* Frosts damage crops in key exporters Russia and Ukraine

* Brazil foods continue to support soybean prices

* US corn planting is 70% complete, government says

CHICAGO, May 20 (Reuters) - Increasing concerns over crop losses in the Black Sea region shoved Chicago Board of Trade wheat futures about 6% higher on Monday in the biggest one-day rally since Russia attacked Ukrainian ports and grain infrastructure last July.

Soybean and corn futures also advanced.

Traders focused on Russia and Ukraine, major grain suppliers where farms have suffered from frosts.

After trading ended, the U.S. Department of Agriculture lowered its good-excellent rating for the country's winter wheat crop by 1 percentage point from last week. Analysts were expecting a 1 point increase.

In Russia, the world's biggest wheat supplier, the price of 12.5% protein wheat scheduled for free-on-board (FOB) delivery in June climbed to $239 per metric ton last week from $221 per ton the previous week, the IKAR consultancy said.

Severe frosts across northern and eastern Ukraine could reduce grain and oilseed harvests, analyst APK-Inform warned.

"Black Sea weather continues to be bullish," said Don Roose, president of Iowa-based brokerage U.S. Commodities.

The most-active CBOT wheat contract climbed 37-1/2 cents to end at $6.88-3/4 per bushel, near last week's 10-month high of $6.97.

Corn rose 8 cents to $4.60-1/2 per bushel, while soybeans closed 20 cents stronger at $12.48 per bushel.

Concerns over crop losses in Brazil's flood-devastated Rio Grande do Sul state helped lift soy futures, traders said.

Excessive rains and heavy flooding have constrained cargo movement at Rio Grande port, the country's fourth-largest for soybean exports. Soy processor Bunge said it is gradually resuming operations in the town of Rio Grande.

"The disruption of logistics has been severe, and the repair of damaged infrastructure would take up to a year," S&P Global Market Intelligence said.

The USDA said U.S. soybean planting was 52% complete, above the five-year average of 49%, and corn planting was 70% complete, compared with the average of 71%. Both exceeded analysts' expectations, as farmers advanced plantings last week following earlier rain delays. (Reporting by Tom Polansek in Chicago. Additional reporting by Nigel Hunt in London, Naveen Thukral in Singapore Editing by Richard Chang and Matthew Lewis)