As we move into another production year, it may be worth recalling some of the challenges of the three most recent years.

In the main, these will be remembered for their droughts, wet spells, planting and harvesting difficulties, very variable yields, mixed quality success and perhaps the fact that they were relatively low disease pressure years.

It may be worth noting that these lower disease pressure years may have contributed to the slowing of resistance development to fungicides, especially in the wet weather diseases.

That said, resistance development has still continued, but at a slower pace.

This is not to suggest that disease was not a problem, or that fungicide treatment was not required or beneficial.

However, less rain in May helps to ease overall disease pressure from troublesome wet season diseases and this may have slowed the further development and spread of resistant isolates, especially in diseases such as septoria, net blotch and ramularia.

Isolates are a segment of the population that show a particular trait, which we mainly assess as susceptibility to different classes of chemistry.

It’s a bit like classing people as blond, bald, fair-skinned, etc, but in the disease world it refers more to elements of the population, or mutants that show decreased sensitivity to different fungicide actives.

Now, as we begin our 2021 season, we must be conscious that further new resistance isolates are emerging in septoria and net blotch

It is important to remember that most fungal populations are made up of a mix of different individuals, which can become groups where they are selected for and are then referred to as isolates.

The choice on what we put into our sprayers is decreasing over time.

Now, as we begin our 2021 season, we must be conscious that further new resistance isolates are emerging in septoria and net blotch. More later.

Continued loss of actives

The loss of pesticide actives continues. This will be our first year to face into disease control in wheat and barley without the support of chlorothalonil.

Its loss will be particularly challenging for the protection of septoria actives that are still effective and for the control of ramularia in barley.

Its removal has given way to the introduction of many new brands of folpet, which is now the multi-site active of choice in the fight against resistance development in septoria and for the control of ramularia.

We must also cope with the loss of other actives such as fenpropimorph, diquat, CIPC, dimethoate, etc.

These and other loses act to decrease the number of tools in the crop growers’ toolbox. It also has the indirect effect of increasing the cost of production because many products that go are from the generic or cheaper actives stable, while those that come arrive with a price premium.

But a separate major concern is the increased risk of resistance development as we narrow the band of actives and modes of action available for weed, disease and insect control.

The fact is the more we depend on chemicals the less dependable they will become in time.

The two big Gs

Research over the past few years has reminded us of the ability of nature to fight back and we are now seeing a considerable increase in the appearance of herbicide resistance in quite a number of weeds, and especially our main grass weeds.

The more often we apply the same mode of action in any pesticide the more we select for resistance development.

While we have herbicide resistance and tolerance in broadleaved weeds too, the grass weeds present the greater longer-term challenge because there are fewer actives and even fewer modes of action to use against them.

So, the first big “G” is grass weeds. The preliminary exploratory work on wild oats, blackgrass, sterile brome, ryegrass, etc, by Teagasc has shown the presence of robust resistance to some families of actives, leaving decisions on product choice and even the viability of growing specific crops questionable. And while we might like to blame imported seed for these escalating problems, it is quite likely that some of these mutations are “homegrown”.

Once we allow numbers of any weed species to be high when sprays are applied, we massively increase the risk of selecting for resistance.

Even where they can be controlled, the additional cost involved means further erosion of crop margins.

Only a decade ago, the British farmer was spending more on herbicides to control blackgrass than we were controlling diseases.

We can ill afford this double cost of grasses and disease so serious farmers must look towards zero tolerance as the best way to tackle this “G”.

The second “G” is glyphosate. While we must still learn to depend on it less for some practices, it is still a very important tool for tillage farming and particularly for the varying practices associated with minimal soil disturbance and carbon preservation.

Once we allow numbers of any weed species to be high when sprays are applied, we massively increase the risk of selecting for resistance

The environmental objectives of lowering greenhouse gas emissions and the loss of nutrients to water can be helped by low or no disturbance systems and glyphosate can be an integral input to enable this to happen.

While the active is globally associated with crop production and Roundup Ready crops in particular, in Ireland the crop sector only accounts for an estimated 30% of total annual use. The 70% is used by a combination of grassland farmers, plus amenity and domestic users.

This active is up for renewal again next year and the anti-glyphosate rhetoric continues (but at a lower level than in recent years). Its re-registration cannot be taken for granted but perhaps it is time that society’s affluent noted the importance of glyphosate in expanding the frontiers of land use to help feed the growing global population so far. To lose it would be highly negative.

To limit its use to professional users could be regarded as sensible.

Farm to Fork and pesticides

The publication last year of the EU’s Farm to Fork strategy pointed the direction for ongoing agricultural policy development.

One of the key elements of this for Irish crop producers is the aspiration to reduce pesticide use by 50%. This is a big ask for crop producers on this island, at least until such time as we are provided with viable alternatives to offset our dependence on pesticides.

However, the fact is that this is the direction of travel in the EU. We may not be obliged to meet the full 50% reduction but, sooner or later, we will have to show decreased usage.

There is little doubt but that integrated pest management (IPM) will be a key element in lowering our dependence on pesticides, but it will not remove it. And this will have to be “real” rather than “token” IPM.

There are many ways in which we can use IPM to reduce pesticide requirement. But the challenge will be to use these methods in combination with lower pesticide use while still delivering cost-effective production.

In terms of disease control, sowing dates will be an issue for winter cereals but so will the spread of planting between winter and spring, the use of good rotations, increased consideration of the location of crops relative to previous stubbles, new research solutions, etc.

In terms of disease control, sowing dates will be an issue for winter cereals

Stubble cultivation will have to become more important as a weed control tool. But this must be done sensibly and with the correct equipment to minimise the risks associated with cultivated stubbles. Stubble cultivation can help to reduce the green bridge and the transfer of disease from one crop to the next and, over time, it is very useful in reducing the weed seed burden in the soil.

Changing the focus

We learned at the tillage conference of the work being done at Teagasc to better monitor aphid flight as a risk assessment tool in the spread of barley yellow dwarf virus in both autumn and spring cereals. If this technology can successfully quantify this virus risk, it can be used to more accurately advise on the need, or not, for aphicides.

For the past few decades, the main emphasis has been on the use of chemicals but this must change. It seems inevitable that research will bring other solutions for growers to help reduce the dependence on pesticides.

The development of fungicide resistance in net blotch is a real threat for barley producers.

One important element in disease prevention is the use of varieties with good genetic resistance to specific diseases. These help slow disease development and help fungicides to do their job. Delayed sowing of winter cereals is another way of slowing disease development but it must be balanced against the risks associated with an inability to plant late in the season due to unfavourable weather.

This obligation to reduce pesticide use was always going to be entangled with genetic resistance, or other characteristics in the varieties we grow.

In this regard, gene editing has the potential to repair a weakness in a variety or to add strength or robustness to a variety’s disease resistance.

Such tools could help our ability to cope with lower chemical usage. But while gene editing is an option for growers in other countries, it remains inaccessible as an option for EU growers as the technology remains under the control of GMO legislation. But the hope is that this may change.

Breeding for resistance

The decision by the European Court of Justice, which placed a range of new breeding technologies under the control of its existing GMO legislation, has effectively prevented the use of gene editing in European society for the moment.

However, scientists and academics have argued that this interpretation is wrong and this initiated a report on new breeding technologies which is due to be completed and presented to the European Commission by the end of April 2021.

It remains to be seen if this report will recommend updating the legislation to accommodate the use of gene editing and other new technologies in crop and animal breeding. But even if it recommends change, it is possible that this will continue to be met with considerable opposition on political and ideological fronts where opinions have become highly entrenched.

The importance of gene editing as a technology was made even more important by recent findings regarding our understanding of standard genetic resistance.

At the Teagasc national tillage conference last February, Steven Kildea outlined how single gene resistance works. But he also showed that the specific septoria mutants that broke the single gene resistance in Cellule are highly specific to that resistance gene.

The importance of gene editing as a technology was made even more important by recent findings regarding our understanding of standard genetic resistance

He then examined the progeny of an older variety, Cougar, which had also used single gene resistance that was cracked by septoria.

He tested that specific mutant on the progeny of that variety and discovered that most of its progeny were also highly susceptible.

This means that these specific mutants can follow that resistance gene down through subsequent generations to remain a problem for that single source of resistance.

This finding provides yet more proof of the importance of having a multi-gene approach to genetic resistance, especially against diseases that have high potential to mutate.

This would be very difficult to achieve without access to gene editing.

So, as we move towards the future in Europe, some of the Farm to Fork objectives would be very difficult to achieve without access to new breeding technologies if we are to avoid significant loss of yield and livelihood.

Evolution continues

Resistance in pest species, whether insect, disease, or weed, continues to be a major challenge in crop production.

Plant protection from fungal diseases has seen the occurrence and multiplication of many less sensitive or resistant types in a number of diseases in recent decades.

This continues to be driven by the frequent use of similar modes of action pesticides and at suboptimum rates.

We have seen a weakening of the azole fungicides over many years and resistance has become more complex.

The strobilurins broke down very quickly to diseases like mildew, septoria, fusarium and ramularia, with net blotch causing a problem for some actives and showing less complete cross resistance. This reinforced our dependence on chlorothalonil which was very important in slowing resistance development and it became essential in the fight against ramularia.

The loss of the strobilurins for septoria control led to dependence on SDHI actives in recent years. This family also carried a risk of resistance development and this is now evident, although many fungi seem not to show perfect cross resistance.

This means that one active can still work quite well while another might be totally ineffective.

So, the arrival of Revysol last year and Inatreq for this season are highly opportune in the fight against specific diseases such as septoria.

The loss of the strobilurins for septoria control led to dependence on SDHI actives in recent years

It is important to remember that resistance is only likely to develop to a product that is used.

Where an active from a family is used infrequently, or only once in the season, the pressure for resistance development is reduced.

For this reason, many new fungicide actives are being confined to one use per crop per season.

Fighting resistance

Finding the balance between reducing pesticide usage and successfully fighting resistance seems like a big challenge in the short term. We will not be able to persuade the diseases to go away but we will hope to be able to develop varieties that have more robust resistance to all diseases going forward.

Recent research at Teagasc has shown how vulnerable this genetic resistance is and that varieties need to be protected using fungicides, just as much as fungicides need to be protected by genetic resistance.

We also need to drive hard to reduce our dependence on chemical weed control and the use of plant growth regulators.

Biopesticides will form part of this solution in the longer term. These are not yet seen to offer full control of any problem in the field but it is inevitable that they will be developed to the point that they will help in this fight.

The hope is that they can help us decrease our dependence on the synthetic products.

Bayer update: resistance across Europe

A conversation with Andreas Mehl of Bayer earlier this year provided an overview of current fungicide sensitivity patterns around Europe.

It came as no surprise that Ireland has the highest level of resistance development across many fungicide families for some diseases but other countries have more severe mutations.

I also noted that cross resistance within many fungicide families remains incomplete.

While we have the highest incidence of resistance in our septoria population, these are primarily soft mutants that are still mainly controlled by SDHIs when applied at an adequate rate and with an effective mixture partner.

There are now many different resistance isolates that show variable sensitivity to individual actives.

Some have very little impact on control, while a few are arguably resistant.

Andreas noted that there are now some isolates (segments of the population that are identifiably different) which combine a number of mutations and these may present a greater challenge over time if they prove to be as fit as wild-type isolates.

In Bayer’s case, Andreas said that the combination of bixafen and fluopyram in Ascra Xpro remains strong in the field against septoria, with one SDHI helping the other and both are helped by prothioconazole. Cross resistance depends on the mutants that are present and this influences field performance.

Triazole issues

Ireland also has the highest background level of resistance or reduced sensitivity to triazoles in some diseases. However, actives like prothioconazole still show quite good field performance.

Resistance levels appear to fluctuate from year to year and while the situation may not be getting better, it is not getting worse either.

Other disease snippets

Control of rust diseases appears to be holding up well across all fungicide families but some show slight variance from year to year.

Powdery mildew is more variable and cross resistance seems to be incomplete across triazoles.

He noted that sensitivity to prothioconazole appears to be quite stable in recent years, while tebuconazole has slipped.

Net blotch concerns

Of all the barley diseases, net blotch would appear to be the one that is most prone to resistance development. We already know that ramularia is highly adaptive but net blotch could be a greater threat.

Some actives already show a severe weakness in some continental countries.

Andreas said that SDHIs such as bixafen have some issues, but that fluopyram is more effective on mutants.

Prothioconazole continues to work well in Ireland but is coming under pressure in certain regions of continental Europe, especially when applied at inappropriate rates.

Many strobilurins continue to be effective but some actives have begun to show problems in places. The F129L mutant is causing problems for some strobilurins but trifloxystrobin still works very well.

Key points

  • The presence of isolates resistant to SDHI fungicides is indicative of the continuous battle between man and nature.
  • The continuous loss of actives increases the challenge to stay on top of the battle against resistance.
  • Our climate leaves it challenging to avoid plant diseases but all farms should operate on zero-tolerance basis for all troublesome grass weeds.
  • Access to gene editing technology is becomingly increasingly important as we battle to reduce dependency on pesticide use.