A season of change, with further alterations likely to come. That’s how Rosemary Bayles of NIAB TAG describes 2012 on yellow rust susceptibility, having completed the update to the diversification scheme to cover the wheat varieties in RL trials.

The revised scheme reflects the impact of new rust races tested during the year, especially the Warrior race, which was first identified in 2011 and has now been examined as part of the UK Cereal Pathogen Virulence Survey, she explains.

The other new race, which was detected on KWS Sterling, hasn’t been tested as thoroughly, but will be in the coming year, she adds.

“It’s at a low frequency in the natural environment and hasn’t been exposed in RL trials yet,” she says. “So it’s an early warning to growers to be aware of it. KWS Sterling is not susceptible to the Warrior race.”

As a result of the update, several varieties have moved out of the lowest risk group, known as DG1, due to their increased susceptibility, notes Dr Bayles.

“We have seen a loss of varieties from this group – namely Alchemy, Beluga, Claire, Horatio and KWS Sterling – although others previously in the group have remained there and maintained their good resistance to the disease.

“They’ve also been provisionally joined by a number of new varieties, although it’s still early days for these ones and less is known about their resistance. Testing on them continues.”

After this, the less resistant varieties fall broadly into three groups – DG2, DG3 and DG4 – with sub-groups within each of these to highlight greater risk. “We’ve used two sub-groups, ‘a’ and ‘b’. Those varieties in ‘a’ are less risky than those in ‘b’,” says Dr Bayles.

Two varieties, however, have been allocated to another group, DG0, because they pose an extreme risk to others, except for those in DG1. Oakley and Torch are now so susceptible to yellow rust, she explains, that their status has been made clear by this classification. Neither is on the Recommended List any more, having been removed when their ratings fell below the minimum standard.

“It’s important to realise that new races can occur at any time,” she says. “So there’s no substitute for monitoring crops regularly, even resistant varieties. The Recommended List can only reflect what’s happened in the previous cropping season, when the effect on adult plants has become apparent.”

The UK’s top selling variety, JB Diego, which was left out on a limb in last year’s diversification scheme, has now been placed in DG1, reports Dr Bayles. “We were still working on it this time last year. But we’ve seen it performing reliably in the field, in the presence of new races, so we’re happy with its DG1 classification.”

The Diversification Scheme should be used to help determine the risk of disease spread in high-pressure situations, she says.

“Growing varieties which are affected by different races can limit the spread of yellow rust. Growers should find the Diversification Group of their first-choice variety and then use the table to identify the risk of the disease spreading to companion varieties.”

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