20 June 1998

FRONTRUNNERS IN RUST

RACES

Forewarned is forearmed.

Gilly Johnson asks NIABs Dr Rosemary Bayles for news of the latest yellow rust sightings.

THE virulent new races of yellow rust are having a field day this summer. But beware: its not only Brigadier thats hosting the disease – a number of promising newcomers are also susceptible, including Savannah.

Growers should think carefully about this autumns variety portfolios, warns NIAB plant pathologist Dr Rosemary Bayles.

To recap on the story so far. There are two rust races currently in the spotlight: the Brigadier race, commonly known as the YR 9/17 strain; and the YR 6/9/17 variant which infects Brigadier and other varieties, but is also a threat to Madrigal and Equinox. The YR 6/9/17 race has been dubbed – unfairly perhaps – as the Madrigal strain.

Dr Bayles was inundated with YR 9/17 samples when Brigadier first succumbed. But she received only a single sample of the YR 6/9/17 last season. This low incidence was to be expected, given that Equinox and Madrigal were still relative newcomers.

Thats now changed. "Weve seen quite a few more YR 6/9/17 samples this year," she comments. The logic is clear – if vulnerable varieties increase in popularity, then the disease will follow suit.

Its a numbers game: the risk from any one strain of yellow rust grows in direct proportion to the size of the area down to susceptible varieties. But weather also plays a role. This seasons mild winter encouraged spore carryover and increased the risk of spread into young crops. Yellow rust can sporulate actively right down to 5íC. And with a high proportion of Brigadier volunteers, set-aside is also acting as a source of infection, says Dr Bayles.

Without defensive diversification, its possible that the YR 6/9/17 could prove more damaging than the Brigadier race, because of the greater range of varieties that are susceptible (see table).

Dr Bayles is keen to keep track of the spread of YR 6/9/17, not just on Equinox and Madrigal, but also on Brigadier and Savannah. Unfortu-nately, many growers are not bothering to send infected Brigadier in for testing this year. They havent seen the necessity, she suggests. "Its only when yellow rust appears on new varieties that infected plants are sent to us. So please – send us your samples."

The indications are that yellow rust has broken out from its traditional homelands in the east. "Weve had reports from all over the UK including Northern Ireland, and Eire. Its very unusual for yellow rust to be seen so far west." The Edinburgh area appears to have escaped – but this is likely to be because Riband, rather than Brigadier, dominates the cereal acreage, says Dr Bayles.

Control of yellow rust hasnt been that easy this season. Although triazoles are effective, its been difficult to spray at just the right time, because of wind and rain, she comments.

Even in NIABs own variety trials, which are given extensive fungicide sprays, yellow rust proved hard to control. "We had to go in more than once to hold the disease in some plots," she admits.

Dr Bayles advice is to make life easier by including varieties with a broad range of different resistances (see p46 in the NIAB cereal handbook). Diversifying the variety portfolio has two benefits:

&#8226 it gives growers a fighting chance of avoiding the spread of infection between neighbouring varieties; and

&#8226 it cuts down the area that will need spraying promptly if disease does strikes a susceptible variety.

On balance, Dr Bayles reckons that this last argument may carry the greatest weight. "Yellow rust spores are airborne and can spread long distances between fields," she admits. "So although vulnerable varieties may be some way apart, its no guarantee that infection will not appear."

Take note of the resistance ratings, which will help dictate spraying priorities. "Theres a huge difference between Brigadier and Savannah, with ratings of 1 and 5 respectively," she says.

Its possible to live with Brigadier in the rotation – just be prepared to spray promptly as soon as the disease appears. "Its sound advice not to include more Brigadier than you could spray in a day."

Some varieties are more resistant to yellow rust as adult plants than at the seedling stage – Riband, for example. The NIAB resistance ratings are based on adult plant resistance – because this is when the disease can have greatest impact on yield.

Some isolated reports have come in of suspected yellow rust on barley. Popular variety Regina is potentially susceptible. "It is a risk," warns Dr Bayles. "Inoculum levels are low because weve had resistant varieties for a while. Theres a strong probability it could build up again with the large acreage down to a susceptible barley." Barley yellow rust does not affect wheat, nor vice versa.

Are there any more unpleasant surprises waiting under the microscopes at NIAB this season? The good news is that Dr Bayles has failed to pick up any traces of new problem strains on winter wheat – so far. But June is a critical month for rust identification, so watch this space….

Think carefully about yellow rust when choosing wheats for autumn drillling – Dr Rosemary Bayles.


Wheat variety

Good resistance to all races: Axona (9); Buster (9); Cadenza (9);

Caxton (8); Chablis (9); Charger (8);

Imp (8); Shiraz (8)

Susceptible to "Brigadier" YR 9/17 race:

Abbot(6); Avans (6); Baldus (5);

Beaufort (5); Brigadier (1);

Harrier (4);Hussar (5);

Reaper (4); Savannah (5);

Soissons(7)

Susceptible to "Madrigal" YR 6/9/17 race:

Abbot(6); Avans (6); Baldus (5);

Beaufort (5); Brigadier (1);

Equinox (4); Harrier (4);

Hussar (5); Madrigal (4);

Reaper (4); Rialto (6);

Savannah (5); Soissons (7)

Susceptible to other races: Consort (8); Hereward (6);

Riband (6); Spark (7)

( ) = general YR resistance rating

Source: NIAB