A new specific off-label approval (SOLA) for the use of diquat to help control outgrade potato dumps has been obtained by the Scottish Agricultural College.

The approval would help fill the gap left by the withdrawal of approval of PDQ (paraquat + diquat) and dichlobenil in recent years, said Gary Collins for the Potato Council, which commissioned SAC to obtain the SOLA.

That had left glyphosate as the only chemical option to control potato dumps, he said. “But glyphosate can take two weeks to kill haulm growing from the dump, which means you can get blight sporulating and being an infection source.”

Dumps could also be a source of virus infection, he added.

To minimise blight and virus infection risk from dumps, growers should aim to minimise what goes into the dump initially, he said. “Over- and under-sized potatoes and those with a poor skin finish have a value, and can be sold for cattle feed or processing. Only rots or greens should be in the outgrade pile.”

Sheeting with black plastic remained the best option for preventing virus or blight spread. “There should be zero tolerance to any green leaf growing in the dump. Covering with plastic will cause any growth to die, and in the heat of the summer, potatoes will rot down.”

Using a knapsack sprayer to apply diquat at 0.4ml/sq m (equivalent to 4 litres/ha). To spray down any green material was another option, prior to sheeting or if growers didn’t want to use sheeting, he said.

“But the SOLA is only for use on permeable surfaces, not hard surfaces,” he said.

Glyphosate should be used where infection risk was low and a slower kill wouldn’t pose any problems, he said.