Tighter rotations and stringent quality requirements are forcing Scottish potato growers to adopt a “belt and braces” approach to skin finish diseases, reports agronomist Dougie Bain of Thistle Agronomy, part of the Masstock Group.

Using both a seed treatment and spraying a fungicide on the soil at planting to get a marketable sample is proving necessary in certain circumstances, he notes.

“Good skin quality is absolutely essential for the pre-pack market.

In the worst cases, where Rhizoctonia (black scurf) and black dot are both serious threats, there isn’t an alternative approach.”

As most of the varieties being grown for packing are susceptible to black scurf, Mr Bain adds that it is common practice to use a seed treatment.

“The disease is both seed and soil-borne.

So although you can test the seed for infection before planting, it’s difficult to predict which fields are harbouring infection.

And rotations are down to four years in some places, which adds to the pressure.”

The introduction of seed treatment RhiNo in 2004 was helpful to growers, believes Mr Bain.

“There’s no delay in crop emergence with RhiNo, and it doesn’t have a 60-day restriction between application and planting.”

RhiNo is doing a good job on black scurf, he confirms.

“It protects the crop at its most vulnerable stage and skin finish is good.

But you have to remember that you won’t get 100% control – it’s doing a holding job.”

At 19/t, the seed treatment is affordable, he adds.

“Where the real expense comes in is if Amistar has to be applied at planting as well.

It goes on at a rate of 3 litres/ha for in-furrow applications, or 6 litres/ha where it is sprayed onto the soil and incorporated.”

Amistar has a role in the control of soil-borne black scurf and black dot, he continues.

“Before you go to the expense of it, consider your rotation and your variety,” advises Mr Bain.

“But in a number of cases, the packer has stipulated that it should be used.”

Full label approval for both diseases was granted in time for last year’s plantings, he recalls.

“Amistar certainly has an effect.

And as potatoes become more concentrated in this area, growers are increasingly going to need all the help they can get to ensure skin quality.”

fwarable@rbi.co.uk