* Fiber slumps nearly 6 pct this week

* Dec/July spread widens to 1-month high

* Cotton falls in sympathy with grains markets - dealers

NEW YORK, May 24 (Reuters) - ICE cotton fell for a fifth straight session on Friday in its longest string of losses since September, pressured by investor selling and rain forecasts that eased supply concerns in the United States, the world's top exporter, dealers said.

The most-active July cotton contract on ICE Futures U.S. edged down 0.29 cent, or 0.4 percent, to settle at 81.49 cents per pound.

The front-month contract closed down each day this week, its longest string of losses since September.

Prices slumped nearly 6 percent since last Friday's close, the spot contract's largest weekly loss since late October. Fiber faced pressure from long liquidation, easing supply concerns in Texas - the largest producing state in the United States - and some weakness in other commodities markets, dealers said.

"With the drop in open interest, we saw significant spec liquidation and with volumes decent, it looks like it's continuing today," said Jerry Marshall of Yiyang Trading in Memphis, Tennessee.

Open interest totaled 187,036 contracts on Thursday, down by 3,426 contracts from the previous session, ICE data showed.

Forecasts of rain in drought-ridden regions of Texas prompted many to step out of their positions ahead of the three-day weekend, dealers said.

ICE Futures U.S. agricultural markets will be closed for the Memorial Day holiday on Monday, and will reopen for regular trade on May 28.

The front-month contract has plunged about 13 percent from a 1-year high of nearly 94 cents in mid-March as investors have abandoned commodities investments in favor of equities.

Cotton felt pressure from falling grains markets on Friday, with soybeans, corn and wheat all down.

Spread trading added weight as open interest moved out of the spot contract.

The December/July spread widened to 2.28 cents a lb, from 1.86 cents previously, the most in a month. The December contract moved into a premium against July earlier this week.

ICE December cotton futures closed up 0.13 cent, or 0.2 percent, at 83.77 cents per lb.

Certified stocks totaled more than 500,000 bales on Thursday, according to ICE data, some of the highest levels since June 2010. (Reporting by Chris Prentice; Editing by Chris Reese)