The re-emergence of the Glasgow brown rust race and an increasing level of complexity in yellow rust races in winter wheat has led to rating changes for Santiago and Crusoe.

Sarah Holdgate revealed that the latest UK Cereal Pathogen Virulence Survey showed rusts were becomingly increasingly diverse, making it far more difficult to predict how varieties will be affected in the field.

Yellow rust

“We have now identified four groups of the Warrior yellow rust race, known as Warrior 1-4, but not all of them are virulent on Warrior,” she said at the recent UKCPVS meeting in Cambridge.

“That gives us a naming challenge and it is something that is being addressed, with a new naming system due to be outlined in September.”

See also: Septoria is the big disease threat to wheat this spring

Warrior isolates have dominated the UK yellow rust population for the last few years, she explained.

“In 2011, Warrior 1 was present at high levels. Since then, it has declined and isolates belonging to the Warrior 3 group have increased, showing a shift in the population.”

The effect of this shift is being monitored in adult plant tests, she reported.

“What is important is that the Warrior race shows major differences to the older UK races of yellow rust – even within each of the four Warrior groups there is a lot of variation.”

This huge amount of diversity makes it very difficult to predict how it will change from here, or how different varieties will react, admitted Dr Holdgate.

The good news is that 20 out of the 41 current Recommended List varieties are resistant to all the isolates tested (see box).

A small group of varieties, including KWS Santiago, has shown resistance to the Warrior type isolates but susceptibility to the older Solstice isolates, she added.

“That’s why Santiago is now more resistant in the field and its rating has gone up from a 4 to a 6.”

Brown rust

The survey, which monitors changes in pathogen populations, also found that the wheat brown rust populations had changed over the last two years, with a race which had previously fallen to very low levels making a comeback.

The race, which was first identified in 2006 and became known as the Glasgow race, is now responsible for 30% of the isolates tested.

“What’s interesting about this is that it was present in 2006 and then it disappeared. Now it has re-emerged to become dominant, and it is a complex type.”

It is probably the reason why Crusoe had higher than expected levels of brown rust last year, but that is being investigated in adult plant tests, confirmed Dr Holdgate. “As a result, Cruose’s rating dropped from a 6 to a 4 in the latest RL.”

Just five winter wheat varieties showed good resistance to all the brown rust isolates tested, she revealed.

Survey changes for 2015

Results of adult plant tests will be reported in September this year, some six months earlier than before, revealed Dr Holdgate.

“There are two phases of testing – seedling and adult plants,” she explained.

“In the past, we’ve reported in March on the presence of any new races, but haven’t explained what effect these will have on varieties until the following year, following the adult plant tests.”

This year, that information will be available just six months later, as soon as possible. “By reporting on any effects on varieties earlier, growers will have more information on which to base their planting decisions.”

Diversification groups

For the first time in 40 years, the grouping of varieties based on yellow rust pathogen populations and variety resistance genes won’t be produced by NIAB this year.

Known as the Diversification Scheme, it aimed to help growers with variety choice by highlighting the sources of resistance and encouraging them to reduce areas of susceptible varieties.

“It worked very well with the older races,” acknowledged Dr Holdgate. “But the arrival of the Warrior race challenged the system – the reaction by varieties to different races is very mixed and quite unpredictable.”

As a result, a meaningful grouping isn’t possible, she stressed. “There’s just too much diversity.

“But the principle of the scheme is still relevant. Grow any susceptible varieties alongside a resistant type, to minimise the risk.”

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