar held by Bayer to highlight the severity of the situation facing growers ahead of the 2024 season, David Cooke of the James Hutton Institute warned of the dangers of relying on sprays containing only a single mode of action in protecting crops.

“Experience with fluazinam-resistant EU_37_A2 and the phenylamides-resistant EU_13_A2 highlights the risk of over-reliance on a single mode of action. What happened in parts of the Netherlands in 2023 was to repeat this mistake. Now, we need to be diligent if the situation is to be contained,” said Dr Cooke.

A sharp decline in the use of fluazinam has served to contain EU_37_A2 although it still exists in isolated pockets, mainly in north Wales. This has led some to suggest reintroducing fluazinam to blight programmes if mixed with a partner belonging to another mode of action group. EU_13_A1 is also still to be found in pockets, typically in the north of England, but the scope to resume using fungicides containing metalaxyl is more limited.

“Isolates of EU_43_A1, a new strain first observed in 2019, have demonstrated intermediate resistance to metalaxyl. This is likely to limit its value in programmes that will also include the CAA mandipropamid and the OSBPI oxathiapiprolin,” he said.

Acknowledgements

Infinito is a registered trademark of Bayer. Infinito contains fluopicolide + propamocarb. Paraat is registered trademark of BASF. Paraat contains dimethomorph. Revus is a registered trademark of Syngenta. Revus contains mandipropamid. Versilus is a registered trademark of Certis Belchim. Versilus contains benthiavalicarb. Zorvec is a registered trademark of Corteva Agriscience. Zorvec Endavia contains benthiavalicarb + oxathiapiprolin.