An extra 10-14 days' growing season can be exploited where Amistar treatment at planting has been used against black dot, research suggests.

Latest results from independent BPC-funded trials show that the interval between emergence and lifting determines the level of black dot infection on tubers, with individual varieties differing in growing period length.

"What Amistar treatment does is reduce the level of initial inoculum," the BPC's Jeff Peters explains.

"It shifts the curve of disease build up in the right direction."

Dr Peters' work shows that in practice growers are buying themselves a two-week delay in disease development with Amistar.

"The conventional wisdom that the longer you leave them in the ground, the greater the risk of black dot, is still valid.

"But the date of burndown or harvest is relatively unimportant."

He calculates that Estima crops have an average 110-day growing period from emergence to lifting to maintain skin infection levels at the pre-pack contract requirement of 10%.

For Maris Piper, it is slightly longer at 120 days.

"Any further black dot development in store can be eliminated with the right management," advises Dr Peters.

"Pull down crop temperatures quickly, by 0.5C a day as soon as the store is loaded. Once down to 3C, there should be no increase in either the severity or extent of infection."

fwarable@rbi.co.uk