Fungicide resistance – an update from Bayer

Resistance to fungicides, as well as other pesticides, remains high on the agenda of all Irish tillage farmers.

While we have seen examples of active resistance develop for decades, it was probably the development of resistance to the strobilurin or QoI fungicides in Septoria tritici in 2001 that magnified the focus on this issue on Irish farms.

Resistance is a complex subject, regardless of the fungus or the family of fungicides used. We have seen triazole actives decline in efficacy for decades, but there was always a new one coming to boost control for a while longer. However, resistance was total and final in the case of the strobilurins and Septoria.

As time goes by, we continue to learn more about resistance, both within the fungi and within the various families of fungicides. While we saw total cross-resistance develop in the case of Septoria and the strobilurins, the same was not the case in other diseases such as net blotch. In this disease resistance to one active did not necessarily result in total cross-resistance across all other actives in that family.

For these reasons, it is important to continue to update our knowledge of this important area.

On a recent visit to Ireland, Andreas Mehl of Bayer provided an update on the progress of resistance development across the EU.

Having relied heavily on triazole actives for decades, prothioconazole (PTZ) is now one of the few long serving azole actives remaining in the market here and is the most widely used. This is now being supplemented by Revysol (mefentrifluconazole), which was introduced a few years ago.

So the story around PTZ is important and interesting across a range of diseases.

This was very much the focus of the conversation with Andreas, as the removal of many other triazole actives left PTZ on its own to do a lot of the heavy lifting against various diseases, which left it at risk of resistance development. However, it still seems to be showing high levels of efficacy.

While this article deals mainly with PTZ, it also focuses on other Bayer actives, with occasional comments on actives from other manufacturers.

The situations with regard to efficacy levels are given on a disease-by-disease basis.

Disease pressure – plus our high levels of fungicide-use – have always tended to drive resistance development in this country. Our resistance levels are often (but not always) among the highest found in the range of countries surveyed. Sometimes we see spikes in resistance, but then these seem to ease back again – and on balance, PTZ would appear to be holding up very well against all the diseases it has good activity against.

Resistance levels are also increasing in many other countries, up to or even exceeding the average levels found in Ireland.

So, on balance, there is an ongoing decrease in sensitivity to PTZ in Septoria across most countries.

Andreas commented that these are mainly rising to the levels found in Ireland some years ago.

He also said that Ireland has been seeing a slight increase in sensitivity in recent years, so there is obviously some form of rebalancing taking place within nature.

The situation here is possibly being aided by the range of anti-resistance strategies we now have being built into all fungicide programmes on all crops.

In summary, Andreas says that there has been a slight drop in sensitivity across Europe in recent years, but there are still no issues anywhere with regard to field performance.

The desthio-metabolite

In recent years, there has been a tendency to look at one of the metabolites of PTZ to provide a more accurate picture as to what the active is really doing. A metabolite is the byproduct of an applied active, which is thought to be active against the target. The product in question is a compound called desthio-prothioconazole (D-PTZ).

In general, the active metabolite is the part with the main fungicidal activity, but in the case of Proline it seems that both PTZ and D-PTZ show somewhat different abilities to control different isolates of the Septoria fungus.

This may help explain the less-than-perfect cross-resistance with other fungicides in the past, while also possibly explaining how the active has remained so effective for so long.

However, despite the apparent robustness of PTZ, Andreas indicated that the high sensitivity shown by Revysol is likely to be very important to the longer-term efficacy of PTZ and vice versa.

Bixafen has weakened

Reasons to be concerned about the use of Bixafen for Septoria control are increasing. Isolates that are more resistant to this active have been found in the Irish Septoria population over the past two seasons.

This is occurring through the expansion of both low and strong mutations (in terms of resistance), which have now been found in the main wheat-growing regions of the south and east of Ireland.

While some other countries currently have fewer concerns about reduced sensitivity, Andreas said these less-sensitive mutants are also now increasing in other countries.

Mutations, such as C-T79N and C-N86S, are regarded as soft mutants and can still be controlled, while double mutants, such as C-H152R and others, pose a more serious threat, being less sensitive to fungicide activity.

However, there are still some questions regarding the fitness of some of these specific mutants.

While there was no specific testing for brown rust isolates in Ireland, testing elsewhere in Europe would suggest that the overall sensitivity of prothioconazole (PTZ) remains stable.

However, the presence of the C-I87F mutation did cause problems for some SDHI actives, but not all. This mutant did cause problems for field control in places across Europe, so there is a need for caution if relying on SDHI actives to control this disease.

Yellow rust

On balance, there seems to be a slight decrease in the sensitivity of yellow rust populations to PTZ. However, Andreas commented that accurately testing for resistance in yellow rust populations is difficult.

PTZ continues to offer reasonable control over mildew on wheat and barley.

It was really the only triazole active to hold its mildew control for so long, which does suggest it is in some way different to the many triazoles that preceded it.

Andreas said that spiroxamine also seems to be doing okay on wheat and barley mildew.

He also commented that, for whatever reason, the use of resistance factors (a calculation to help assess the degree of sensitivity a fungus has to a chemical active) does not necessarily reflect the field-performance of actives for powdery mildew control.

The story around net blotch is perhaps the most concerning resistance-issue overall, which has been building for a number of years now.

Andreas indicated that there are now some troubling concerns in parts of Europe around resistance in net blotch populations.

On PTZ, the situation here would appear to be deteriorating – perhaps because other actives are offering less protection than they used to do.

While Andreas feels that PTZ is still adequate, he also described it as being “definitely under pressure”, so barley growers should keep an eye on this disease.

Andreas described fluoxastrobin (in Fandango) as being quite poor against net blotch now, but that trifloxystrobin is still quite good against the disease.

He indicated that all SDHI actives appear to be weakening and said Bixafen now has a real control issue, especially where the double mutants are prevalent.

He went on to say that fluopyram generated some real concerns in 2021, but it appeared to have been less of an issue in 2022.

The main problem now is that there are just not enough good actives that are effective against net blotch to help protect each other against this fungus.

These findings also present a cause for concern with seed dressings, especially on winter barley.

While there is no evidence of any level of failure with seed dressings, the risk must be regarded as high if the evolution of mutations continues for foliar infection.

On Ramularia, Andreas reported that the strobilurins are no longer active and that this largely holds true for the SDHIs too.

While the impact of DMI (demethylation inhibitor) fungicides (triazoles) remains a bit variable, they still provide some level of activity against Ramularia.

It is somewhat comforting for wheat growers to know that most of the actives we use appear to be holding up well to resistance development.

However, there is no denying that sensitivity is continuing to decrease across most of Europe, and that any further loss of fungicide actives will only make this situation worse.

It seems that the real concern now relates to our continued ability to control net blotch in barley. Some troublesome mutations have been found in parts of the continent and we know they have a tendency to move about, given the opportunity.

This must be watched carefully with every care taken at field-level to minimise the risk of infection, especially with early-sown spring crops.

While caution is always necessary, it seems that in Ireland we can continue to expect adequate field performance against all diseases from sensible fungicide mixtures in the year ahead.