* Hot, dry weather seen trimming soybean yields
* Market awaits Pro Farmer tour results
* Wheat eases on Ukrainian export efforts
CHICAGO, Aug 21 (Reuters) - Chicago soybeans rose for a fourth straight session on Monday, reaching a three-week high as hot, dry U.S. conditions fueled concerns over crop stress.
Wheat fell as Ukraine sought new Black Sea avenues for export, adding pressure to corn futures. Markets were awaiting results from the annual Pro Farmer crop tour, which will examine corn and soybean fields across the Midwest this week.
The most active soybean contract on the Chicago Board of Trade (CBOT) ended 8-1/2 cents higher at $13.61-3/4 a bushel, having earlier touched its highest price since July 28.
CBOT corn lost 10-1/2 cents to $4.82-1/2 a bushel, while CBOT wheat fell 13-1/2 cents to $6.25-1/2 a bushel.
Extreme heat is expected across large swathes of the U.S. Midwest this week, with temperatures above 100 degrees Fahrenheit (37.8 degrees Celsius) across the U.S. Plains.
Sweltering temperatures, combined with a lack of rain, could damage soybean crops during a crucial development window.
"People are saying the rain we had in the first week of August made the crop," said Mark Gold, managing partner at Top Third Ag Marketing. "I think this heat will do an awful lot of damage."
Weekly U.S. Department of Agriculture crop ratings, published after the market close rated the U.S. soybean crop 59% good-to-excellent as of Aug. 20, unchanged from the week prior and one percentage point below analyst estimates.
Corn conditions fell by one percentage point to 58% good-to-excellent.
"It's not wanting to rain for maybe three weeks. If that forecast holds, you're going to back this bean crop off," said John Zanker, market analyst at Risk Management Commodities.
Soybeans also found support after exporters sold 159,350 metric tons of U.S. soybeans to unknown destinations earlier on Monday, according to the USDA.
Exporters also sold 111,770 metric tons of corn to Mexico, the USDA said.
The latest military incidents in the Russia-Ukraine war sparked short-covering on Friday as the threat of further disruption to Black Sea grain trade hung over the market, though a new plan to insure Ukrainian export vessels travelling through a newly-tested Black Sea export corridor added pressure.
"Just because they get insurance doesn't mean Russia won't shoot at them. We'll have to see how that pans out," Gold said. (Reporting by Christopher Walljasper; additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Paul Simao and Cynthia Osterman)
