* Wheat firm after Monday's losses on Black Sea supply pressure

* U.S. corn, soybean harvest slower than expected

CHICAGO, Sept 20 (Reuters) - Chicago wheat futures climbed on Tuesday after a day-earlier slide, underpinned by renewed fears about Black Sea supplies, traders said.

Soybeans and corn also firmed, supported by slower-than-expected progress in the U.S. harvest.

The most-traded wheat contract on the Chicago Board of Trade (CBOT) was up 47-1/2 cents at $8.78 a bushel by 11:09 a.m. CDT (1609 GMT), poised for its biggest daily gain since July 8.

CBOT soybeans added 22-3/4 cents to $14.84 a bushel, while corn gained 13 cents to $6.91-1/4 a bushel, after climbing to $6.93-1/4, its highest price since Sept. 13.

Concerns over Black Sea shipments flared after Russian-installed leaders in occupied areas of four Ukrainian regions set out plans for referendums on joining Russia this week, a move that Ukraine dismissed as a stunt by Russia after recent crushing losses on the battlefield.

"There's also talk that President (Vladimir) Putin may block a renewal of the Ukraine/Russia safe passage deal, set to expire in November," said Terry Reilly, senior agriculture futures analyst at Futures International, referring to Russia's leader.

Corn and soybean futures followed wheat higher, supported by degraded crop conditions and slower-than-expected harvest pace.

The U.S. corn harvest was 7% complete as of Sunday, the U.S. Department of Agriculture (USDA) said after Monday's market close, below an average estimate of 10% in a Reuters poll.

The U.S. soy harvest was 3% complete, lagging estimates of 5%. Condition ratings for both soybean and corn crops declined, the USDA said.

"We set the stage for lower yields this year," said Bill Lapp, the founder and president of Advanced Economic Solutions. "Front-end demand is good, and that is helping."

A slow Chinese import pace and a surge in export sales by Argentina were tempering U.S. soybean export sentiment. (Reporting by Christopher Walljasper; additional reporting by Gus Trompiz in Paris and Enrico Dela Cruz in Manila; editing by Paul Simao)