Modern fungicides have contributed greatly to the success of intensive high-yield cereal production systems across northwestern Europe over the last three decades.

The triazole fungicides have dominated fungicide programmes in each of these decades, with notable key roles for other key fungicide groups, including strobilurins and, more recently, the SDHI chemistry. However, the last 30 to 40 years have shown us that the discovery of new fungicide modes of action are scarce and, in recent decades, are proving to be somewhat elusive.

Given the well-documented sensitivity reduction issues with triazole fungicides in the past five to seven years, specifically on Septoria tritici in wheat, it is not surprising that we have seen low to moderate field efficacy performance in local trials in the last three seasons.

This now means that the best possible management and stewardship of the SDHI fungicides is critical, especially for septoria control in wheat.

There are a number of reasons for this. Specifically, the SDHI chemistry has a single site mode of action, hence the SDHI products have a medium to high resistance risk for many key diseases.

Also, resistant strains have already been found for a number of important fungal pathogens worldwide, including for net blotch on barley in 2012 and 2013 in Europe.

Independent research, both locally and across western Europe, has shown that SDHI fungicides show excellent activity for the control of many important diseases on cereal crops and can contribute to very good disease control management on each of these crops.

This article provides an overview of the key properties of the SDHI active ingredients currently available for use on cereal crops and offers brief comments on the strengths and weaknesses apparent across some of the more important disease issues on cereal crops in Ireland.

The Bs – Boscalid and Bixafen

While stronger and higher efficacy SDHI actives have more recently been introduced, other important carboxamide products, such as Boscalid, were the first of the recent SDHI fungicides introduced back in 2005. This has many useful attributes for use in both wheat and barley. In wheat, Boscalid provides good to very good efficacy for eyespot but robust dose rates are required to achieve the higher efficacy.

Boscalid has a more preventative profile for foliar disease control as it is only weakly curative. It is sold in pre-formulated mixtures with epoxiconazole but this now has limited curativity expected due to the emerging triazole insensitive septoria strains.

Hence the use of Boscalid-containing products like Venture Extra and Cauldron need to be very well timed on wheat. The products are most likely to be used at T1 timings.

Bixafen is also a very versatile SDHI fungicide for wheat. It is included in products like Aviator and Boogie Skyway for wheat, with prothioconazole as the key triazole partner.

Each of these Bayer products show strong broad-spectrum activity for key diseases at each of the T1, T2 and T3 timings on wheat, linking the strengths of both the SDHI and triazole components.

The efficacy of prothioconazole on both stem eyespot and ear blight offers options for using the products as a T1 or a T3 option.

Bixafen chemistry has a very good profile for use at the mid-season (flag leaf) timing on wheat with a good curative plus protectant balance in Aviator.

At robust application rates of 1.0+ litres/ha, the good to very good persistence of Aviator is a notable characteristic.

The loss of triazole efficacy has affected the curativity of this fungicide also and, hence, good fungicide timing has become even more critical.

There is a note of caution in situations of active rust infection where the more modest efficacy of prothioconazole is a reason to opt for products with a higher rust efficacy profile.

Many field trial studies and on-farm experiences have shown that Bixafen is an excellent SDHI for barley disease control with strong intrinsic activity on net blotch and ramularia and it also offers good additive benefits to the strong prothioconazole efficacy on rhyncho and the ear disease complex.

While careful dose rate selection is important for Siltra, as for all other fungicides, the product has shown good rate flexibility in low to moderate disease pressure scenarios.

Curative Edge – Xemium and Penthio

Xemium has a strong profile for septoria control on wheat and has a strong intrinsic activity on this disease and the Adexar and Librax products are profiled as key options for the mid-season timings on wheat crops.

The most notable strength of Xemium is good to very good curative activity and this is a very valuable property in a scenario where triazole curativity is being weakened and eroded by new triazole strains.

However, there is a need to use the SDHI + triazole products with careful spray timing to protect the chemistry going forward, as too much curative use in medium to high disease pressure scenarios makes the chemistry vulnerable. The message is that curativity is good and valuable but this needs to be used very carefully. Librax has been a strong product in trials in 2012 and 2013, with the sharp activity of both Xemium and metconazole evident in the high efficacy achieved. In contrast, the weak additive effect of epoxiconazole in mix with Xemium has been a concern in 2012 and 2013 trials.

The most strongly curative product option for wheat is a tank mix of Imtrex (Xemium) plus Gleam, with rates chosen to balance the SDHI and triazole components for curativity and persistence.

There is also an important dose message with Xemium. As for all other SDHI-based fungicides, there is a need to keep dose rates up on wheat to achieve good performance, especially with the weakened triazole contribution.

With Xemium, there is more opportunity to carefully reduce dose rates in barley programmes and the BASF offer in barley has stepped up notably this year following the introduction of Ceriax.

This combines SDHI + strobilurin + triazole components and there is a lot of complimentary activity in the product. Its field performance in high disease pressure scenarios in recent years has been very impressive.

Ceriax also offers a robust resistance management strategy to manage the imminent risk of net blotch resistance emerging to SDHI chemistry. Xemium has shown very good efficacy on key barley diseases, including rhyncho, and offers an interesting option for barley disease control in this newly introduced three-way formulation.

Treoris is a novel SDHI plus chlorothalonil mix, which has the benefit of being available for use with a choice of straight and co-form triazole fungicides in both wheat and barley. For disease control on wheat, the SDHI component, penthiopyrad, has a nice balance of curativity and persistence and in both T1 and T2 slots.

Treoris has been shown to be a very flexible product both in terms of timing and dose partner. This DuPont product has shown a good disease efficacy and yield response package when used with good triazole partner products in wheat trials, equivalent to the leading BASF and Bayer products.

Preferred triazole partners are likely to be Gleam and straight epoxiconazole at early and mid-season timing options.

The straight penthiopyrad product, Vertisan (trialled as LEM), is recommended for three-way mixtures with contact and triazole partners.

Treoris is also a very flexible SDHI-based option for use on barley and the chlorothalonil component sets the product up as a T2 option, where the partner triazole and/or strobilurin can be selected based on rhyncho / rust risk, etc.

Again, good dose response flexibility has been evident on spring barley from extensive trial testing of the product and good dose flexibility from tank mixes with prothioconazole has also been observed.

IZM for persistence!

Izopyrazam (IZM) has been a very intriguing SDHI in research studies, which often provide clear evidence of excellent broad-spectrum disease control properties combined with durable persistence in wheat and barley trials.

The reduced efficacy of epoxiconazole had a notable effect on the curativity of the Seguris product and, given the more protectant profile of IZM, this has affected the timing flexibility of the product.

Seguris is best used in more protectant situations with situations requiring more curative activity left to products discussed earlier.

However, Seguris has a very good profile for well-timed, early-season use where limited curativity is required and where a strong persistence performance is targeted. The ideal application timings for the product are early- to mid-stem extension.

The IZM-based product on barley is Bontima. This is a much more straightforward product with good broad-spectrum disease control and specific strengths against net blotch and ramularia but it also has good activity on rhyncho. The product fits well into sequences with prothioconazole-based treatments.

SDHI benefits on wheat, barley

With so many SDHI-based fungicide options on the cereal fungicide market, it may seem, at first, an unwarranted question as to what is the benefit of the chemistry for cereal crops and specifically on winter wheat.

The recent sequence of very difficult and unpredictable growing seasons, combined with a very dynamic situation with reduced triazole efficacy on septoria, has made detailed ratings of the relative contributions of triazole, SDHI and contact fungicides to a winter wheat programme difficult.

The contribution of the contact component, typically chlorothalonil, is currently outstanding and, with modest pricing, an excellent cost-benefit can be expected.

The basic triazole contribution to the fungicide programme is also still a valuable part of the total fungicide response but is expected to be an increasingly diminishing proportion with a reducing cost-benefit compared to three to five years ago.

The SDHI benefit is probably best presented as being inversely proportional to the triazole benefit, e.g. as the triazole benefit reduces over time, there is an incremental beneficial response to SDHI use.

There is some impressive trials data to support this presentation of the triazole-SDHI yield response balance.

There is a very clear message from the Irish trials over the past three to four years. This shows reducing triazole contribution to disease management programmes in winter wheat and, correspondingly, increasing dependence on the SDHI component, in combination with strong chlorothalonil performance.

Recent years have not been good for wheat yields with difficult autumns, wet, disease-ridden summers and bad harvests combining to reduce area grown, hinder potential yield levels and severely impact on key quality parameters.

The SDHI chemistry may have, at times, been over-sold and marketed over-enthusiastically, but the family has probably suffered more from the fall-off in triazole efficacy than was first acknowledged.

The use and performance of individual SDHI actives is inextricably linked to their partner triazole fungicides. This recent triazole slide has, in part, hindered the profiling of SDHI products, and the dynamics of triazole sensitivity / insensitivity to emerging septoria strains has also been a complicating factor.

In this context, to not recognise the potential of SDHI chemistry to positively enhance wheat production prospects under Irish conditions in a difficult crop protection scenario in the short- to medium-term future would be a mistake.

The future reality is that if and when there is a major resistance breakdown on a key fungal disease locally or internationally, then SDHI efficacy will be quickly and irreversibly lost, irrespective of previous usage on individual farms.

So there is a clear message to use the chemistry sensibly and effectively whilst adhering to good agronomic practices regarding application timings, partner product choices, dose rates and spray programme sequences.