16 August 2001
Blackleg roguing? Not worth it

By Andrew Swallow

ROGUING seed crops of potatoes is time-consuming, costly and can do more harm than good, according to a Scottish Agricultural Colleges disease specialist.

The advice from Stuart Whale was given at a Scottish Crop Research Institute/SAC Potatoes in Practice event held near Dundee recently.

“Why are we roguing and why do we have tolerances for blackleg in the crop if it bears no relation to seed loading?” He asked.

The current practice is for seed growers to walk through the crop removing plants infected with blackleg. But within weeks infection could re-emerge.

Mr Whale added that walking through a crop when canopies had closed could spread more disease than the roguing removed.

The Department of Rural Affairs fertiliser guide book, RB209, was also questioned at the event, which was sponsored by the British Potato Council.

The book recommends 130kg/ha of phosphate is applied across all crop types at soil index 3, higher than SACs recommendation for maincrop.

This itself could be too high, according to SAC potato specialist Barry Mitchell, who suggested growers could be applying more than was necessary.

Those growers with soil phosphate indices of 3 or over (high on the SAC scale) could cut down to a 50kg/ha insurance dressing, or even possibly none at all.

“We are not seeing a response in [BPC-funded] trials to phosphate applications,” he said.

“At face value, they suggest that if we withhold the phosphate there would be no detrimental effect on tuber number or yield.”