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SDHI stewardship takes on greater significance

In light of recent confirmation of septoria tritici resistance to SDHI fungicides

In light of recent confirmation of septoria tritici resistance to SDHI fungicides, stewardship of these products was a focus at the AHDB Agronomists’ conference at the end of last year. Dominic Kilburn writes.
How SDHI fungicides are used this spring will be of paramount importance for both optimal disease control of main winter wheat disease septoria tritici and maximising the long-term efficacy of products.
That was the key message of ADAS plant pathologist Jonathan Blake (left), speaking at the AHDB Cereals & Oilseeds Agronomists’ Conference late last year.
Following Rothamsted Research’s reporting in November of low frequencies of septoria isolates with reduced SDHI fungicide sensitivity in UK field populations, in December, Teagasc in the Republic of Ireland also confirmed the detection of field isolates of septoria with reduced sensitivity to SDHIs.
“Preliminary information indicates that there is reduced sensitivity to SDHIs and it does suggest a step change in sensitivity compared with isolates so far,” commented Mr Blake.
“We don’t know any more at the moment but based on the mutations observed, and previous laboratory studies cross resistance to SDHIs is expected,” he added.
In light of these latest reports, stewardship of SDHI fungicides this season takes on a greater significance, continued Mr Blake, who stressed that fungicide strategies this spring would be key. “The other methods of controlling septoria must be maximised to the full including the use of multisite and azole products, and even strobilurins still have a small effect.
“There should be effective, targeted use of SDHIs; so, where possible, use one application in the season rather than two, or none at all if you can get away with it,” he advised.
“Also, grow varieties with resistance to septoria as the more we have to use fungicides to treat the disease the faster we will lose them. Minimising the use of SDHIs will slow the evolution of resistance.
“Sustainable product use is vital,” he said.

Wheat fungicide trials
Mr Blake presented the latest AHDB fungicide disease control and response trials data beginning with a look at activity of fungicides against septoria tritici in wheat. He said that, putting 2015 in context with RL trials data from 2002-2015, yield response to fungicide treatments were, on average, 1.97t/ha, which, although 2015 was not perceived as a high pressure season, was close to the long-term yield response average.
“Septoria is still the main cause of yield loss in wheat and, despite 2015 being a lower disease pressure year compared with 2014, it was still not fully controlled. In June, on average 50 per cent of crops were infected with septoria and there were noticeable infections at leaf 2 in some parts of the country, particularly the south west, so it’s clear we still have a job to do,” explained Mr Blake.
Fungicide protectant activity against septoria was trialled on six sites in 2015 with Ignite (epoxiconazole) and Proline (prothioconazole) showing similar activity although Proline was slightly ahead this year. Bravo (chlorothalonil) at 50 per cent dose rate looked strong when compared with the full dose azoles.
Individual SDHI actives Imtrex (fluxapyroxad) and Vertisan (penthiopyrad) were very active in 2015 trials, although Mr Blake reminded conference attendees that the two SDHIs can only be used in a mixture with at least one fungicide with an alternative mode of action, and which has efficacy against the target pathogen.
Of the SDHI mixes, including Aviator Xpro (bixafen + prothioconazole); Adexar (epoxiconazole + fluxapyroxad); Vertisan + Ignite and Librax (metconazole + fluxapyroxad), all gave very strong and similar protectant activity. Librax’s performance, with activity matching up to Adexar, was a surprise seeing as it uses what is generally considered a less septoria-active azole in the mix, suggested Mr Blake.
In terms of yield response to fungicides in 2015, there was a bit more separation between Proline and Ignite and the SDHIs were “where we expect them to be”, he said.
When considering septoria protectant activity over the past three seasons (2013-2015) Phoenix (folpet) was similar to epoxiconazole but chlorothalonil was the more active across those seasons while excellent protection was seen from the SDHIs.
When it came to septoria eradicant activity across the same three seasons, considerable changes in the performances of Proline and Ignite were recorded, confirming the gradual decline of these azoles and their curative efficacy.

Changes in activity
“We need to look over a range of seasons to highlight efficacy changes with these two azoles and, taking epoxiconazole as the example, its protectant activity has fallen from as much as 90 per cent control in 2009, dropping below 80 per cent in 2010-2012 and dropping further in 2013-2015.
“Prothioconazole’s efficacy across the same seasons has suffered almost exactly the same change however the most noticeable difference has been its curative activity which now sits at around 35-40 per cent control.
“This all fits closely with what fungicide resistance monitoring work shows,” he added.
Mr Blake suggested that variability in curative activity observed in these trials, relates to activity decline that occurs as the disease went through its latent phase. This was of little surprise, highlighted Mr Blake as success, or otherwise, depended on when the crop was treated.
“Best curative control of septoria is when crops are treated up to the mid-point of the latent period, about 10 days after infection and although back in 2005 that meant up to about 90 per cent control with triazoles, today it is more like 40 per cent with triazoles.
“When we apply fungicides during the season we apply them soon after leaf 1 emerges but about 10 days after leaf 2. So we need to make sure the SDHIs are in the mix,” he added.

The rusts
Turning to yellow rust control in wheat Mr Blake pointed out that 2015 was quite a “delayed epidemic'” season compared with 2014.

Despite this, high levels of yellow rust were present in AHDB fungicide performance trials, and Ignite and Comet (pyraclostrobin) were very active on yellow rust in 2015 trials with partial activity from the SDHI straights Imtrex and Vertisan.
“Any epoxiconazole-mixed SDHI was strong,” he pointed out.
In terms of yield, according to Mr Blake Comet showed less of a yield benefit compared with Ignite, while Adexar provided the best yield results out of the SDHI mixes.
He said that the mild conditions experienced during the winter to date (speaking pre-Christmas) were similar to what was seen ahead of the brown rust epidemic in 2007 and above average temperatures this winter might trigger early epidemics, he claimed. “Gator, Grafton and Crusoe are particularly susceptible varieties but Ignite and Comet were very active against brown rust in inoculated AHDB fungicide performance trials in 2015, which provided a good test.
“SDHIs are more active on brown rust than yellow rust and although Caramba (metconazole) was not quite as effective as Comet and Ignite it was close none-the-less.
“It’s worth remembering that azoles applied at T0 in 2007 had a big effect on brown rust,” he added.

Wheat summary
Septoria- SDHIs the most active chemistry- Librax at least as effective as other SDHI/azoles (only one year in trials)- Activity of azoles has declined but they still support the SDHIs in curative situations- Multi-sites add valuable protectant activity to programmes- Both azoles and multi-site actives are required to protect SDHIs

Rusts- Epoxiconazole highly effective in curative situations- Strobilurins more active than SDHIs (especially on yellow rust)

Barley fungicide trials
Switching to barley, Mr Blake hailed 2015 yields as “fantastic”; achieving an average of 10.7t/ha treated yield across AHDB trials. The 14-year average yield benefit to barley in fungicide performance trials is 1.5t/ha – exactly what was seen in 2015, he highlighted.
Starting with rhynchosporium protectant activity in 2015, he said that straight SDHIs Zulu (isopyrazam) and Vertisan showed good activity but Imtrex was stronger. Proline performed better than Comet, and both the SDHI mixes Siltra Xpro (bixafen + prothioconazole) and Adexar, performed similarly well.
Across three years (2013-2015) Imtrex and Proline again performed very well while a Vertisan + Proline mix performed as well as Siltra Xpro and Adexar.
In across years (2013-2015) eradicant trials, activity from all products was very similar to what was seen in terms of protectant results.
Imtrex, Zulu and Vertisan performed very well and similarly in net blotch protectant trials (2012-2015), while Proline was better than Comet. The mixtures; Siltra Xpro and Vertisan + Proline were closely matched and performed very well.
Again, as with rhynchosporium, eradicant trials results proved very similar to protectant across the same products.
In powdery mildew trials (2013-2015), Mr Blake suggested that Proline offered better disease control than that of the SDHI straights Imtrex, Zulu and Vertisan, and, again, the two mixtures Siltra Xpro and Adexar saw very good and very similar control.
Matching Proline, Bravo performed better than the straight SDHIs, in ramularia fungicide activity trials (2011-2015). Less control was seen by Ignite however.
Of the mixtures, the best control came from Siltra Xpro, while Adexar and Bontima (cyprodinil + isopyrazam) gave similarly good control.

Barley summary- Siltra Xpro giving good broad spectrum activity- Adexar/Imtrex or Vertisan + Proline similar on rhynchosporium and net blotch- Proline consistent efficacy- SDHIs showed good net blotch activity- Ramularia was controlled by the SDHIs, Proline or Bravo- Avoid over reliance of SDHI + azole – other mixtures are available

Oilseed rape
Average oilseed rape yields in 2015 were just below 6t/ha across AHDB trials, commented Mr Blake, who suggested it was a “phenomenal result”. Although there have been no untreated yield trials during the past six years; the nine years between 2002-2010 had produced an average of 0.58t/ha yield response to fungicide treatments.

Across eight sites and four years (2011-2014), Proline has been highly effective at controlling phoma stem canker, he said. In a highly curative situation in 2015 trials, both Cirkon (prochloraz + propiconazole) and Orius 20 EW (tebuconazole) had a higher canker index than Proline and SDHI combinations Refinzar (penthiopyrad + picoxystrobin) and Pictor (boscalid + dimoxystrobin) – the latter two coming with label restrictions that need to be taken note of, highlighted Mr Blake.
In light leaf spot trials across five sites (2014-2015), Orius 20 EW, Orius P (prochloraz + tebuconazole) and Proline were highlighted in terms of maintaining low levels levels of light leaf spot infection, as did Refinzar, while all products gave a good yield response.
“Light leaf spot control is more about product timing rather than product choice,” he said.
To conclude, Mr Blake focused on sclerotinia trials in Kent and Herefordshire (2006-2008) with Filan (boscalid), Folicur (tebuconazole), Compass (iprodione + thiophanate), Pictor, Priori Xtra (azoxystrobin + cyproconazole), Proline and Amistar (azoxystrobin) all performing well. Filan needs to be mixed with other modes of action to reduce likelihood of resistance, he reminded those in attendance.
For sclerotinia disease control and yield benefit on one site in 2015 – Proline, Amistar, Filan and Pictor all performed similarly well.
“Sclerotinia is a disease that can halve the yield of oilseed rape and so it’s reassuring that all available products offer good control and yield responses.
“In epidemic seasons you see a benefit in going beyond the half label rate,” he added.

OSR summary- Phoma control can generally be achieved using half rates in a two-spray programme- Treatment timing is key for good light leaf spot control – monitor on field-by-field basis- Spray timing critical for sclerotinia control – benefits from using higher doses- Non-azole options available for phoma and LLS control – note restrictions

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