WASHINGTON — The 2013 corn harvest was 4% complete as of Sept. 15 in the 18 major states, below the 2008-2012 average of 10%, the U.S. Department of Agriculture said in its Crop Progress report released yesterday.

The crop continued to make progress in its development but lagged several key five-year averages, continuing to show effects of late planting because of excessive rains in much of the Corn Belt earlier in the season.

The U.S.D.A. said 22% of the corn crop was mature as of Sept. 15, up from 9% the previous week, but well below the 41% five-year average for the date. A total of 81% of the crop in the 18 major states had dented as of Sept. 15, down from the 86% five-year average but up sharply from 64% the previous week.

The condition of the crop deteriorated slightly compared to the previous week but remained much better than it was on the same date a year ago, when the nation’s midsection was in the grips of a severe drought. The U.S.D.A. said 53% of the crop was in good-to-excellent condition as of Sept. 15, down from 54% good to excellent the previous week. A total of 18% of the crop was deemed very poor to poor, up a percentage point from 17% the previous week, the U.S.D.A. said.

The soybean crop, which, like corn, experienced late planting because of an excessively wet spring, continued to make progress in dropping leaves, although the progress lagged the five-year average significantly. Some market participants remained concerned that a possible early freeze may damage a soybean crop that was being harvested a couple of weeks later than normal.

The U.S.D.A. said 26% of the crop had dropped its leaves in the 18 major states, down from 35% as the five-year average. The percentage of the crop that had dropped its leaves as of Sept. 15 ranged widely by state, with as little as 6% dropped in North Carolina to as much as 70% dropped in Louisiana. Only 7% of the crop had dropped its leaves in one of the top-producing states, Iowa, and only 13% had dropped in Illinois, another big soybean producer, in the latest week.

The soybean crop condition deteriorated modestly in the week ended Sept. 15, the U.S.D.A. said. In the 18 major states, 50% of the crop was rated good to excellent, down from 52% the previous week, and 18% was very poor to poor, up from 16% the previous week.

Winter wheat seedings in the 18 major states as of Sept. 15 were 12% complete, equal to the 2008-2012 average, the U.S.D.A. said.

The spring wheat harvest in the six major states was in advance of the five-year average. The U.S.D.A. reported 90% of the crop was harvested, up from the 87% five-year average.