Reduced triazole sensitivity and field control of septoria

By Steven Kildea Teagasc, Oak Park

Ireland is geographically positioned to produce some of the highest-yielding cereal crops in the world. As the island is constantly buffeted by the southwesterly gulf-stream and its associated rain, we rarely suffer from the extremes in weather often experienced elsewhere, even among our European neighbours.

But we do suffer from wet season diseases, which can be very damaging to yield, and the control of problems like septoria tritici in wheat is critical as it can reduce yield by up to 50%.

Over the past decade or more, we have witnessed increasing problems with the development of resistance or reduced sensitivity in septoria in particular to a number of fungicide actives, with strobilurins and triazoles worst affected.

The importance of ensuring the long-term effectiveness of our fungicides is, therefore, practically and financially important, and not just academic.

Resistance vs reduced sensitivity

In 2002, septoria developed resistance to the strobilurin fungicides. In the field, this meant that septoria was able to develop following strobilurin application, even where the fungicides were applied protectantly.

This was clear to be seen in crops throughout the country, where foci or hotspots of septoria began popping up expectantly in early July.

The strobilurins brought disease control in cereals to a new level. Treated crops could remain green, capturing sunlight well into July. The emergence of these hotspots clearly showed that these fungicides no longer provided the much-needed disease control and could not be relied upon to do so into the future.

In the lab, the impact of resistance was very easy to detect. Septoria with the mutation leading to resistance (G143A) grew even in the presence of high concentrations of the pure fungicide.

The difference between strobilurin sensitive and resistant strains can be upwards of 1,000-fold.

This means that to kill a resistant strain, it can take 1,000 times more chemical than to kill a sensitive strain.

Fungicides work by disrupting essential systems in the target fungus. To do so, they often bind to specific sites (single site fungicides) involved in these systems.

For example, the strobilurins bind to a protein in septoria (and in many other pathogens, and hence resistance evolved similarly in numerous other diseases) that forms part of the process involved in generating the energy the disease needs to grow.

The binding of the fungicide disrupts this process and has lethal effects on the fungus, as it can no longer produce energy.

We now know that the development of the mutation G143A in septoria changed the structure of the strobilurin target and blocked the fungicide from binding.

The rapid spread of the mutation G143A within fields, and eventually nationally in the septoria population, resulted in almost complete loss of efficacy of the entire group of strobilurins, whether applied in protective or curative situations.

Triazoles

So how does this differ to what has happened, and continues to happen, with the triazoles?

Firstly, the targets of the strobilurins and the triazoles are very different. The triazoles target a process involved in generating fungus-specific fats needed by the disease to give it the stability and structure to rapidly grow on and in susceptible plants.

In addition, while all triazoles target the same site, they differ in structure and size. This can influence how exactly they interact with septoria.

This can explain differences traditionally observed between the triazoles in their efficacy against the disease.

The development and commercialisation of numerous triazoles since the early 1970s, culminating with Opus and Proline, is also likely to have imposed slightly different selective pressures on septoria populations over the years, which may have influenced where we are now.

In the lab, this can be as a gradual increase in concentration of fungicide required to kill septoria.

For example, from a two-times increase in chemical rate required in year one, to four-times in year five and to a 10-times increase in concentration by year 10.

As this evolution has been stepwise in nature, i.e. taking gradually more and more fungicide to kill the disease, it can be referred to as reduced sensitivity.

Because of this, the dramatic effect which was seen with the strobilurins will not necessarily be readily observed in the field with triazoles.

The mechanisms by which septoria tolerates different concentrations of triazoles can also help explain what is actually happening.

Similar to strobilurin resistance, changes in the target site are the principal means by which septoria is overcoming the toxic effects of triazoles. However, unlike the strobilurins, it appears that it takes numerous changes or combinations in the target to affect the binding of the triazoles.

As septoria has accumulated these changes over a period of years, the sensitivity to different triazoles has decreased or reduced over this same time span. We can, however, see that, as with the strobilurins, these combinations spread locally initially but, eventually, nationally.

The question is at what point does reduced sensitivity affect field control?

Persistence and curativity are first affected

In an ideal world, every one triazole molecule applied to wheat should prevent one septoria protein from doing its job. Where this is the case, even under normal conditions with sensitive populations, the quantities of fungicide needed will differ in protectant and curative situations.

A septoria spore contains fewer target proteins then an actively growing fungus and, therefore, protectant applications should require lower quantities of fungicide.

Looking at it another way, there will always be a point at which it will be impossible to achieve curative control, even when the population is sensitive. There will just be too much fungus present and too many targets to be inhibited.

Persistence of efficacy can be looked at similarly. Once a fungicide is applied, its concentration does not remain static, it will always decrease as the fungicide is broken down in the plant.

The persistence of protection or curativity is then related to the amount of fungicide applied and the rate at which the fungicide decays.

Protectively, this relates to the decay of the fungicide on the leaf, which will be largely dependent on weather conditions and the growth of the plant. The more rain there is, the more the fungicide will get washed off the leaf. The more actively a plant is growing, the greater the amount of leaf material that needs protection and hence the fungicide is diluted, leaving a lower concentration per unit of leaf area.

Curatively, this can also relate to how much disease is present for the fungicide to kill. Curative activity can be viewed as the fungus locking up the fungicide.

Now introduce into these scenarios septoria with reduced sensitivity. Depending on the sensitivity level, the initial levels of protection may not be adversely affected – there is sufficient fungicide on the leaf surface to kill any spores that land following application.

As the fungicide decays, however, the point at which the disease can survive the fungicide will occur sooner for septoria with reduced sensitivity and hence persistence is affected.

And, as more fungicide is required to kill the disease, curativity will also be affected. So the effective dose will increase as the level of reduced sensitivity increases. This may continue to the point where the strains can be called resistant.

Have Irish triazole sensitivity changes affected efficacy?

In 2008, strains with reduced sensitivity to Opus and Proline were first found in the northeast. Under lab conditions, these strains grew at greater concentrations of both chemicals than any others previously examined at Oak Park.

The lab tests showed that these changes were greater for Proline than for Opus. Subsequent analysis revealed the presence of the mutation S524T, in combination with mutations which were in the majority of the Irish septoria at that time.

Under lab conditions, these combinations of mutations didn’t seem to adversely affect the sensitivity of the disease to either Folicur or Caramba.

Detailed trialing in the northeast in 2009 confirmed that these strains affected the efficacy of both Opus and Proline. Their impact was, as expected, greatest on both curativity and persistence of activity. However, unlike the lab findings, both fungicides were affected equally in the field (Figure 1).

The trial results shown in Figure 1 show comparable efficacy between Proline and Opus three-spray programmes, especially in populations with reduced sensitivity.

However, control using Folicur alone was much poorer and the sequence of different actives fared much better, especially in the reduced sensitivity population.

So the differences between the triazoles (Opus, Proline versus Folicur, Caramba) could still be used to help manage septoria, either in alternations with either group or in mixtures (Gleam and Prosaro).

Fast-forward five years and we now have strains of septoria with reduced sensitivity throughout the country. The loss of efficacy experienced in trials in the northeast in 2009 can be experienced everywhere in the country. Furthermore, we now have strains that appear to break the previous grouping of triazoles, i.e. they show reduced sensitivity to Opus, Proline, Folicur and Caramba.

Examination of these strains has confirmed that the mutations which adversely affected Opus and Proline have combined with those that affect Folicur and Caramba, while additional novel mutations and resistance mechanisms have also appeared.

Fortunately, most of these strains do not show higher levels of reduced sensitivity to the individual triazoles than has been present in the population for some years now.

We can still get a level of control from the triazoles when applied correctly (from a range of trials, this is estimated to be 55% to 65%). This is much lower than when the products were first introduced, hence good rates of triazole are now required.

Is triazole inclusion still important?

The short answer is yes. Together with a multisite (such as chlorothalonil or flopet), they still remain the cornerstone of all T1, T2 and T3 applications. Their inclusion at T3 is easily justified for Fusarium head blight control.

With the arrival of the newer SDHIs, it has been argued that their inclusion at T1 or T2 is less than clear. However, the SDHIs, which are clearly now the leaders in terms of septoria control, will develop resistance and this is just a matter of time. While it is irresponsible not to try to protect them, when resistance does develop it is likely to have an immediate efficacy effect, similar to what was observed with the strobilurins.

Anti-resistance strategies, such as mixing effective chemistries, should not only be viewed as a means of protecting chemistries, but also as insurance strategies for disease control when resistance does develop. While the inclusion of a multisite such as chlorothalonil with SDHIs is a must, their inclusion alone is of limited value.

Multisites can only protect against the selection of strains which have not yet arrived on the leaf. In the absence of an additional fungicide with curative activity, an SDHI resistant strain, which may well have infected the leaf as little as only one day before spraying, will be rapidly selected by the SDHI fungicide.

This is where the triazoles have an essential role to play. Therefore, it is important to match the rate of triazole to the activity needed.

In most situations, the rate of triazoles in the preformulated SDHI mixes will provide the curativity needed. However, if timings get stretched due to poor weather conditions and unprotected leaves have been exposed to infection events for a considerable period, increased curativity is likely to be required. And, as such, the rate of triazole may need to be increased to match the SDHI activity.

Essential to get timings correct

The fact that curativity and persistence of the triazoles have been affected gives further reason to ensure timings are as correct as possible and their usage is limited to when they are really needed. Unnecessary usage of triazoles at T0 clearly reduces their effectiveness at the much more important timings.

There is no need for curativity on leaf five – it will have done its job of providing energy for the upper leaves to grow long before septoria develops and it does not contribute to grain filling as it is shaded by the leaves above.

The protection provided by a multisite on leaf four and part of leaf three is all that is required and this will only be worthwhile if the T1 is delayed.

Applying fungicides on a calendar basis is no longer acceptable and is a waste of money. As yield is created from the flag leaf down, programmes should reflect this.

A T1 application, timed to ensure complete fungicide coverage of the third last leaf, not only protects this leaf but also protects a good chunk of the second last leaf (with SDHI and triazoles applied at T1 you are also getting curative control of disease on the fourth last leaf – how important this is to final yield is always questionable).

With current chemistries (SDHIs and good rates of triazoles), a correctly timed T2 will protect the flag leaf and top-up disease control on the second last leaf. Unfortunately, our climate does not always allow precise timings and intervals inevitably get stretched. However, poorly timed applications or ill-conceived programmes (poor choice of fungicide) will only compound these issues.

If a spray is delayed, the difference can be made up by increasing the product rate or using a stronger product. As programmes must start with a solid foundation, knowing exactly what leaf is out at T1 is vital. As crops have come through a mild winter, this year’s growth stages (node development) may not always match the expected leaf.

We have few enough fungicide products available and few enough spray days to get round our crops. If you know what you’re doing in choice of product, it is still possible to achieve perfectly good disease control with our traditional timings of third last leaf fully emerged, flag leaf fully emerged and an ear spray.

And there is the possibility of a multisite at leaf four to buy some timing flexibility in the first main spray.

Aiming to apply more sprays will provide a bigger challenge to get round and cover all of the crops with unnecessary fungicide and hasten the loss of the fungicides we currently have.