When ramularia resistance to SDHIs and azoles was announced last year it flagged up a clear warning for the need to steward current chemistry.

Coming on the back of the discovery of net blotch mutations the year before, SRUC’s Professor Fiona Burnett said that resistance management has to be ‘front of mind’. How that is achieved was the subject of a recent SRUC/Bayer resistance update.

Prof Burnett noted that that although ramularia is a relatively new disease, it is now a global problem. In 2017 it made its way to Australasia with the disease being found in Tasmania. Nothing appears to stop it, and it isn’t limited to barley.

Estimates suggest that losses in the UK are in the region of £10m with ‘spot for spot’ losses similar for spring and winter crops.

One reason for that spread she said was that the disease has ‘every trick in the book’. “Being seed-borne it has spread to all parts. But it grows within the plant, survives on stubbles, blows on the wind and bounces up between leaves,” said Prof Burnett.

In many ways she considered it akin to septoria, but a 'big trick' in the ramularia book is its ability to stay hidden in the plant for longer. “It’s there all the time and during much of the season it is benign – it has a unique ability to evade detection by the plant’s natural defences. But as soon as plants are under stress it becomes necrotrophic. Senescence is a significant stress – it’s why we often see the disease late,” she said.

More work is needed to understand what triggers this but researchers believe it is to do with a toxin ramularia produces, rubellin. This accelerates cell death causing cells to shut down and the rapid appearance of extensive spotting that is characteristic of the disease.

This response is triggered by stress and early signals from the plant about leaf senescence. Other risk factors, such as temperature fluctuation, radiation intensity, waterlogging, and nutrient deficiency, can have the same effect.

Leaf wetness has also been identified as a potential factor, but again Prof Burnett said more work is needed: “Risk is sometimes linked to leaf wetness from stem extension to ear emergence. But we also see the disease where leaf wetness hasn’t been a factor.

"There’s more going on which is ‘work in progress’. Sowing date, previous crop, other disease threats and cumulative temperature are all part of the picture.”

With variety resistance probably some way off, control is still dependent on fungicides. That ‘front of mind’ approach begins with correct diagnosis of the disease. Identifying it from other stress symptoms isn’t easy and it also can be confused with net blotch, tan spot or old mildew symptoms.

Her checklist for accurate identification is that reddish/brown lesions are rectangular, develop a ring of chlorosis and that necrosis is right through the leaf, something that you don’t get with physiological spotting, which is restricted to the upper surface. Another pointer is that lesions are restricted by leaf veins.

So, accurate diagnosis and targeting will allow growers to make full use of the fungicide arsenal, which she noted was still diverse for barley. Diversity is needed, with concerns around net blotch and rhynchsporium sensitivity too.

“We need to be increasing the use of actives in balanced mixtures, based on accurate diagnosis of the target. Azoles, SDHIs, strobilurins, cyprodinil, spiroxamine and chlorothalonil are all options to be mixed and alternated. It is important that we don’t expose chemistry via unnecessary use or without adequate support in balanced mixtures,” she pointed out.

In recent seasons, some growers have turned to T0 sprays to bolster protection in winter crops but she is concerned regarding chemistry choice. She suggested that use is limited to high risk situations and that it is possible to avoid azoles and SDHIs.

“If you need a T0 make use of other actives in the chemistry arsenal. Don’t over expose a key active when you can use something else,” she said.

SRUC lab data showed a decline in efficacy of SDHIs and prothioconazole in 2016, which was confirmed again in 2017 and illustrated in SRUC and AHDB field efficacy data where only chlorothalonil gave control. This was supported by the monitoring carried out by Bayer, which illustrated the widespread occurrence of SDHI and azole resistance.

But Prof Burnett warned that chlorothalonil is no panacea for ramularia. “It’s certainly vital for control but I have concerns on ‘blanket use’. There’s no correlation between the amount of ramularia early on and severity later in the season.

"What’s more important is the level of stress plants are under and the environment. That’s why accurate diagnosis will help growers build a picture of their regional risk.”

Bayer’s Dr Kerry Maguire added that although the picture isn’t rosy, it isn’t all doom and gloom. She points out that although the company’s resistance monitoring has picked up a sudden decline in azole and SDHI sensitivity, the picture is patchy.

“Our monitoring data is drawn from every European region and at some sites we saw a sudden fall in sensitivity. But it differs from region to region – against some mutations we still see full sensitivity.”

Last year's fungicide trials, with Prime Agriculture, at Woodbridge, Suffolk, reflected this with prothioconazole-based mixtures still giving strong yield responses. “Historically, this is a strong site for ramularia. Fandango (prothioconazole + fluoxastrobin) applications at T1 and T2 of 1.0 l/ha and 0.75l/ha respectively delivered close to a 1.0 t/ha response over untreated.

"Adding chlorothalonil at the T2 application increased this to just short of 1.4 t/ha. It was a similar situation where we used two doses of Siltra (prothioconazole + bixafen). Siltra at 0.6 l/ha and then 0.4 l/ha + 1.0 l/ha chlorothalonil delivered an extra 1.6 t/ha over untreated plots.”

Dr Maguire agreed that prothioconazole remains the cornerstone of T1 and T2 mixtures, despite a loss of activity against ramularia. “No other azole really gets close to the broad spectrum activity of prothioconazole, and our monitoring suggests it is stable against net blotch and rhynchsporium. In both winter and spring barley we have a range of diseases that can impair yield and quality. Using appropriate partners for a range of disease targets is one way of protecting our most important active for barley disease control,” she concluded.