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Triazole shifts show need for protectant approach

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Warnings of the decline in triazole fungicides’ curative activity against septoria are becoming louder. Martin Rickatson asks what this may mean for this spring’s spraying, and next season’s wheat management as a whole.

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Prevention is better than cure. It is a simple mantra, but one which is becoming ever more relevant to wheat fungicide plans. After the high pressure 2013-14 season and signs of septoria’s shifting sensitivity to triazoles, the efficacy of the chemistry which forms the basic building block of programmes is under scrutiny. Relying on it to get crops out of trouble in situations where septoria is already ensconced is rapidly becoming a non-option.

 

As a result, there would appear to be a consensus among agronomists, agchem firms and seed breeders that wheat growers should be taking a more proactive approach to disease control. But how serious is the decline in triazole activity, and what practical measures should farmers be taking to mitigate its effects?

 

While he doesn’t dispute the need for growers to think carefully through their crop and chemical planning, the evidence of a fall-off in triazole efficacy needs to be taken in context, believes Chris Bean, technical director at Zantra.

 

“While some farmers are reporting significant drop-off issues, primarily this evidence is coming from HGCA trials which, by their nature, are based on single product applications and fixed timings. In practical on-farm situations, farmers will – should – be using with structured programmes which incorporate partner products and are applied according to leaf emergence.

 

“But the evidence shouldn’t be ignored – there is a decline in curative activity. The message with septoria has to be to anticipate and not chase, to protect and not expect to cure. Therefore there is a need for greater accuracy in terms of timing. If triazoles and complementary fungicides are to work to best effect, that means application not by calendar date or even by growth stage as such, but by accurate assessment of leaf emergence. This should ensure all of the leaf layers within the canopy are adequately protected.

Septoria graph

Timings

“Big gaps between the four key timings (T0-T3) just cannot be risked. Assuming leaves 4, 3, the flag leaf and the ear (at early-mid flowering) are properly targeted with an appropriate fungicide treatment, there shouldn’t be a need for further applications, unless disease pressure is particularly high – if it’s a bad yellow rust season, for example.”

 

It is ensuring the gaps between those timings do not go beyond recommendations which is crucial to ensuring product efficacy and therefore sustaining the useful life of key chemical groups over the longer term, suggests Mr Bean.

 

“The most critical of these is the time between T1 and T2. Ideally the interval should be around 21 days and certainly no longer than 28. T0 should be applied around 14 to 21 days prior to T1, and T3 around 16-18 days post-T2.

 

Weather and other workload will have an impact, but for yield and quality it’s important to get timing as correct as possible. The only leaf within the canopy not directly hit at early emergence will be leaf 2, but good product choice at

T1 should ensure this shouldn’t become a major worry.

 

“An additional spray at leaf two emerged – a T1.5 – is an option, but is likely to be of greatest benefit in high risk years when spray days are limited in any case. If there’s a need to run through a high-risk variety to apply a herbicide or growth regulator at this stage then a protectant product could be added, but not if it compromises T2 timing. Focus instead on getting T2 timing right.”

 

SDHIs have a lot to offer as partner products to triazoles, in terms of both enhancing their activity here and now, and protecting their efficacy over the longer term, says Mr Bean. But even the most expensive fungicide programmes will disappoint if not timed correctly, he says.

 

“An SDHI at T2 is now a must, but demands accurate timing. This newer chemistry appears to deliver the best results when applied to a fully-emerged flag leaf, which again demands a robust approach at T1. While nothing offers the curative activity on septoria which the triazoles did back in the late 1990s, the SDHIs based on either penthiopyrad (e.g. Vertisan, Treoris) or fluxapyroxad (Adexar) appear to be slightly more eradicant than those based on bixafen (Aviator) or isopyrazam (Seguris).

 

“Alongside timing, applying the correct rate is essential. Aside from being an anti-resistance measure, a robust rate will show an additional return on investment in extra yield. Trials consistently show increasing epoxiconazole rate from 0.5 litres/hectare to 0.75 litres/ha delivers up to an additional 0.5 tonnes/ha, and an additional 0.3t/ha from a small increase in SDHI rate.”

Varieties

Varieties

There is some scope in ensuring a farm’s spread of varieties contains differing septoria resistance scores, but a single point may not make much difference.

 

“That said, some Group 4 wheats – Revelation, Relay, JB Diego – plus Skyfall in Group 1 looked cleaner than others last year.

 

Prioritising spraying in order of variety scores is advisable.”

 

With farmers in bad black-grass areas being urged to delay drilling to give time for stale seedbeds to work properly, there is also an advantage to be gained here for septoria control, he notes.

 

“Crops drilled by the end of the third week of September and the first week in October can be like chalk and cheese in disease development terms, with lush early growth favouring development, especially of septoria. Early drilling may be fine for less-susceptible varieties in non-black-grass areas, but elsewhere delayed sowing can help give septoria less opportunity to gain a significant hold.”

 

Melanie Wardle, of Keystone (isopyrazam + epoxiconazole) maker Syngenta, agrees with Mr Bean that delayed drilling has a role to play as part of a multi-pronged septoria control strategy.

 

“Cultural control is as important with septoria as it is with black-grass,” she says.

 

“That is especially true if you’re in the more septoria-prone West, where mixed farms may also have a lot on at spraying time.

 

“Delaying the start of drilling in high-pressure areas from September 20 to October 10 can make a significant difference. It can be done on more disease-prone parts of the farm as a management tool. But don’t over-inflate seed rates to compensate, as thick crops are more disease-prone.”

 

Like Mr Bean, she sees no likelihood of high pressure causing any widespread need for an additional application.      

 

“Product persistency and accuracy in meeting the standard timings is what matters.”

Adjustments

Neil Paveley, crop protection lead at ADAS, also echoes Mr Bean’s belief additional applications should, in the main, not be necessary with a well-timed programme, and are not the solution to declining triazole efficacy. But with increasing pesticide regulation, resistance development and the lack of a new broad-spectrum mode of action on the horizon, he says cereal growers must look at adjusting the way they approach their agronomy if the industry is to prolong the life of triazoles, and of SDHIs.

 

“Cereal diseases are evolving and developing resistance much more quickly than, for example, the potato blight pathogen. That’s why it’s not simply a case of more frequent spraying, as with potatoes and blight, but more accurate and targeted spraying.

 

“Fungicide resistance development, and the speed at which it occurs, is ultimately determined by on-farm treatment decisions. Studies have shown not only that adding mixture partners with different modes of action in a fungicide application has by far the biggest effect on decreasing the selection for resistance, but also that robust doses are key to limiting insensitivity development.  

 

“But it’s also essential to balance the mixture correctly to avoid increasing the selection pressure for resistance development to either group. Products with multi-site activity play a role here, particularly for early protection against septoria.

 

“At the same time, varietal resistance forms a part of all of these, representing in itself an additional mode of action. Variety resistance makes the intensity of a spray programme less critical.”

 

Greater septoria resistance alongside quality and yield traits is something plant breeders are working on, says John Miles, of KWS, citing the recent introduction of Group 2 type KWS Lili, which is rated 6 and has yield figures of 105% of controls. Limagrain’s Revelation and RAGT’s Skyfall, among others, also score 6, while the latter’s soft Group 4 Cougar, added to the 2015-16 Recommended List (RL), is the first on the RL to score a 7 for septoria tritici resistance.

 

“But remember that, providing recommended fungicide programmes are followed, to both protect against resistance development and guard against the disease itself, then the investment will be returned, even on a relatively resistant variety,” he says. 

 

“Variety resistance should be seen as another tool in the box, with a more resistant type a little like investing in a higher-capacity sprayer. It shouldn’t alter your spray programme, but it can provide a level of risk management and greater flexibility, allowing weaker crops to be prioritised.

 

“If your priority is to maximise yield and income – and yield has to remain top of the list when selecting varieties – then a more resistant variety shouldn’t be seen as a route to cutting input costs. High-scoring types still show a return on investment from a full fungicide programme, and with wheat prices as they are we’re in a time when small percentages count.”

 

Growth habits

Growth habits

Celia Bequain, head wheat breeder at RAGT, believes not only should Cougar’s inherent protection give farmers some flexibility, but its growth habit should also help them here, in it does not tend to produce lush forward growth.

 

She says: “Its growth habit means Cougar is flexible as regards sowing date.

 

It gets off to a good start, but is then a slow to intermediate developer.

 

“The relatively small difference between its untreated and treated yields is of practical value and is an indicator of a degree of flexibility if fungicide timings are delayed by a day or two because of poor weather or ground conditions.”

 

RAGT breeders used DNA marker selection to obtain Cougar’s fungal and pest resistance traits, as well as its quality attributes, says Ms Bequain.

 

“Its parentage is Robigus and Tuscan, the latter being where the septoria resistance comes from. New gene-marker technology helped us to quite quickly pinpoint the specific traits we were seeking.”

 

Taking a protectant approach – key points

  • For septoria control anticipate and do not chase, aim to protect and do not expect to cure
  • There is a need for greater accuracy in terms of timing
  • An SDHI at T2 is now a must, but demands accurate timing
  • Newer chemistry appears to deliver the best results when applied to a fully-emerged flag leaf, which demands a robust approach at T1
  • Prioritising spraying in order of variety scores is advisable, but a single point may not make much difference
  • Plant breeders are working on greater septoria resistance
  • Variety resistance should be seen as another tool in the box, but it should not alter your spray programme
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