Arable News

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Decline in triazole performance continues

The two-day AHDB Agronomists’ conference got under way in Northants late last year. Dominic Kilburn reports on cereal and oilseed rape fungicide performance trials results presented at the event.

Indications that the sensitivity of septoria isolates to triazole chemistry had stabilised appear now to be in doubt, according to new data released at the recently held AHDB Agronomists’ conference.

Everyone will be familiar with the decline in triazole performance against septoria between 2009 and 2015, pointed out NIAB’s deputy director Stuart Knight, however this appeared to have slowed down between 2015–2017. “We know that the level of protectant control of septoria offered by a full dose of epoxiconazole or prothioconazole has decreased over time, typically 40–50 per cent in recent years compared with 80-90 per cent in the early 2000s, but 2018 data shows another dip in performance to around 30 per cent control.

“It’s only one year, so we are not sure if it’s a blip or not, but we eagerly await the next set of data. We cannot be complacent – perhaps we are still losing triazole performance over time,” he warned.

When comparing SDHI fungicide performance over-years (2013–2018) when controlling septoria, Mr Knight said that following the first indications of a fall in protection in 2017, 2018 SDHI data is also showing a clear reduction in efficacy since 2013.

“It should be noted though that these trials were assessing the performance of straight SDHIs (Imtrex and Vertisan) as single sprays in very testing conditions and this is not how they should be used in the field. They are much more robust in mixtures with azoles rather than as straights,” stressed Mr Knight. “This recent decline doesn’t mean that SDHIs are no longer of value, but it does mean we need to learn how to preserve their activity as best we can,” he stated.

Product comparisons

Mr Knight also presented data comparing a range of fungicide products (mixtures and straights) against key diseases in wheat, barley and oilseed rape.

Starting with septoria in wheat, 2018 trials data suggested that better protection had been achieved from the SDHIs Vertisan (penthiopyrad) and Imtrex (fluxapyroxad) than the azoles Proline (prothioconazole) and Bassoon (epoxiconazole). Proline however had achieved better control than Bassoon, and Imtrex better than Vertisan.

SDHI mixture products (left) in septoria protection trials all performed similarly, and much better than the azoles or straights alone (right)

Also compared with the straights was chlorothalonil, which gave similar protection to the straight SDHIs but provided increased control compared with the triazoles, highlighting the importance of its inclusion in fungicide programmes for wheat.

Mixture products in septoria protection trials included Elatus Era (solatenol + prothioconazole), Ascra Xpro (bixafen + fluopyram + prothioconazole) and Librax (fluxapyroxad + metconazole), and all performed similarly and much better than the azoles or straight SDHIs alone.

“It emphasises again the much improved protection of the mixtures over the straights,” said Mr Knight.

Looking at over-year protection data (2016–2018), azoles and straight SDHIs performed similarly as they did in the single (2018) year data. Mixture products couldn’t be separated in terms of performance and, again, out-performed the individual compounds, he noted.

Curative

Turning to curative performance against septoria, the over-year (2016–2018) rank order for the straight azoles and SDHIs was the same as in the protectant data, however the performance gap between the azoles was narrower, with Proline showing only a slight advantage over Bassoon.

Chlorothalonil wasn’t included in the analysis as it has no curative activity on septoria, added Mr Knight.

In terms of the mixtures over the same time period, they performed similarly well, with Librax pulling away slightly from the other two (Ascra Xpro and Elatus Era) largely due to very good control from Librax in 2017. However, as in the protectant scenario, the mixtures performed better in a curative situation against septoria than the individual azoles and SDHIs.

Yield 

Comparing over-year (2016–2018) yield performance in the presence of septoria, Mr Knight said that a similar separation in ranking occurred between the straight SDHIs and azoles as seen in the protectant and curative disease control.

In addition, Bravo (chlorothalonil)-treated crops yielded similarly to those treated with the SDHI straights. 

“The mixtures were again all relatively close in performance, with maybe some slight advantage to Ascra Xpro, however it’s very clear that mixture products are vastly outperforming the straights in terms of delivering yield,” commented Mr Knight.

Yellow rust

Although he cautioned that the 2018 yellow rust control data was from only one trial, Mr Knight said that Bassoon performed very well as expected and Proline was close behind. In this trial, strobilurin Comet (pyraclostrobin) was included and it also performed well.

“Straight SDHI Imtrex gave a useful reduction of yellow rust but, as we have said before, yellow rust is not the main strength of SDHIs. They do contribute but not to the extent of strobilurins and azoles,” said Mr Knight.

Right: Bassoon (epoxiconazole) performed as well as expected in yellow rust trials and Proline was close behind. With rust-active azoles featuring in the SDHI mixes (left), it was no surprise these products also performed well.

All mixture products including Elatus Era, Ascra Xpro, Librax and Priaxor (fluxapyroxad + pyraclostrobin) performed well and were similar to the azoles on their own. “Aside from Priaxor, the mixtures mostly contain rust-active azoles and so the SDHI elements of the mixes are not adding that much to the azole performance in terms of disease control,” he said.

For the over-year (2016–2018) yellow rust comparison it was a similar picture for the straight products, while the mixtures also included Keystone (epoxiconazole + isopyrazam). Again, it was hard to differentiate between products in their performance, added Mr Knight.

In terms of yield in the presence of yellow rust, as expected, Imtrex gave the least response of the straights and Bassoon the greatest – reflecting the disease control data.

“There was more of a yield benefit to the mixtures compared with the straights; with Elatus Era giving a slightly greater yield response to the other mixtures tested. So although the SDHIs don’t seem to add much to the azoles in terms of yellow rust control, they are adding to yield.”

Briefly looking at brown rust control (2018), of the products tested Elatus Era and Librax were both very effective and their performances couldn’t be pulled apart, he pointed out. Both were better than their component azoles on their own (prothioconazole and metconazole respectively) highlighting that, in a brown rust situation, SDHIs are adding significantly to the control.

Fusarium

In the 2018 wheat fusarium trial, prothioconazole, tebuconazole and Unizeb Gold (mancozeb) were included, as well as new product Soleil (bromuconazole + tebuconazole).

Soleil, Folicur (tebuconazole) and Proline all did a similar job in terms of reducing the proportion of heads visually affected with head blight. Unizeb Gold also gave a reduction in symptoms and, although not quite at the level of the azoles, it was a good contribution in terms of control, suggested Mr Knight.

“In addition to visual field testing, Harper Adams University carried out testing of DON and DNA levels to find out what was going on behind the head blight symptoms, and, although DON levels were low, the work revealed some interesting DNA results,” he explained.

“Again, we must apply caution as it was only one season but it’s helpful in understanding what we are seeing with the visual symptoms. The azoles gave a clear reduction in DNA for true fusarium species compared with untreated, although Unizeb Gold gave no reduction.

“However, for microdochium DNA it was a different picture. There was some activity from the azoles, albeit not consistently better than the untreateds, but Unizeb Gold gave a clear reduction.

“So visual symptoms hide the fact that some products are more effective on fusarium and some more so on microdochium, and that might influence how effective those products might be in reducing DONs had we had a wetter season.”

Barley

Switching to fungicide performance in barley, Mr Knight pointed out that, in 2018, straight products Imtrex and Proline performed well against rhynchosporium, while Comet made a useful contribution.

Imtrex and Proline performed well against rhynchosporium (right) in barley disease trials, while Comet made a useful contribution. The performance (left) of Priaxor (fluxapyroxad + pyraclostrobin) highlighted the importance of being able to use products other than azoles and still get good control, said Mr Knight.

The mixture products, including Siltra Xpro (bixafen + prothioconazole) and Elatus Era couldn’t be separated and performed similarly to the straight SDHI and azole.

Vertisan (penthiopyrad) was included in the straight product line up for the over-year (2016-2018) comparisons but didn’t perform as well as Imtrex, highlighting a difference between straight SDHI performances in relation to rhynchosporium.

Strob/SDHI product Priaxor was an addition to the mixture products for the over-year data and provided similar levels of rhynchosporium control to the SDHI/azole mixtures.

“It’s important to be able to use products other than azoles and still get good control,” added Mr Knight.

Net blotch and mildew

In a single 2018 net blotch trial, Proline kept disease levels to a minimum (5 per cent disease present at full label dose), out-performing straights Imtrex and Comet. “The latter two did a good job, but 2018 reflects some of what we have recently seen with SDHIs and strobs performing just below the level of azoles,” he said.

“Kayak was also included in the straights to monitor what its active cyprodinil was contributing. It’s not a new product but it still gives a useful reduction although not quite to the level of the other products.”

The mixtures (Siltra Xpro and Elatus Era) performed similarly to Proline.

In the over-year analysis (2016–2018), Proline was strong again in terms of net blotch control while the mixtures (including Priaxor) demonstrated robust activity and more of an advantage over Proline compared with the 2018 trial.

Mr Knight said that it was also encouraging to see that Proline was still giving very good protection against mildew after many years of its use in barley.

“Similarly Cyflamid (cyflufenamid) and Talius (proquinazid) are continuing to do a good job on mildew, so there are good options here.

“With prothioconazole in mixture product Elatus Era, which was also tested, we expected it to give good control of mildew and indeed it did, very much matching the straights,” he added.

OSR data

Mr Knight concluded his presentation with a brief look at 2018 results from two phoma stem canker control trials; one with a moderate canker index (Herefordshire) and the other with a high canker index (Norfolk).

At the moderate site, Proline and Plover (difenoconazole) performed similarly at 40 per cent reduction in stem canker at full label rate, however Filan (boscalid) was more effective than the azoles with a 60 per cent reduction.

At the high index site it was a similar picture with perhaps a bit more separation between the azoles in favour of Proline, but the key thing to note again was Filan’s effectiveness in its level of disease reduction, concluded Mr Knight.


  • Written by: Farmers Guide
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