Be sensible with SDHIs

However, with the good weather set to continue into the next week, arable growers were warned about the risks of overloading T1 and T2 sprays, if previous sprays were missed, and scorch.

Speaking at CropDoctor North, agronomist, John Murie said that T2 sprays for winter barley are traditionally always around Fife Show (third weekend in May), with the same sprays for spring crops around the Highland Show.

The rapid growth witnessed over the past month has as a result, seen some growers apply T1 sprays at the end of the first week in May in Perthshire, with T2s expected from this weekend onwards.

"Normally there can be a three to four week gap between the various sprays but such has been the growth over the past month that this gap has been reduced to nearer two weeks between the T1 and T2s," said Mr Murie.

But, he also highlighted the problems associated with the poor sowing conditions last autumn.

"It has been a challenging time for winter crops due the later sowing dates and crops not sown in the best of conditions, but there is still potential for good crops but also a lot more average crops.

“We all know that winter wheat has that ability to recover providing you get enough plants through. But, I do have a concern with some of the winter barley in this area (Carse of Gowrie, Perthshire). Some have lost too many tillers to fully recover.”

He added that growers who missed a T0 spray should not look to bulk up on T1 sprays as this adds further stress to plants which are already likely to be under pressure, with septoria and mildew witnessed in some winter wheat plants.

Instead, he advised growers to maintain dose rates to protect both barley and wheat heads.

“I think there is enough there for a reasonable crop, it’s about justifiable spend. Maybe switch the SDHI to a strob.”

Bayer’s Grant Reid agreed that a strob would be a good alternative in less promising winter crop situations.

“Siltra is the more potent option and offers broader spectrum activity. But with drier conditions helping check many diseases that broad spectrum activity might not be required. Rhynchosporium hasn’t gotten away this season, and the gap to T1 is short. A product like Fandango (prothioconazole + fluoxastrobin) is a good choice in such situations,” he said.

While net blotch and rusts have not been seen in the area, Mr Murie said there is some Rhynchosporium but not at significant levels. That said, he warned against cutting back or delaying spray timings as the risk of disease spread is still high following the recent heavy dews.

“A number of crops didn’t get a T0 due to weather or excessive workloads, a key growth regulation timing. For those looking to load PGR and trace elements with T2 fungicides it might be better to split a PGR out.”

Mr Murie also warned growers to check PGR labels as cut off dates vary even for products that may contain the same active. That applies to wheat too with many varieties susceptible to Septoria and root lodging.

With crops racing through growth stages, he added that winter wheats will get a azole + SDHI + CTL mix at T2. However, he also urged growers to apply T2 fungicides at the correct growth stages rather that calendar date from T1.

“You only have to look in the base of plants to see what pressure was there early in the season. This warm weather might keep Septoria in check currently, but we only need a few showers and Septoria could suddenly take off.”

In the battle to protect crops from the disease, and steward chemistry he said growers should do all they can to achieve optimum target leaf coverage.

“As well as using azoles, SDHIs and multisites together and at robust rates, we must do all we can to achieve effective spray deposition. Angled and alternate facing nozzles, water volume and travel speed are all import considerations.

Grant Reid agreed and pointed to work Bayer undertook with delayed T1 sprays at Callow, Herefordshire, last season.

Bayer delayed T1 applications 10 days post GS32 to see what impact it had on Septoria control. “Despite reduced Septoria pressure there was a visible effect as crops approached flowering. By the time we got to GS59 and ear emergence you could see the impact. The plots where we delayed T1 applications had less green leaf area (GLA). And that was with a potent product like Aviator (prothioconazole + bixafen).

At the same time the company looked at the impact of variety rating and drilling date on Septoria infections. It recorded Septoria DNA on leaf two in varieties Trinity (5.3) Revelation (6.4) and Siskin (6.8) at GS39 and ten days later in untreated plots.

“There was a clear pattern between Septoria DNA picogram levels and variety rating. In Trinity, some plots had readings of 25 DNA pg. No plot of Siskin was above 5 DNA pg. But what was particularly interesting, was the DNA pg in early sown plots. This increased for every variety and some Trinity plots had DNA pg levels close to 250.”

Across the new Queensferry Bridge, winter crops at Boghall, Midlothian, have also raced through the growth stages where Septoria could be found in the base of treated wheat plots and was more evident in untreated. Rhynchosporium, mildew and brown rust were found in untreated winter barley, while treated plots were fairly clean.

“We’ve got Septoria and mildew in some wheat varieties, and yellow rust is popping out in the usual suspects, but, the difference is that plants are protected," said the SRUC's Prof Fiona Burnett.

And while some winter wheat T1s were only applied in the third week of May, there is still a need to come back with a T2 to protect leaf layers according to Agrii agronomist, Greig Baird.

“We’re now in the critical phase, protecting the most important leaves for yield production, and there’s nothing more important than the flag leaf. If you hold off then you risk exposing the flag leaf to potential infection. The medium term weather forecast is dry but Septoria can still spread via leaf contact. And in some crops it hasn’t got far to go.”

He will be applying SDHIs with stacked azoles to reduce selection pressure. The choice of primary azole will depend on other disease pressures with epoxiconazole favoured for susceptible yellow rust varieties, prothioconazole where mildew still lingers.

Prof Burnett agreed and advised growers to target T2 wheat sprays at GS39, and to stick with azole + SDHI + CTL mixtures, despite the protective position growers find themselves in. “One compromise in timing shouldn’t lead to another,” she warned adding that CTL would provide the best possible protective properties, and support azole and SDHI chemistry. Something she considers vital.

Looking at winter barley, Prof Burnett believes azole + SDHI + CTL is probably the pick of fungicide choices. “CTL is now vital for Ramularia protection, but azole + SDHI ticks all the remaining disease boxes. There isn’t a huge difference in price between Siltra (prothioconazole + bixafen) and Fandango (prothioconazole + fluoxastrobin).”

The fact some winter barley crops didn’t get a T0 spray hasn’t caused too many issues for Mr Baird as he reckons robust T1 mixes and clement weather means spray gaps are unlikely to be stretched. He also still feels a strobilurin has a place at T2 when used with prothioconazole, if Rhynchosporium is the target alongside Ramularia.

Mr Reid agreed but added that Fandango is still a good option. “I think Siltra is an obvious choice for the T1 timing. But if disease pressure has been checked by robust T1 strategies then it does offer an alternative.”

For those that took the opportunity to apply a PGR and a holding spray of CTL, as with wheat he said he wouldn’t use it as an opportunity cut back sprays or reduce doses. “It didn’t look great in March when some winter crops were really struggling, but now they have picked up soil nutrients they look pretty promising. I wouldn’t give the opportunity for disease to get back in.”

Looking at spring barley, spotting was found in the crops at Boghall, which fortunately was not found to be Ramularia which is easily misdiagnosed, Professor Burnett said.

“Don’t predispose you have the disease. Classic Ramularia symptoms are spots that go through to the underside of leaves, the rectangular shape, lesion restriction by the veins, the reddish colour and the halo. If you’re not seeing this then it is possibly a stress factor.

“Knowing what you’ve had in previous seasons gives you an indication of how susceptible your area is to Ramularia and regular monitoring of crops is always a good idea,” she said

A key stress is flowering, which is often why the disease is picked up later, therefore timing is important. “You need to be on ahead of key stress factors, such as when the crop moves towards senescence,” added Ms Burnett.

Hence, reducing stress factors can help according to Mr Baird. “Healthy plants withstand the disease better – I think all growers can do a lot to limit other stress factors. Morpholine fungicides can be a trigger and growers need to be alert to hot mixtures, and nutrient deficiency is a key plant stress.

“Spring crops suffer particularly due to the shortness of the season. Which is where we are now, with late drilled crops going into good seedbeds and favourable weather.”

Notably, all agronomists worry about a blanket move to CTL throughout the programme. “It’s typically a disease we see at flowering. If the threat is there then it makes CTL an obvious addition at T1 but it’s not a given,” adds Prof Burnett. But, despite SDHI resistance to Ramularia she wouldn’t move away from azole + SDHI + CTL mixtures at T2, due to it covering all disease threats.

Side Panel

Be sensible with SDHI stewardship in wheat

According to Prof Burnett the industry is in a moving situation with SDHIs and she stressed the need to support the chemistry with azole and multisite as partners. She is confident the use of chlorothalonil is a win:win for growers and the wider industry as it adds to disease control seen in trials and to the yield. It is also proven to reduce selection pressure against SDHI mutants.

All consider any kind of mandatory guidelines as impractical but want growers to apply best practice at all times. For Agrii that means only selling straights with an appropriate azole partner. “Having the option to mix a product like Imtrex (fluxapyroxad) with the best azole partner is hugely beneficial for effective targeting of the disease pressures you are facing. But we must ensure the azole is used at an appropriate dose,” said Mr Baird.

All also doubt that SDHIs have a role at T3. Prof Burnett said use them for what they are designed for – disease control.

“T1 and T2 is where the key disease threats lie. That is where they are intended to be used. Late green leaf retention can be achieved in other ways and to expose SDHIs when they aren’t needed for disease control is to hasten any decline.”

Mr Baird says for those looking for late greening, alternatives exist. “If that’s what you are looking for then add a strobilurin to a product like Kestrel (prothioconazole + tebuconazole). Prothioconazole itself will protect against Fusarium and provide foliar top up. I would rather see a strobilurin added to prothioconazole over an SDHI being taken out of a primary disease timing,” he concluded.

Mr Reid acknowledges that Ascra does have greening properties but questions whether taking SDHIs out of primary disease timings is wise. “We’ve done work with ADAS at our Callow site measuring upper canopy green leaf area decline to 35% GLA after GS39. What it has shown is that plots treated with Aviator at T1 followed by Ascra at T2 retained a minimum of 35% GLA for 18 days over untreated plots. But this was five days longer than plots treated with Proline275 (prothioconazole) at T1, followed by Aviator at T2.

"Over three seasons this has been worth an extra 0.45t/ha per day. I’m sure this in part is due to the physiological properties of these fungicides but also their disease control properties,” concluded Mr Reid.