WASHINGTON — The 2013 corn harvest was 4 percent complete as of Sept. 15 in the 18 major states, below the 2008-2012 average of 10 percent, the US 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 USDA said 22 percent of the corn crop was mature as of Sept. 15, up from 9 percent the previous week, but well below the 41 percent five-year average for the date. A total of 81 percent of the crop in the 18 major states had dented as of Sept. 15, down from the 86 percent five-year average but up sharply from 64 percent 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 USDA said 53 percent of the crop was in good-to-excellent condition as of Sept. 15, down from 54 percent good to excellent the previous week. A total of 18 percent of the crop was deemed very poor to poor, up a percentage point from 17 percent the previous week, the USDA 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 USDA said 26 percent of the crop had dropped its leaves in the 18 major states, down from 35 percent 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 percent dropped in North Carolina to as much as 70 percent dropped in Louisiana. Only 7 percent of the crop had dropped its leaves in one of the top-producing states, Iowa, and only 13 percent 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 USDA said. In the 18 major states, 50 percent of the crop was rated good to excellent, down from 52 percent the previous week, and 18 percent was very poor to poor, up from 16 percent the previous week.