Keeping seed potato crops free of viruses is becoming increasingly difficult. However, alternative approaches are showing promise in Scottish trials.

Controlling aphids is a key part of potato seed production, as they can carry damaging viruses such as mosaic virus.

But this is becoming more difficult with the loss of products such as Biscaya (thiacloprid). There is also increasing aphid resistance to pyrethroids.

See also: How to grow potatoes using minimal tillage

To look at alternative approaches, farmer-owned-co-operative Scottish Agronomy is collaborating with Jim Reid who grows potatoes at Milton of Mathers Farm, near St Cyrus, and has been involved with seed potato trials for over a decade.

Together they have been looking at the benefits and practicality of spreading straw mulch and applying mineral oils to the crop canopy and comparing this integrated approach with that of using a pyrethroid insecticide.

Spreading chopped wheat straw as a mulch before emergence of potatoes acts as a natural defence.

Applying a mineral oil to the canopy early on in the crop development creates a coating on the leaf surface, so when an aphid lands on the leaf, the oil dislodges the virus particle from the aphid mouth parts.

Two trials were conducted in 2021, one on Mr Reid’s farm close to Montrose and another sister trial in Fife, and it was found that applying straw mulch led to a 40% reduction in mosaic virus.

Using an integrated programme applying mineral oil decreased the virus by 55%.

The same trial was repeated in 2022, and using straw mulch alone accounted for a 49% reduction in virus. Where a mineral oil was added, it provided 54% control.

“Interestingly, where we used a pyrethroid insecticide, it increased mosaic virus relative to a control of no treatment and was 17% less effective,” says Mr Reid.

“By adding straw, you are not only discouraging aphids landing in the crop, but you are adding organic matter to the following crop as well,” he adds.

Other defence techniques being trialled at Milton of Mathers, include drilling spring barley around the seed crop and planting wildflower corridors stretching from headlands to the middle of the field to entice natural aphid predators into potato seed crops.