10 July 1998

Severe disease challenge shows resistance benefits

Severe disease and lodging

pressure spells trouble for

growers, but suits variety

testers just fine.

Charles Abel, Andrew Blake

and Andrew Swallow report

the latest variety advice to

emerge from a challenging

season as discussed at the

NIAB Varieties and Seeds

Day at Cambridge.

Continued on p52

INTENSE pressure from diseases this season has highlighted the benefits of good variety resistance in wheats, according to NIAB.

But while yellow rust has grabbed the headlines, it is worth remembering the yield implications of failing to control the others, plots at last weeks Varieties and Seeds day demonstrated.

With wheat at only £70-£80/t and growers trying to reduce costs, perhaps by trimming three-spray programmes to two and maybe two to one, inherent disease resistance is a valuable asset, says pathologist John Clarkson.

"It has been a really bad year for foliar disease, one of the worst I can remember. Yellow rust has been found right across the country and even in Ireland. It is a similar picture to when Slejpner was affected except this time more varieties are affected because many have Rendezvous in their parentage."

Brown rust levels on susceptible varieties like Buster have also been high. "That is slightly surprising because the disease generally likes warmth, but it over-wintered well. But we have had less than we expected on Rialto, which might be to do with a new race."

Septoria tritici, the main yield sapper in all six varieties on display bar Buster, has been more aggressive than for several years, he adds. "But mildew levels have dropped. It does not like the wet."

To demonstrate potential losses due to disease, all the more likely this season when sprayers were often prevented from operating because of bad weather, Mr Clarkson and colleagues used paint to make their point.

For each variety the likely loss due to each disease was calculated from known interactions. "We can calculate the mean loss in yield for each one point reduction on the resistance ratings scale," he explains.

The percentage of each plot at risk if untreated was then painted in disease-matched colours on the cut stubble, blue representing mildew, brown Septoria tritici, yellow rust yellow, and brown rust pink.

The exercise (see table) shows nearly a quarter of Ribands output could be eaten into by Septoria because of its poor resistance rating. Likewise Busters similar rating against yellow rust leaves 17% of its yield at risk.

At the other extreme the rust threat to unsprayed Hunter, a variety now outclassed for reasons other than disease resistance, is zero.

"It just shows that in a variety with good disease resistance, like Hunter, there is potentially much less to lose than in one like Riband," says Mr Clarkson.

Disease rating/yield loss interactions in winter wheat

Mildew Septoria tritici Yellow rust Brown rust

Rating Yield loss Rating Yield loss Rating Yield loss Rating Yield loss

Buster 5 6% 5 14% 9 % 3 17%

Equinox 6 5% 5 14% 4 12% 9 0%

Hunter 7 3% 6 10% 9 0% 9 0%

Reaper 7 3% 6 10% 4 12% 9 0%

Rialto 7 3% 5 14% 6 7% 4 12%

Riband 7 3% 3 24% 6 7% 4 12%

Painting the disease picture. Different colours show the potential impact of disease on Riband (right) and Hunter, says NIABs John Clarkson.