14 April 2000

Scab is main enemy, reckon Scots

Potatoes are a key crop for

many Scots growers, both seed

and ware. Andrew Swallow

travelled to the Borders for

some timely advice from

SAC and to visit a grower

group taking a new approach

to seed sales

POWDERY scab is the biggest concern for most Scottish potato growers, according to a survey of both ware and seed producers.

But control of the disease is never complete, and growers must adopt a risk reduction approach, says SACs Barry Mitchell.

In the survey, members of the SACs Association of Potato Producers were asked for their number one agronomic concern.

"Three out of four said powdery scab. That may be driven by the seed producers but many ware growers rate it as the number one problem too."

Accordingly, Association research is investigating new ways to tackle the skin finish problem which can cut saleable yield by up to 50%.

Trials with chemigation delivering treatments such as zinc through irrigation brought positive results for some diseases, but unfortunately not with powdery scab.

"The weather did not favour powdery scab development, despite the site having a history of powdery scab," says Mr Mitchell.

The work will be repeated this year. In the meantime growers best bet is a step by step risk assessment approach, he says.

"That starts with seed inspection, ideally before grading as obviously infected tubers may have been graded out. Growers dont want to plant seed with no visual symptoms but that is nonetheless infected with spores. You should visit your seed supplier as soon as possible after harvest," he suggests.

About 100 tubers selected at random from each stock should be washed and inspected.

In some seasons it may be difficult to secure scab free supplies of seed, especially for more susceptible varieties such as Saturna, Cara or Estima, he concedes. But growers should avoid planting such seed in virgin potato ground as that could establish infection, which survives at least 18 years in the soil.

"Rotation is not a realistic control option," he comments.

Growers planning to plant on virgin potato ground might also ask whether livestock have been fed with potatoes on the farm. Fields have been known to become contaminated from applications of manure from stock fed on potatoes, he warns.

In the crop, tuber infection occurs when soils are saturated. That means care with cultivations to avoid and correct compaction is crucial. Low soil zinc content also seems to be associated with the disease, and incorporating zinc in the beds can reduce the severity. "But it wont cut incidence and the reduction in tuber surface area affected varies from 0 to 60%."

At about £600/t and 40kg/ha, treatment costs £24/ha (£10/acre) and the return is not guaranteed. There appears to be no merit in zinc treatment for soils with over 6 ppm of zinc, he adds.

Irrigation where powdery scab is a concern can be tricky. "As a rule of thumb we say apply 12mm of water at a soil moisture deficit of 18mm for common scab control. That leaves 6mm of leeway. But that might not be enough to avoid saturation with the inaccuracy of some rain guns or if rain arrives unexpectedly."

Most seed growers prefer not to irrigate crops, he says.

Booms and trickle irrigation could be the way to increase accuracy of water delivery.

POWDERY SCAB STRATEGY

&#8226 Start with clean seed.

&#8226 Use resistant varieties on infected soils.

&#8226 Ensure good drainage.

&#8226 Check soil zinc status.

&#8226 Extra care with irrigation.

SACs Barrie Mitchell (left) and Martin Baird of Scott Country Potatoes match varietal characteristics, including powdery scab susceptibility, to individual fields. This free draining, light silt field will be planted with Saxon.