Scientists work to tackle rose grain aphid

THEy may not be as visible as locusts and they may not leave such a trail of destruction but the rose grain aphid is increasingly being blamed for spreading virus in potato crops.

Yesterday at trial sites outside Cupar Dr Brian Fenton, from the Scottish Crop Research Institute, said that up to one million of these bugs and their close relatives, the bird cherry oat aphid could pass through a potato crop within a week.

The problem is that their grazing habits help spread disease. "They can land on an infected plant and pick up the disease in seconds and then transmit it to another plant within a few more seconds," he stated.

Up until now most attention in potato fields has been concentrated on another bug, the peach potato aphid which can transmit the leaf roll virus. However, such has been the success in dealing with this yield reducing disease and in dealing with the vector that it is now quite unusual to come across the peach potato aphid in Scotland.

On the other hand, there have been an increasing number of seed potato crops failing to come through field inspections because of mosaic problems.

These crops have been infected with a potyvirus and the infection was likely transmitted by the rose grain aphid or one of its relations according to Dr Fenton.

Because there are so many of the aphids and because they move across fields in feeding waves, normal spray control methods are not effective.

As a result, Eric Anderson potato consultant with Scottish Agronomy has been carrying out trial work which involves spraying a highly refined paraffin oil on the plants.

This oil acts as a repellent to the passing bug and after three years of trial work, indications are that it might provide a solution to the problem. Anderson admitted that there would be yield loss of up to 10 per cent but that this was not significant for seed growers as it only put back the cutting off period by a week or ten days.

The Scottish Agricultural Science Agency (SASA) has been wary of the mineral oil treatment because they felt it might mask other problems in the potato crop.

But Dr John Kerr who heads up the potato section at SASA, said that it now looked as if there was no problem in applying this protective layer. He added that Swiss visitors to SASA last week said they were using this repellent and it was effective. However, they are dealing with much higher levels of infestation and their experience may not translate to this country.

Growers attending the event were also told of the mechanics of the new testing regime for potato cyst nematodes, which in a previous era were called eelworm.

As from the beginning of this month growers will require to pay for their land to be sampled. The range of costs varies with the scale of the field involved and also its previous history but, for a 20 hectare field, will run between 165 and 1,035.

The new regime will involve many more samples being taken and the samples being twice the size they have been. To cope with this, SASA have invested in new equipment to deal with the estimated 30,000 lots requiring testing annually.