Agronomist reports last season on the difficulties of controlling yellow rust was put down to a combination of late drilling, winter weather and changes to the pathogen.

But further digging into the data has suggested that fungicide choice might have been a factor.

Chemistry specialist, Bayer, had always cited that formulation as a key factor in fungicide performance and when reports of yellow rust began to circulate, the company took the opportunity to check fungicide performance at its Cawood trial site, in North Yorkshire.

Using the variety, JB Diego, the company applied a single application of various fungicide treatments and followed up with disease assessments approximately one week and one month post application.

This revealed that there are few ‘all-rounders’ in the yellow rust fungicide armoury with most actives ideally suited to protectant or curative situations, but rarely both. It also confirmed that formulation had a bearing on the performance on product performance.

How these curative and protectant fungicide properties differed was quite marked. Strobs stand out for their curative properties but proved to have limited persistency. Some strong protectants offer little against established disease.

What it came down to was formulation and speed of active on the pathogen. “It’s common practice to include a strob in a fungicide mixture to boost yellow rust control but their movement on and through the leaf means they are short-lived on leaf surfaces," pointed out Bayer’s Craig Simpson.

"Our first assessments showed only small differences in disease levels between pyraclostrobin, epoxiconazole, tebuconazole and prothioconazole treated plots. But second assessments, one month after application, revealed high disease levels in strob-treated plots.

“Strob speed is an asset where the disease is established as the fungicide acts quickly against the pathogen. But for a fast-cycling disease like yellow rust it leaves plants exposed to new infection,” added Mr Simpson.

With strobs at the bottom of performance tables when it came to persistency, it was no surprise to see Elatus (prothioconazole + benzovindiflupyr) at the top of the list. But this persistence also comes at a price, and that is impaired kick-back.

“It offers protection over multiple cycles but it is weaker on established disease. In such situations, a strob or fast-moving azole is probably the better option,” he added.

Formulation was the reason for variable performance with tebuconazole – a popular T0 choice to damp down the disease. Bayer's Folicur (tebuconazole) was compared to Toledo (tebuconazole) and the first assessment, just one week after application, found the Toledo treated plots had 5% more yellow rust infection severity compared to the Folicur.

This confirmed what the company suspected, that SC (suspension concentrate) formulations don’t act as fast as its formulated tebuconazole. Prosaro, a Bayer co-form of prothioconazole + tebuconazole, was also compared to a mixture of generic prothioconazole + generic tebuconazole formulation.

Whilst they both performed well at the first assessment, the later assessment revealed 5% more yellow rust in the generic formulated plots, demonstrating its limited persistency, compared to Bayer’s EC formulation.

“Given the uncertainties and unpredictability of factors affecting yellow rust, both tebuconazole and prothioconazole proved themselves as the best all-round actives against the disease delivering a good balance of persistency and curativity, particularly when well formulated,” he noted.

When it came to wheat disease threat No 1, he said it probably depended on region and variety choice, but he added that any programme also had to protect against both septoria and yellow rust.

“For most, septoria is probably the key risk, but growers should always way up all risks – some varieties are very prone to mildew and eyespot. A broad-spectrum approach is always sensible and with a product like Ascra (prothioconazole + bixafen + fluopyram) you cover the two key wheat foliar diseases and the stem-based complex.”

Hutchinsons regional technical manager, Cam Murray, pointed out that it was a triple whammy that drove yellow rust pressure in 2020. “We had plenty of late drilled wheat and ideal conditions on the coast with a series of sea haars, that clearly helped," he argued.

“But its sexual reproductive ability has led to wide genetic diversity and a disease that has an ability to adapt to a wide range of environments. When you think that the spores can travel long distances on air currents circulating across continental Europe –– it is no wonder a new race, or races could be present in the UK.”

The disease had been widely reported on a range of winter wheat varieties throughout this past winter. He noted that the cold snap had checked the disease, but unlikely to have eradicated it.

With favourable conditions the disease could easily return, especially as again some wheat drilling was delayed this season. “Typically, we see adult plant resistance kick in at around GS37 but with later drilled crops being behind in the early part of the season this can have a more pronounced effect.

"Yellow rust is a disease that cycles rapidly and the damage can be done quickly. Under ideal conditions, it will complete its lifecycle in 10 -15 days. Where septoria will take most of the season to do its real damage on the main leaves, yellow rust can do it in a couple of weeks,” said Mr Murray.

What 2020 taught us is that yellow rust resistant ratings cannot be relied upon and disease control strategies have to be built around septoria and yellow rust. “Regardless of the weather, septoria is always there, although wet weather in April and May will increase the risk.

"What is important now is to factor in yellow rust protection with varietal resistance questionable – the programme needs to protect against both diseases.” Ascra is a good all-rounder for septoria and yellow rust.

Deciding your fungicide strategy also depends on variety septoria rating. That is because it will dictate what actives are used at the key T1 and T2 timings.

“We’ve seen a sensitivity shift in key actives and currently our strongest option is the new active mefentrifluconazole (Revysol). Although mefentrifluconazole has activity on yellow rust he feels it’s not quite to the heady heights of epoxiconazole.

"However, it still provides adequate protection, and at the T2 timing adult plant resistance will usually have kicked in which will help in the overall programme," he said.

T1 and T2 choices are likely to be based on mefentrifluconazole + fluxapyroxad (Revystar), prothioconazole + bixafen + tebuconazole (Sparticus), prothioconazole + bixafen + fluopyram (Ascra). Prothioconazole + tebuconazole (Prosaro) with fluxapyroxad (Imtrex) also having a place where more resilient varieties are in the ground.

“Revysol will mainly have a place at T2 and possibly at T1 where you have a weak variety against septoria, like KWS Barrel (rated 4.2), under severe pressure."

With better varietal resilience, such as Extase (8.0), he said there was more choice to tailor fungicide inputs. “Here, prothioconazole + bixafen + tebuconazole or a prothioconazole + tebuconazole with fluxapyroxad mix will provide the necessary septoria control and good yellow rust activity.”

But he also advised growers to consider eyespot and mildew ratings. “Eyespot is a disease that ‘comes and goes’ but Sundance and Graham are rated at just 3. You have to build that level of risk in when planning a disease control programme.”

He also considered it wise to start the programme with yellow rust protection at the T0. “Adding triazole, or a strob isn’t a huge cost to the fungicide bill.

"Where there is active yellow rust, then tebuconazole is a strong option. Where the disease still has to express itself then a strob is more suited. The problem with a strob at the T0 is that with just two applications allowed it could prevent use at T1 or T2 if planned for the T3,” he concluded.