Potato price increases more than threefold between farm gate and shelves

In an article entitled "Grocery report's disparities only small fry",  Paul Sheehan notes: "The figure you need to know is the price for a kilo of potatoes from the farm gate: 55 cents. That is the most common bulk price sold to wholesalers. There are variations of potatoes, farms, regions and prices, but 55 cents a kilo is the most representative price at the farm gate. When I checked at my local shopping centre on Friday, the cheapest potatoes at Coles were $1.99 for a kilo of brushed potatoes. It was the same price next door at Harris Farm, the gourmet produce market. But at Aldi, in the same shopping centre, a two-kilo bag of brushed potatoes cost $1.99, exactly half the price per kilo at Coles or Harris Farm. The great question is: why does the price of the potatoes increase more than threefold between the farm gate and the shelves at Coles or Woolworths, a price double that charged by Aldi?"