Potato seed is the main source of black scurf and it is therefore important to consider treatment, scientists at SAC warn. "While the overall level of infection on seed stocks is a lot less than last year, with a perceived threshold of one infected tuber in 100 to have an economic impact on the crop, treating seed is an important decision to make," said the SAC's head of crop services Stuart Wale.

Black scurf is reportedly visible on 45 per cent of potato seed stocks, which means that the other 55 per cent of stocks potentially harbour symptomless infection.

Dr Wale notes that soil temperature after planting is a critical factor in determining the resulting level of stem canker in the harvested crop. "As a rule of thumb, a soil temperature of 7°C or more will minimise the infection risk by hastening emergence,"said Dr Wale. Warning that soils can still be that cold into April he added that by then it is too late to do anything about seed-borne disease.

He said that most stocks warrant treatment. "Unlike other diseases, the rhizoctonia fungus can be effectively controlled using a seed treatment," said Dr Wale. "If seed is infected it will attack the subsequent crop."

The disease can also be soil-borne, but it is less easy to quantify. Delivering RhiNo (flutolanil) applied to the seed tuber as a liquid formulation before planting will protect growing tissue as it emerges. "The longer rhizoctonia infection can be held-off, the better," said Wale, commenting that as stems emerge they are stronger and therefore better able to fend-off infection.

Implementing rotations with five years or more between potato crops will act to reduce the incidence of soil-borne infection. "Rhizoctonia can persist in the soil for years and is known to appear where there is no history of potato production," he concluded.