More changes in yellow rust in wheat

11 November 2017

A close up of an infected leaf showing the yellow pustules and the clear stripes that are characteristic of the disease

Yellow rust is one of those wheat diseases that, once experienced, is never forgotten. Severe infections strip green leaf away almost like a herbicide and decimate yield – badly affected varieties in SRUC untreated trials often yield as low as 3 tonnes per hectare compared to the 10 tonnes upwards of their unaffected, fungicide treated neighbours.

One of the issues with yellow rust is how adaptable it is and how quickly races of rust change and overcome the resistances of widely grown varieties. In an interesting contrast fungicide efficacy has been very stable with azoles, strobilurins, SDHIs and even older groups like morpholines retaining their efficacy. But while fungicides offer good protection if applied ahead of infection, varietal resistance is obviously one of the key defences against yellow rust. The UK has a good history of breeding innovation and the introduction of varieties with good yellow rust ratings. But because new races emerge rapidly it is imperative to keep on top of news of which races are common and to understand how this changes your assumptions about the varieties that you have selected.

Yellow rust was less of an issue in 2017 compared to the rampant problems in 2016 but it was still relatively common in Scotland where spray intervals were stretched. The results of the 2017 UK Cereal Pathogen Virulence Survey have just been released and show that the new races picked up in 2016 were confirmed again but there have been no further major shifts in the rust races found which is good news. That means that the ratings for varieties haven’t shifted again as they did last year when several varieties such as Myriad tumbled from a high rating to a new low as they succumbed to a new race.

A new naming system has been introduced so that yellow rust races are no longer referred to by the variety on which they were first found – as an illustration of why this no longer works the ‘Warrior’ race had evolved to be so complex that it contained many different races some of which didn’t even infect Warrior! So if you are worried that the ‘Blue 7’ and ‘Red 24’  races of rust talked about now are something new and scary then be reassured. They are the races already picked up in 2016 and confirmed again in 2017 and were previously named  the ‘Kranich’ and ‘Invicta’ races. The new naming system simply takes the key groups of races by colour and names each new variant of it with a sequential number as they occur.

It is really important to keep abreast of new races as things can change rapidly, but at the moment varieties in the ground now look like they will perform as expected from their yellow rust ratings given in recommended list tables. It is worth noting that many varieties have seedling susceptibility though, so may show symptoms before their adult plant resistance kicks in. Costello, KWS Crispin and KWS Siskin are the only varieties with both adult and seedling resistance and the UKCPVS 2017 survey reports that no candidate varieties coming through the UK testing systems have seedling resistance. Varieties known to be weak and lacking adult plant resistance i.e. rated 5 or less may need early fungicides to protect them in the spring if yellow rust is reported early, as often happens now in a warm autumns and winters.

Fiona Burnett (SRUC) for the Farm Advisory Service

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