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Fungicide performance under the spotlight

The 2020 spring fungicide campaign is nearly upon us – and the AHDB Agronomists’ Conference provided some useful pointers for cereal fungicide performance during the previous year, including data on brand new chemistry. Dominic Kilburn writes.

Wheat disease septoria.

ADAS principal research scientist Jonathan Blake began his presentation of fungicide performance in wheat (2019) with a focus on key disease septoria tritici, comparing straight products and SDHI/azole mixtures in a protectant situation (across five trials).

Proline (prothioconazole) provided moderate levels of control in 2019, while Imtrex (fluxapyroxad) was a little more effective – with Mr Blake reminding delegates that, on farm, Imtrex can only be used in mixture with at least one fungicide with an alternative mode of action that has efficacy against the
target disease.

Bravo (chlorothalonil) was also included in these trials and even at half label rate (1-litre/ha) it was providing more control than a full dose of prothioconazole and comparable efficacy to Imtrex – quite a shift in the performance of these two products, he suggested.

Moving across to the mixture products, two standard SDHI/azoles were compared; with Ascra Xpro (bixfen + fluopyram + prothioconazole) showing a little separation from Elatus Era (benzovindiflupyr + prothioconazole) over the past year.

“New product Revystar XE (mefentrifluconazole + fluxapyroxad) performed very well within these trials to the point where even a half dose of Revystar, in a protectant situation, was doing more than a full dose of either Ascra Xpro or Elatus Era.

“Mefentrifluconazole is a strong new active ingredient,” pointed out Mr Blake.

In mixed protectant and curative septoria trials in 2019 (across four trials), it was a similar picture but with Imtrex a little closer to Proline in overall performance, and Ascra Xpro tracking closer to Revystar in the mixtures.

Curative activity

Mr Blake said that curative trials (2019), which focused on the lower leaves well below those that usually receive a spray, provided products with a very tough test.

“In a curative situation, we don’t get a huge amount out of Proline or fluxapyroxad any more. When we look at the mixtures, Ascra Xpro and Elatus Era are tracking quite similarly and we are still seeing an advantage from Revystar,” he noted.

Over three years (2017–2019), the picture is the same with Revystar, even at quite low rates, providing a high level of control from what is being seen from Ascra Xpro and Elatus Era, said Mr Blake.

In septoria yield trials (2019) – which he said was a tough test of product persistence – of the singles, Proline and Imtrex performed similarly. However, for the mixtures, Revystar has given an additional 0.3t/ha yield lift compared with Ascra Xpro and Elatus Era.

The same advantage from Revystar was seen in the three-year (2017–2019) yield trials, he added.

Yellow rust in wheat was widespread in 2019.

Yellow rust

Mr Blake said that yellow rust in wheat was widespread in 2019 and with a large number of varieties assessed by the RL, accordingly some ratings have changed – including Zyatt from an 8 to 7, Bennington (6 to 5) and Viscount (7 to 6).

He highlighted research work carried out in the past by his ADAS colleague, Peter Gladders, which came to the conclusion that late sowing is a factor for an increase in yellow rust in the spring. “And so, for those growers that have drilled late this season, it’s one to watch out for,” he commented.

Yellow rust fungicide trials in 2019 (Terrington, Norfolk), using susceptible variety Reflection, confirmed that Bassoon (epoxiconazole) is still very active and “doing a good job” on yellow rust even at quarter or half rates.

“Prothioconazole is close behind, showing good activity and is more effective than either Comet (pyraclostrobin) or Amistar (azoxystrobin) – the two strobilurins we tested in 2019.

“Imtrex adds some activity to yellow rust but isn’t quite up to the same level of activity as of Amistar, Comet or the azoles,” he added.

In terms of the mixtures, Mr Blake said that Revystar adds something over Imtrex (one of the partner products it contains) but is not quite as strong as the SDHI/azoles in Ascra Xpro and Elatus Era, which both track the same path and provide good yellow rust control.

“We noticed that some treatments do give us four weeks’ control of yellow rust rather than three; Elatus Era is one that is highly active and offers more persistency of control, and we see the reverse rank order in this trial for the mixtures that we saw in the equivalent septoria yield trials.

“Over the three-year yellow rust yield trials, Elatus Era again stands out with its persistent control.”

Brown rust

As with yellow rust, there have been some brown rust resistance score changes on the new RL, including Firefly (8 to 6) and Viscount (9 to 8), however Skyscraper and Spotlight increased their ratings from 5 to 6 and 6 to 7 respectively.

Brown rust was slow to develop in 2019 following a cool spring, commented Mr Blake, and in trials featuring susceptible variety Crusoe, Comet did a reasonable job. “Proline is not very active on brown rust and while Imtrex is not good on yellow rust, it has good activity on brown rust.

“Across the mixtures, therefore, anything featuring fluxapyroxad – Librax (fluxapyroxad + metconazole) and Revystar – also showed good activity in trials.

“At full rate, Ascra Xpro did a decent job but it may be a case of adding something to it in high pressure brown rust situations.”

Yield results broadly reflected what was seen in control trials, with Elatus Era and Revystar the leading actives on brown rust.

Fusarium

Comparing Proline, Soleil (bromuconazole + tebuconazole) and multisite fungicide Unizeb Gold (mancozeb) in two-year fusarium trials, Mr Blake suggested that the latter reduced fusarium head blight quite well, while the bromuconazole (in Soleil) added to tebuconazole’s activity.

Soleil’s activity on fusarium tracked closely to that of prothioconazole, he added.

DON (deoxynevalenol) levels coming out of these trials were also looked at, according to Mr Blake, who said that Proline halved the level of DON, while Unizeb Gold did very little. “Mancozeb has good activity on microdochiums but not on true fusariums. However, microdochiums can affect yield and cause headblight symptoms and so mancozeb might have a place in a T3 strategy, used in combination with other actives if targeting fusarium
and microdochium.

“Soleil reduced DON levels compared with prothioconazole this year, which perhaps suggests that bromuconazole is adding something additional in terms of fusarium control.”

Powdery mildew

Reassuringly, Mr Blake pointed out that there are a number of products which have good activity on powdery mildew, including Proline and fluxapyroxad in Imtrex.

“All of the SDHI/azoles we tested looked to be controlling powdery mildew fairly effectively, with between 60–80 per cent control from a half label rate.

“Maybe powdery mildew-specific products should be considered in high pressure situations, but at T1 and T2 regular products should do enough to keep the disease in check.”

Barley

Turning to barley, he said that the picture seen for a long time with rhynchosporium remained the same in that there are several products with good activity, making the disease straightforward to control. Both fluxapyroxad and prothioconazole do a decent job on rhynchosporium, even at half label rate, and any products containing these two actives perform well. “Strobilurin Comet has good activity and, in the mixtures, Priaxor (fluxapryroxad + pryaclostrobin), Siltra Xpro (bixafen + prothioconazole) and Elatus Era are all also doing a good job at quite low rates.

“It is reassuring that they are all working well,” Mr Blake commented.

Where tan spot was seen in 2019, it was effectively controlled with a quarter dose of Proline while Siltra Xpro gave good control. Imtrex showed useful activity as well, he added.

Mr Blake pointed out that prothioconazole controls powdery mildew in barley at the quarter rate, and this factor has precluded the need for any mildew specific products. “However, if there is a requirement for this then Cyflamid (cyflufenamid) is very active on powdery mildew, perhaps more so than Talius (proquinazid). Imtrex, when used, will also add activity.

“Unsurprisingly, mixtures containing prothioconazole, including Elatus Era and Siltra Xpro, look very active on that pathogen.”

Net blotch

Mr Blake said that the main change with net blotch is the dose rate at which good control is achieved. “There are a range of actives providing a good level of control; including Imtrex, Proline and Comet, and, while mixtures Siltra Xpro and Elatus Era used to give us very good control at the half label rates or even lower, we now think that you may have to apply more to get that level of control.

“We are seeing a gradual change in sensitivity of populations to SDHIs and azoles and, in high pressure situations with net blotch, there is a need to apply higher doses to maintain control.

“We also see that in terms of yield, where there may be a need to apply above the half label rate to achieve an adequate yield response,” he advised.

Mr Blake concluded with ramularia – a disease he said was a key pathogen in winter and spring barley and which, for now, was well controlled by chlorothalonil at quarter or half rates. For 2019 trials, Revystar was compared alongside Proline and chlorothalonil and Mr Blake said that it adds useful activity, perhaps offering an alternative to CTL once it’s gone, although unlikely at the same price as a half rate of Bravo, he suggested.

“Interestingly, in the past we’ve seen less activity from Proline but in 2019 it did add some activity to ramularia control.”

SDHI and azole efficacy

While it is clear that since 2003 there has been a gradual shift in the sensitivity of septoria populations to SDHIs, Rothamsted early season monitoring trials (2019) using SDHI Bixafen have shown a smaller shift than seen in previous years, explained Mr Blake. “There was a large shift between 2017 and 2018, but less so between 2018 and 2019,” he said. “Farmers certainly noticed that there was a loss of control in the field during 2019 but that may have been due to higher disease pressure.

“We think that populations are now dominated by moderately insensitive strains – T79N and N86S – which are dominating but not increasing.

“Of slight concern however is the H152R isolate which could possibly be increasing and is a worry for the future,” he added.

Looking at Imtrex over four years (2015-2019), Mr Blake pointed out that in 2015 it achieved 90 per cent control on average from full or half label rates, and now that was closer to 60 per cent control, however there has not been a significant shift in sensitivity in the past year.

Of the activity of SDHIs mixtures (2015–2019), he suggested that there has been a decline but this has been gradual. “In recent years there has been a slight separation of SDHI azoles within Elatus Era which is slightly less effective now compared with 2017.”

Rothamsted early season monitoring of epoxiconazole and prothioconazole (2019) was almost identical to 2018 in that no further evolution of these azoles was seen. “This is reassuring me that we may have reached the point where the CYP 51 enzyme isn’t mutating any further and isn’t able to develop any further resistance to azoles. If that is the case, the decline in performance of prothioconazole and epoxiconazole may level off.

“Prothioconazole continues to offer 35–40 per cent control in a protectant situation and if there is some stabilisation then we are hopeful it may continue to protect new chemistry going forward.”


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