Scottish potato growers have been warned to be ready to adjust their blight control programmes in the face of two new strains of the disease which have the potential to overcome normal control measures.

Although Scottish growers might have so far had an easier ride on controlling the disease this season than producers in England, where wet weather and flooding has held up spraying, growers were given a stark warning that the situation could change with the arrival of damp conditions likely to encourage development of the disease.

Speaking at this week’s AHDB strategic potato open day, in Perthshire, plant pathologist Dr David Cooke, of the Hutton Institute, said that the two strains of most concern to the industry were the so-called 36_A2 strain which is highly aggressive and the 37_A2 strain which has reduced sensitivity to fluazinam, one of the most widely used fungicides.

Dr Cooke said that the challenges in managing potato blight (phytophthora infestans) had always been the main focus of fungicide programmes for potato growers and routine applications to prevent establishment of the disease often took place every seven days.

However, he said that while growers were getting used to using the Hutton Criteria forecasts to help predict when their crops were most at risk, they should also make sure that their fungicide choice would be effective and that they were treating with the right product.

He said that the ability to do this was now on offer through the ‘fight against blight’ campaign which allowed growers to send samples for DNA testing using the straightforward FTA card system which allows different strains to be identified within a few days, rather than only at the end of the season as had been the case in previous years.

He said that ‘Blight scouts’ drawn from members of the industry who were routinely walking potato fields during the season could submit samples which would allow a better picture of the spread of different strains.

But with this week’s change in weather favouring blight development, he said that although few samples had yet been received from Scottish producers, two-thirds of those received from England had been of the aggressive 36-A2 strain – and the resistant 37-A2 strain had been identified as being present in Scotland for the first time in a trial crop in Ayrshire last year.

“With prevention being far favourable to control, it is important that growers know that their programmes will be effective – and knowing the strain present in their fields will help them make informed decisions,” said Dr Cooke.

He said that while the normal range of active ingredients could be used to control the 36-A2 strain, knowing that the more aggressive strain was present would allow producers to either use more robust – and probably expensive – treatments or reduce spray intervals to every five days.

While there are 13 actives available for treating blight, fluazanim was by far the most widely used and knowing that the insensitive 37-A2 strain was present could allow the use of alternatives which would stop the disease getting a foothold – or becoming silently established and causing storage losses.