* Swift planting, rainfall boost U.S. corn harvest outlook * Technical selling accelerates declines * Soybeans, wheat follow corn lower (Adds closing prices) CHICAGO, May 25 (Reuters) - U.S. corn futures tumbled to a one-month low on Tuesday as timely spring planting and beneficial rains across most of the Midwest boosted crop prospects. Losses accelerated as corn prices fell below recent lows and breached other technical chart support levels. Soybeans and wheat also dropped on stronger harvest prospects as well as spillover pressure from the tumbling corn market. “Corn is just dragging everything down,” said Ted Seifried, chief ag market strategist at Zaner Group. “We had put some weather premium in the market because of dryness in the West. But now that we’ve gotten rains and a better-looking forecast, we’re taking that weather premium back out.” Chicago Board of Trade July corn futures dropped 37 cents to $6.20-1/4 a bushel, the lowest since April 22. New-crop December corn was down 24-1/2 cents to $5.15-3/4 after breaching chart support at its 50-day moving average. July soybeans fell 11 cents to $15.11-3/4 a bushel, while CBOT July wheat shed 5-3/4 cents to $6.56-1/2 a bushel. In a weekly update issued after Monday’s market close, the U.S. Department of Agriculture (USDA) said farmers had planted 90% of intended corn acres as of Sunday, above the five-year average of 80% and just below an average estimate of 91% in a pre-report Reuters poll. Brisk planting, coupled with regular rain in the U.S. Midwest, has taken attention away from global supply tensions, although cool spring temperatures have raised some question marks. U.S. soybean planting remained ahead of the average pace of recent years, although it was a touch below analyst expectations, the USDA data showed. Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Sonya Hepinstall and Andrea Ricci Our Standards: The Thomson Reuters Trust Principles.