13 October 1995

A better test of potato scab resistance brings reliability

POTATO growers should have a much better and earlier idea of powdery scab resistance of new varieties in future.

A new test introduced at the National Institute of Agricultural Botany now makes reliably consistent results a reality.

Until recently powdery scab resistance was measured by planting tubers on land known to be infected with the disease, says Tom Dixon, head of NIABs potato section. But unless field conditions favoured its development, differences between varieties could be hard to pick up, he explains.

The new test, trialled on 50 varieties in 1994 and 48 this year, uses single tubers planted in pots containing infected compost and conditions to favour the disease.

Estimating the amount of disease present from the symptoms arising remains a skilled task, Mr Dixon acknowledges. Powdery scab can appear as pustules and cankers. "The difficulty in some instances is distinguishing between the pustular form and common scab symptoms."

This years figures highlight a big range of susceptibility, though he stresses the need for several seasons results to provide an accurate picture for specific varieties.

The crisper Hermes, in the first year of independent variety trials (IVT), and the recommended maincrop Sante were most resistant in 1995, with just 0.1% and 2.7% of their surfaces respectively affected by pustules.

At the other end of the scale, Anna, a white maincrop with good resistance to common scab, was 50.2% pustulated by powdery scab in this years pot tests. Some US introductions – the processing types FL 910 and FL 1771 from breeder Fritolay – were also badly affected, says Mr Dixon.