The nature of septoria is to evolve

For almost three decades septoria tritici blotch, commonly known as septoria, has been the most damaging disease of Irish winter wheat crops.

The disease is easily identified by the straw brown-coloured lesions it forms, which are peppered with small black fruiting bodies called pycnidia.

These lesions can be detected in Irish crops throughout the growing season but they can become a real problem in the later stages of grain fill.

The pathogen involved, Zymospetoria tritici, is relatively slow at moving through its lifecycle and from first infection through to production of new spores can take a couple of weeks.

For this reason, the disease is unlikely to kill tillers or cause significant damage over the winter months.

Disease development

Unlike the vegetative part of the season, where the speed of new leaves emerging will outpace the development of the disease, once the plant moves into its reproductive stage the number of new leaves is finite. These last leaves form the upper canopy and intercept the sun’s rays to provide grain fill. What Z. tritici lacks in speed of development, it more than makes up for in spore formation.

Each of those tiny black pycnidia that litter each lesion have the potential to produce thousands of spores called pycnidiospores. Having cycled every couple of weeks over the winter and spring months the potential quantities of spores that can be in any individual wheat field is incomprehensible.

Only a small amount of these will actually make their way on to the upper canopy, but those that do have a bountiful supply of leaf material to infect and make their home, eventually leading to typical septoria lesions.

These lesions kill part of the leaf, and so remove a part of the plant that is converting the sun’s energy into carbohydrates which we aim to harvest in the grain. The earlier this death occurs during grain filling and the greater the levels that occur, the bigger the impact on grain yield.

Control of septoria is all about preventing this from happening.

Although it may sound relatively simple, Z. tritici has a number of quirks in its biology that make it a formidable adversary.

Firstly, it has evolved to be suited to the mild damp climate that is highly favourable to winter wheat production.

In the absence of these conditions septoria will struggle and is likely to be only a minor disease, but it is these same climatic conditions that generate the potential for yield that we strive to achieve. So, accepting this, what can we do to limit its potential impacts on yield in any given season?

Minimise infection pressure

After deciding to grow a winter wheat crop, the next decision is where to grow it. Based on current margin potential, wheat tends to be in the first slot after a break crop, be that a non-cereal or cereal break such as oats.

Z. tritici has a number of quirks in its biology that make it a formidable adversary

Rotation is the most basic and successful disease management strategy. However, the capacity of crop rotation to restrict the development of septoria is limited. This is where the most important quirk of the biology of Z. tritici comes into play. In addition to the asexual clonal pycnidiospores, which are rain-splashed through crops, Z. tritici also has the ability to produce smaller wind-borne ascospores.

These are created when two strains of Z. tritici meet and undergo sexual reproduction. This allows for recombination or mixing of genes to create variability in the species. These ascospores are then shot up into the air and spread to the wider environment in the wind to the next crop.

As Irish crops harbour a lot of lesions throughout the season, we can be confident that this occurs all season long. However, it is no surprise that ascospore production peaks in early- to mid-autumn when the next crop is being planted.

As these spores move in the wind, rotating a wheat crop from one field to the next will have limited impact on the potential to avoid these spores, hence the limited capacity of rotation to reduce initial infections in autumn.

Once production and release of these ascospores has peaked in mid-autumn the amounts released will decrease.

Even though we can still detect ascospores well into December, the later we sow winter wheat crops the lower the chances of these ascospores landing in wheat crops.

Unlike delayed sowing, the impacts of varietal resistance can continue throughout the season

While delayed sowing date helps to tackle some grass weed problems, it is also a major advantage in delaying and reducing autumn septoria infection. However, this is not a silver bullet for septoria control.

While ascospore infection will be significantly reduced, they will still be about and, under the right conditions, the infection they initiate will allow inoculum to reach levels that are sufficient to cause significant problems come May and June.

A resistant variety

However, we should look at this differently – what could I do that would make the situation worse? Sow the wrong variety too early and you can be guaranteed a problem that will be difficult, if not impossible, to extinguish come summer.

So what is the right sowing date? For septoria control it is simply as late as is practically possible without impacting yield. This could be from early October through to early December, depending on location.

A more disease resistant variety will further maximise the benefits afforded by delayed drilling and further reduce disease development over the winter months.

Unlike delayed sowing, the impacts of varietal resistance can continue throughout the season, as the variety continually restricts the further development and spread of septoria.

Improved genetic resistance

Over the past two decades we have seen a steady increase in septoria resistance and varieties such as Madrigal, Savannah, or Tanker, which were common at the turn of the century, would be unlikely to be considered today due to their septoria susceptibility.

Ideally, we would like to be able to reduce our dependency on fungicides as a consequence but it is not that simple.

This is partly due to the intense disease pressures Irish crops can experience.

Even where higher levels of varietal resistance are used, the potential remains for septoria to result in significant yield losses if control measures are relaxed.

Also, varietal resistance can, in some instances, be shorted lived due to the ability of Z. tritici to mutate. Excellent septoria control is only worth its value if we can grow a crop that yields a product that can be sold.

We demand a lot of different things from a wheat variety. As breeders get better at bringing these various things together in a complete package, we can expect more tailored varieties in the future.

Major gene resistance

Massive gains can be made by the inclusion of major gene resistances for septoria control. Such resistance types are liked by plant breeders as they provide strong resistance and can be relatively easy to track within breeding programmes.

However, such methods of genetic varietal resistances come with a risk. Simple changes or mutations in the pathogen can disrupt the resistance and enable the pathogen to invade the plant.

The most recent example of this was the breakdown of the Stb16q septoria resistance gene present in Cellule.

Until 2019, Cellule displayed some of the best resistance to septoria, which was a joy to see in the field.

In early May 2019 something had gone wrong as serious levels of septoria were evident in the variety.

While only a small proportion of the Irish Z. tritici population was likely to overcome the Stb16q resistance in Cellule, these had a free run in this variety and were quickly selected for in commercial crops. As genes such as Stb16q are brought through breeding programmes, strategies for how best to deploy them will now be required. This is likely to include the agronomic aspects described previously and also the careful use of fungicides.

Essential fungicides

Over the past five seasons significant changes have occurred in both the availability of key septoria fungicides and the efficacy of those remaining. However, control of septoria in commercial crops has, in most instances, remained very good. This has been in a large part due to having the right weather at the right time, combined with increased awareness regarding correct application timing.

Chlorothalonil was also a key component in that success but this can no longer be used in the EU.

We must now reassess fungicide programmes. This includes assessing how much the older azoles, such as prothioconazole or tebuconazole, and the suite of current SDHIs will contribute in light of resistances to each, and of course the commercialisation of the new azole Revysol by BASF and the QiI Inatreq by Corteva Agriscience.

An alternative multisite

The first thing to assess is whether there is really a need for an alternative multisite in fungicide programmes and if it would be cost effective.

The application of any fungicide is a risk management tool, protecting the upper canopy from potential infections and yield losses. If a fungicide does not do this then, of course, its inclusion should be seriously questioned.

The alternatives to chlorothalonil do not bring the same level of efficacy or cost benefit so why should we include them?

Over the past two decades, chlorothalonil has been included in fungicide programmes for a combination of disease control and resistant management.

The latter has not changed, if anything with azole and SDHI resistance dominating our Z. tritici populations, resistance management by including a low resistance risk fungicide such as a multisite should be as important now as it ever was.

Managing resistance

Resistance management itself cannot be simply separated from disease control – they are one and the same thing. However, we know that including a multisite such as folpet or sulphur in carefully considered fungicide programmes does provide improved disease control and, as such, resistance management. This will mean paying more this season to achieve the same so why do we have to do it and can fungicide programmes be tailored to reduce cost?

A fungicide programme is a yield risk management tool whereby we apply fungicides to protect yield

Resistance management is often viewed solely as a strategy to ensure the future activity of a fungicide.

We are all aware that if one person fails to implement an anti-resistance strategy everyone will inherit the problem, so where is the benefit?

We do not know exactly when or where fungicide resistance will develop.

Given the number of potential spores that exist in any given crop, the potential for resistance to develop in your field is as likely as in your neighbours’ field.

A fungicide programme is a yield risk management tool whereby we apply fungicides to protect yield. As most actives in fungicides are single-site, they will be at a greater risk of resistance development.

Were this to happen in either of the key septoria timings, the potential for significant loss of control is a real possibility in a typical Irish season. This is where the value of resistance management truly comes in and is often overlooked.

There will be some hit on control as folpet is not chlorothalonil, and neither is as effective as the new azole or a QiI. It provides a level of control that will be reflected in the final yield.

Their inclusion is far from altruistic, although we do know that their inclusion will delay or reduce selection for resistance when it does happen, which will be good for you and your neighbour.

Resistance with new actives

With the commercialisation of Revysol in 2020 and Inatreq in 2021, the two new actives join a fairly crowded wheat fungicide marketplace. Resistance in Irish Z. tritici populations has eroded a good chunk of the efficacy of the older azoles such as prothioconazole, tebuconazole and metconazole.

Equally, with moderate SDHI resistance now completely dominating the population, a similar erosion of efficacy has occurred to the various SDHIs available.

Combined, we can expect our older SDHI azole combinations to provide 50% to 65% efficacy, some subtle differences may exist between them.

We can expect the Revysol combinations (with the SDHI Imtrex) and the Inatreq combinations (with various azole partners) to provide closer to 80% and 85% control, undoubtedly placing them top of the list for inclusion in wheat fungicide programmes.

A carefully considered fungicide programme should reflect the variety and the disease risk that have been built in following the earlier decisions as to where and when to sow the crop. While not all crops will require Revysol or Inatreq, others may well require both.

What will be the overall disease risk, the potential yield, and, more importantly, the return on spend?