GRAINS-Corn slides to near 2-month low on U.S. weather

* Corn, soy dive as summer growing conditions to improve

* Active U.S. wheat harvest approaches, weighs on prices (Recasts, updates prices, market activity; changes dateline, previously Paris/Singapore)

CHICAGO, June 22 - Corn tumbled 3.4 percent on Monday, and U.S. grain and soybean futures extended pre-weekend losses as crop prospects improved, with greenhouse-like conditions blanketing the U.S. Midwest.

"The heat that is forecast for the next two weeks is deemed beneficial for the crop," said grains analyst Dan Cekander of Newedge USA in Chicago.

The Midwest, the heart of the world's food belt, is expected to heat up into the 80s to 90s degrees Fahrenheit this week. The first hot spell of the season follows heavy weekend rains, creating the perfect climate to boost young corn and soybean seedlings.

Chicago Board of Trade corn futures tumbled to their lowest point in nearly two months on the crop outlook while soybeans slipped to their lowest level since late May.

CBOT corn for July delivery was down 13-3/4 cents, or 3.4 percent at $3.85-1/2 a bushel past midday.

July soybeans were 28-1/2 cents weaker, or 2.4 percent at $11.50-1/2 a bushel.

Traders also eyed the price spread between old-crop July soybeans and new-crop November soy, which remained volatile -- stretching July's premium over November to about $1.80 given the tight domestic supply of soy before narrowing to $1.67 as traders took profits on trading the spread.

WHEAT SLIPS AS ACTIVE U.S. HARVEST APPROACHES

U.S. wheat prices were pressured by expectations for the winter wheat harvest to gain momentum this week.

After a wet weekend across the U.S. southern Plains the next week to 10 days should be mostly clear -- giving wheat farmers a big window to run combines across Kansas, the top U.S. wheat state.

Chicago July soft red winter wheat was 12-3/4 cents a bushel lower, or 2.3 percent at $5.42-1/2. Kansas City July hard red winter wheat was 9-1/4 cents lower, or 1.5 percent at $6.05-1/2.

Additional selling in the grains stemmed from the dollar strengthening and weaker oil and equities as investors were cautious ahead of economic data and a Federal Reserve meeting this week.

A firming dollar is typically a sell signal for commodities as it increases the cost of U.S. commodities for overseas buyers.

"It's a factor of the strength in the dollar and the tone set by the crude oil market, which is coming under a bit of pressure," said Gavin Maguire, analyst with EHedger, a Chicago brokerage.

"Crude trading $2 (a barrel) lower is a real blow to the soybean bulls. We're now seeing some capitulation from weak-handed longs."

The weakness in the grains as well as the crude oil was reflected in the Reuters-Jefferies index of 19 commodity futures, down 1.8 percent by midday.