CHICAGO, Aug 25 (Reuters) - Chicago Board of Trade corn and soybean futures rallied on Tuesday on rising export demand from China and concerns that deteriorating crop conditions will curtail the size of the U.S. fall harvest, traders said.

Soybeans topped out at a seven-month high while corn jumped 2.6% to its highest in more than six weeks. Both commodities closed just below session peaks.

Wheat futures also posted sharp gains, hitting their highest since July 24 on bargain buying after falling on Monday.

Corn futures notched their third straight day of gains after the U.S. Agriculture Department said good-to-excellent ratings for the U.S. crop sank to 64% in the week ended Aug. 23, down 5 percentage points from the prior week.

"Ratings losses exceeded expectations yesterday afternoon and more will be likely after a dry, scorching week for the corn belt," Matt Zeller, director of market information at StoneX, said in a research note.

Chicago Board of Trade December corn futures were up 9-1/2 cents at $3.54-1/2 a bushel. The most-active contract hit its highest since July 10.

CBOT November soybeans settled 14-1/2 cents higher at $9.20-1/4 a bushel.

U.S. soybeans were rated 69% good-to-excellent, down from 72% last week.

USDA on Tuesday said Chinese importers bought more U.S. soybeans and booked their largest single-day U.S. corn purchase in nearly a month as Washington and Beijing reaffirmed their commitment to their Phase 1 trade deal.

Senior U.S. and Chinese officials, who spoke by phone on Monday, see progress on resolving problems over the deal and say both sides are committed to its success.

"It could lead to higher demand for U.S. agriculture products," said Ole Houe, of agriculture brokerage IKON Commodities. "It has a real chance of changing things."

CBOT December soft red winter wheat finished up 7-3/4 cents at $5.35-1/2 a bushel. (Additional reporting by Michael Hogan in Hamburg and Naveen Thukral in Singapore, editing by Kirsten Donovan, David Gregorio and Tom Brown)