WASHINGTON — The portion of the corn crop in the 18 major states rated good to excellent as of Aug. 18 deteriorated by three percentage points compared to the previous week, the U.S. Department of Agriculture said in its latest Crop Progress report.

The segment of the crop rated very poor to poor gained two percentage points in the latest week as well, the U.S.D.A. said.

A total of 61% of the crop was rated good to excellent as of Aug. 18, down from 64% the previous week, while 13% of the crop was rated very poor to poor in the latest week, up from 11% in the previous week, the U.S.D.A. said.

Both the percentages of the corn crop in the dough stage and the dent stage gained ground in the latest week but still lagged the five-year averages. The U.S.D.A. said 52% of the crop in the 18 major states had reached the dough stage as of Aug. 18, down from the five-year average for the date of 65% but up from 32% as of Aug. 11.

A total of 11% of the crop had dented as of Aug. 18, down from the 30% five-year average but up from 5% dented in the previous week, the U.S.D.A. said.

The good-to-excellent portion of the soybean crop as of Aug. 18 also deteriorated modestly in the latest week. The U.S.D.A. said 62% of the crop was rated good to excellent, down from 64% the previous week. A total of 10% of the crop was rated very poor to poor as of Aug. 18, up a percentage point from Aug. 11, the U.S.D.A. said.

A total of 72% of the soybean crop had reached the stage of setting pods as of Aug. 18, down from the 81% five-year average but up from 58% the previous week.

The market was preoccupied with the possibility that a spate of hot, dry weather predicted this week for the Western Corn Belt might have a negative effect on the soybean crop as it continues to set pods. Jerry Gidel, chief feed grain analyst for Rice Dairy in Chicago, said 35% to 40% of the soybean crop was growing on land where rainfall had been well below normal in the last month. He also noted that old-crop supplies were extremely tight and that some of the crop planted in the northern Midwest near the end of June might not make it to harvest. But he downplayed the threat of an early freeze that has been much talked about in the market, saying temperatures near harvest would have to fall to 28 degrees for enough time before cold would do real damage. Although he cited these fundamentals as bullish for soybean prices, he said evaluating the true condition of a soybean crop was extremely difficult, perhaps the most difficult of all agricultural crops to assess.

While the Crop Progress report showed that the hard red winter wheat crop is almost completely harvested (96% as of Aug. 18), the spring wheat harvest is still in its beginning stages.

The U.S.D.A. said 18% of the crop in the six major states was harvested in the latest week, well behind the 38% five-year average but up from 6% harvested as of Aug. 11. The crop’s condition stayed the same — 66% — in the good-to-excellent category as of Aug. 18. A total of 7% of the crop was rated very poor to poor in the latest week, down from 8% the prior week.

A total of 70% of the rice crop was in good-to-excellent condition as of Aug. 18 in the six major states, down from 71% the previous week, the U.S.D.A. said. The rice harvest was under way with 10% harvested in the latest week, almost on a par with the 11% five-year average for the date, the U.S.D.A. said.

The oats harvest in the nine major states was 68% harvested as of Aug. 18, down from the 79% five-year average but up from 51% harvested as of Aug. 11, the U.S.D.A. said.

The peanut crop in the 8 major states virtually completed the pegging stage in the latest week at 97%, the U.S.D.A. said. The crop’s condition, though, deteriorated, with 61% rated good to excellent as of Aug. 18, down from 65% the previous week, and a jump to 33% fair from 28% fair the previous week, while 6% was rated very poor to poor, down from 7% the previous week, the U.S.D.A. said.