Modern fungicide use in Ireland has been dominated by systemic products with broad-spectrum activity. Over time, these have provided excellent and reliable solutions to a wide range of damaging diseases. Systemic fungicides from triazole, morpholine, strobilurin and carboxamide (SDHI) chemistry have been the key components of crop protection programmes over the last four decades and have provided very effective and reliable disease control.

Well-established fungicide timings such as T1, T2 and T3 in cereal crops are based primarily on the three- to four-week protection offered by these chemistries. These systemic actives tend to have very specific modes of action (MoA) to disrupt a pathogen’s life cycle in a very precise way – they are site-specific in their activity.

This very specific MoA makes these chemistries prone to resistance development. Many serious resistance issues have emerged over time following widespread exposure of pathogen populations to the specific chemistries. Mutations emerged in various important diseases to reduce the effectiveness of the chemistry. Sometimes this resulted in complete resistance development to a specific mode of action resulting in failure to control that specific disease.

This was most dramatic when the strobilurins failed against Septoria tritici in 2002. The emergence of the G143A mutation effectively resulted in the total loss of the chemistry for septoria control.

The robust contacts

The alternative crop protection options to the systemic fungicides are the contacts and biologicals. Contact chemistries contrast with the systemics in many ways so they offer a very complementary contrast in activity to help control.

The activity offered by the contact fungicides is best described as non-specific – it is very general in its nature. They work very differently and their benefits need to be measured and understood in a different way.

The single most important contact fungicide used on wheat and barley crops over the past three decades has been chlorothalonil (CTL) and its use became more important over time. This is because of the various resistance issues which emerged, specifically in the control of wet weather diseases such as Septoria tritici on wheat and ramularia leaf spotting on barley.

In both crops, the widespread use of CTL at key fungicide timings made a very important contribution to disease control and to grain yield, quality and profitability. The competitive pricing of CTL was of practical importance for its popularity and usage.

CTL had been under review by the European Food Safety Authority on foot of public health and environmental concerns. The result was that its registration has been discontinued by the European Commission and its removal from use is imminent. It now seems unlikely that we will have CTL after 2020, if even that long.

Similar concerns for future approval licences are being expressed for other contact chemistry, the dithiocarbamates, of which mancozeb is the most familiar. This type of chemistry has not been used much on cereals in recent times, but it could be a possible alternative to CTL, even if less effective.

So, in the short term it looks like the contact fungicide element of a programme will be based around folpet when CTL is gone. However, this is subject to it retaining its approval based on its risk classification and its ability to meet the requirements of the EU registration process.

Folpet on wheat

Many farmers and agronomists have had limited experience of folpet since its commercial introduction. This is because of the widespread preference for CTL due to its competitive pricing and general performance.

Folpet can be summarised as having somewhat reduced efficacy compared with CTL and with a significantly higher cost. Any discussion around the future role of this contact active in wheat and barley must take place in this context.

The scenario for contact fungicide use in the future is that it will be both similar and different to the recent past. Folpet has plenty in common with CTL and the time to use folpet is the same as CTL. That’s the simple part and it is straightforward.

Other aspects are more technical. In wheat crops, folpet rates of 750g/ha (1.5l/ha) are needed to get comparable efficacy on septoria to CTL at 500g/ha (1.0l/ha). However, this rate is unattractive commercially when CTL is available. However, when CTL is gone, the optimum folpet rate is likely to be 500g to 600g/ha, which, in part, means accepting a reduced contribution from the contact component at specific timings.

Also, with CTL-based products used typically three times-plus on winter wheat in Ireland, it is realistic to expect that the use of folpet may be confined to two applications and so reducing the overall contact contribution in the fungicide programme. This is based on both cost and efficacy factors.

The key message is that folpet will be useful, effective and important in future disease control in wheat. But it is not CTL and it will not simply be a like-for-like substitution on wheat.

Concerns on barley

In barley, there is a well-founded anxiety regarding disease control. This is specifically for ramularia control based on the recent emergence of resistance issues in both the SDHI and the most effective triazole chemistry. CTL is highly effective for ramularia control in barley, with 90%-plus efficacy excepted from a well-timed T2 fungicide. Recent field trials have shown variable efficacy at 40% to 70% in 2017 and 2018 trials from triazole/SDHI combinations.

Folpet is a good option for ramularia control, but it does not match the high efficacy of CTL. Realistically, one can only expect a 65% to 75% control level in a moderate ramularia infection from a well-timed 500g/ha application. A split application would be needed to get more robust control in a high-pressure leaf blotching epidemic in a wet summer.

A series of detailed field trials are planned for 2019 and 2020 to fully understand the best practices using folpet in a no-CTL future.

Biologicals

Biological products are under increasing scrutiny and are increasingly the focus of R&D spend. This is partly based on smaller-scale successes in horticulture, with limited favourable outcomes in cereal crops from products such as Optiplant (on septoria), Soil Set (on take-all) and Nutriphyte products.

The role of biological fungicides is uncertain for key aggressive pathogens such as septoria and yellow rust in wheat and for rhyncho and net blotch in barley. However, the increasing understanding of the mechanisms as to how biological products can be effective in pathogen epidemic management - for example prevention, delay or reduction of pathogen incidence - can make an important contribution to future disease control.

Whether the contribution of biologicals is peripheral, minor or major in the next decade remains uncertain. It is worth noting that some future fungicides, including those currently in development, have a classification and environmental profiling close to that of biological, so the definition of chemical or pesticide is more uncertain moving into the future.

Focus on sulphur

One other interesting focus of recent research across western Europe has been the re-evaluation of foliar sulphur for disease control. Historically, sulphur-based products were widely used for mildew control and there were questions about its possible role in reducing other disease levels. This is again the focus of research.

Work in France found that repeated use of sulphur over an extended period typically resulted in 50% less septoria. More recent work in field trials in the UK and Ireland produced promising field performance from high rates of sulphur applied at eight- to 10-day intervals.

Summary

Predictions of uncontrollable disease epidemics in the absence of chlorothalonil are unlikely. I am confident that cost-effective and successful disease control practices will be continued into the future. These will be helped by a very useful contribution from contact chemistry, assuming contact products continue to be available.