WASHINGTON — Old crop corn and soybean futures posted strong gains Monday while wheat futures were modestly higher, with all three trading mixed overnight. Prices were supported by concerns about late planting of corn, soybeans and spring wheat, by tight old crop supplies of corn and soybeans, and by the poor condition of the hard red winter wheat crop.

In its May 13 Crop Progress report, the US Department of Agriculture said corn planted in the 18 major states as of May 12 was 28 percent completed, up from 12 percent a week earlier but far behind 85 percent at the same time last year and 65 percent as the 2008-12 average for the date. Planting in top producing Iowa was only 15 percent completed compared with 79 percent as the average, and in Illinois, the second largest corn state, planting was 17 percent completed compared with 64 percent as average.

According to the Univ. of Illinois, corn planted in that state after May 10 may see an 8 percent yield loss, after May 20 a 15 percent loss and after June 1 a 25 percent loss.

Corn emerged in the 18 states was 5 percent as of May 12 compared with 52 percent a year earlier and 28 percent as the five-year average.

Soybean planting stood at 6 percent as of May 12 compared with 43 percent last year and 24 percent as the average, the USDA said. Planting in top producing Iowa was only 1 percent completed compared with 30 percent as the average for the date, and planting in No. 2 Illinois had yet to begin compared with 19 percent as the average.