* Fiber posts largest 1-day gain in over a month
* Technical support seen at 100-day moving average
(Reuters) - Cotton futures gained for a third straight session on Monday, as a speculator selloff slowed in the face of rising financial and grains markets and on concerns over weather-related planting delays in the United States, the top exporter, dealers said.
The most-active July cotton contract on ICE Futures U.S. gained 1.49 cents, or 1.8 percent, to settle at 85.74 cents per pound, posting its largest daily gain since mid-March.
Global financial markets rose as the formation of a new government in Italy eased concern over euro zone stability.
The Thomson Reuters-Jefferies CRB index, a benchmark for global commodities, continued to climb up from recent lows touched during a selloff earlier this month.
Grain markets were higher, with corn futures surging as forecasts of continued rains in key growing regions of the United States prompted worry over delayed plantings and reduced supplies from the world's largest producer and exporter.
"Cotton could see plantings problems, too, in any of the fields along rivers. The weather is giving people some caution," said Peter Egli, director of risk management for Plexus Cotton Ltd, a British-based medium-sized merchant.
While delays to plantings supported cotton prices on Monday, a loss of corn acreage could ultimately translate to a greater cotton acreage and pressure fiber prices, traders said.
Early U.S. government estimates have pegged plantings at a four-year low, as cotton continues to lose area to more lucrative crops.
Dealers said cotton was also buoyed by technical support. Even as prices slipped last week to the lowest since late February, the July contract has not fallen below recent uptrend lines and its 100-day moving average.
Some mills took price dips last week as an opportunity to buy, dealers said. That buying and a lack of investor selling have driven gains during the last three sessions and helped prices recover from two-month lows touched last Wednesday.
During the first quarter, cotton rallied about 18 percent as speculators boosted their bullish stance in cotton futures and options to a five-year high last month.
As noncommercial dealers have dialed back their investment, prices have slumped. The spot contract is down more than 11 percent from a 1-year high of nearly 94 cents touched in mid-March.
Open interest totaled 162,290 contracts on Friday, the lowest level since December, ICE data showed.
Prior to the recent speculator-driven rally, fiber posted two years of loss as lower-priced, synthetic alternatives eroded demand and global surpluses grew. (Reporting by Chris Prentice; Editing by Phil Berlowitz)
