Image copyright Getty Images Image caption One local farmer described the sabotage as "food terrorism"

Canadian potato farmers have offered a reward of 500,000 Canadian dollars (US $400,000; £255,000) for information about an unknown saboteur.

Farmers on Prince Edward Island say that needles and nails have been pushed into the potatoes as they are grown.

Since the objects first began appearing in potatoes in October, rewards of 50,000 and 100,000 Canadian dollars have been offered with no success.

Recalls of potatoes have already cost farmers millions of dollars.

Nails have been discovered by customers in bags of potatoes sold in stores.

The Royal Canadian Mounted Police are investigating and have appealed for anyone who finds damaged potatoes to report them.

"Anyone who finds any foreign metal objects in a potato is asked to refrain from throwing out the potato, metal object or the bag it was located in when purchased," they said in a statement.

The Prince Edward Island (PEI) Potato Board, who represent farmers, have increased the reward for information leading to a conviction to 500,000 Canadian dollars until 15 August.

After that tipsters have until the 31 October to claim a 100,000 Canadian dollar reward.

The board's chair, Alex Doherty, said that recent incidents had caused the "industry to increase the profile of this reward in order to maximise the chance that those responsible will be brought to justice".

Potato farming is the island's main industry and the PEI Potato Board say it is worth over a billion dollars a year.

'Food terrorism'

The island's farmers have begun installing metal detectors and scanners in an attempt to halt the problem.

"It's food terrorism," farmer Alex Docherty told the UK's Guardian newspaper. "The people doing this are cowards, lower than a snake wearing snowshoes. These are really evil people."

The cost of the news scanners is put at five million Canadian dollars by the PEI Potato Board. The local government has announced they will contribute two million Canadian dollars to the new equipment.

Farmers say they cannot relax because of the sabotage.

"The fear is always there for other people [who] will want to do the same thing because somebody got away with it," Randall Neiuwhof told CBC.