7 July 1995

No new yellow rust ratings until 1996

WHEAT growers will have to wait until next year for new resistance ratings on varieties affected by the novel yellow rust strain (Arable, June 9).

The new disease race gave visitors to last weeks Varieties and Seeds Day at the National Institute of Agricultural Botany plenty to talk about. It overcomes the resistance provided by the YR17 gene in Rendezvous, a variety widely used by breeders.

But until comparative field tests are carried out next season the numbered ratings for previously resistant but now affected recommended list varieties, like Brigadier, Hussar and Beaufort, will be replaced by asterisks. This will draw growers attention to the potential risk from the new strain, explains NIAB pathologist Dr Rosemary Bayles.

"We are now considerably more confident that our predictions are being borne out in the field," she says. Five naturally infected crops of Brigadier, all within the usual high risk areas of the east, have been detected. Correct timing for the potentially "explosive" disease is critical, she says.

A "snapshot" view of demonstration plots at Cambridge, where yellow rust is present on several untreated varieties, backs Dr Bayles observation that polythene tunnel tests are highlighting "quite a range of infection levels". It is likely that several varieties in National List trials will prove susceptible because of the widespread use of the Rendezvous resistance gene in breeding programmes, she adds.

Least susceptible

Of those affected in the plots, aside from Brigadier and Hussar, Beaufort "looks the least susceptible at the moment," according to NIABs Phil Stigwood. Of the two list candidates showing symptoms, Chianti has slightly more obvious signs of the disease than Reaper. But the findings are from one site only, he stresses.

Much more work by the UK Cereal Pathogen Virulence Survey team needs to be done before reliable field ratings can be given.

The NIAB plots are a "fairly dramatic demonstration" of the power of the disease, says NIABs head of combineable crops, John Ramsbottom. This years events will clearly alter advisers perceptions, he says. But their influence on future management practices remains to be seen.

With correctly timed triazoles able to give good disease control, main NIAB advice is to limit the area of susceptible varieties to that which can comfortably be treated in the time available.

The new strain of yellow rust is bound to affect some of the newer varieties coming through NIAB trials, says Dr Rosemary Bayles.