March white sugar was down $3.70, or 1 percent, at$385.20 a tonne by 1100 GMT.
Dealers said technical signals turned bearish after prices were unable to hold above the 10-day moving average and slipped below the key $390 support level.
Fundamentally, focus was on Thailand, where harvesting is currently underway and production is expected to increase.
"Mostly fine weather is forecast for Thailand, conducive to the start of harvesting," analyst Green Pool said in a weekly note. "Let's see what happens when the Thai mills have been in operation for several months, concentrating on higher paying white sugar."
Thai refined sugar supplies, in combination with sharply higher EU beet production, are expected to guarantee ample global white sugar stocks in the current 2017/18 season.
March raw sugar fell 0.08 cent, or 0.5 percent, to 14.90 cents per lb.
Dealers said the technical structure was damaged after prices failed to follow through on gains made early last week, leading to a disappointing close on Friday.
COCOA
March New York cocoa was down $13, or 0.6 percent, at $2,028 a tonne.
Prices were pressured by a stronger dollar, which snapped a three-day losing streak after the US Senate approved a tax overhaul over the weekend.
This extended gains from Friday when prices slumped to the weakest in nearly two months under the pressure of crop-friendly weather and strong arrival figures in top grower Ivory Coast.
March London cocoa was down 6 pounds, or 0.4 percent, at 1,483 pounds a tonne, hovering above a 2-1/2 month low touched in the prior session.
COFFEE
March arabica coffee slipped 0.70 cent, or 0.5 percent, to $1.2885 per lb.
January robusta coffee fell $10 or 0.6 percent, to$1,743 a tonne.
Indonesia exported 13,347.6 tonnes of coffee beans in November from Lampung province, the main coffee growing area, a 57 percent drop from the same month last year, data showed on Monday.
Coffee production from the world's four biggest growers could fall by more than 1 million bags in 2017-18, as higher output in Vietnam, Colombia and Indonesia only partially makes up for a smaller harvest in Brazil, US attache estimates show.