The U.S. Department of Agriculture report said 30 percent of the corn in the 18 states that produce most of the nation's crop is now considered in poor or very poor condition. A week ago, it was 22 percent.
Indiana and Illinois have been particularly hard hit. The USDA said 61 percent of Indiana's corn is now rated poor or very poor, compared to 50 percent last week. In Illinois, 48 percent of the corn is rated as poor or very poor, compared to 33 percent a week ago.
Nationwide, the amount of corn rated good to excellent also is dropping, to 40 percent this week from 48 percent a week ago.
Corn surged 31 cents to $7.74 per bushel in afternoon trading.
The soybean crop is stressed too and worries that it will suffer drove prices to a new record on Monday. In morning trade, prices peaked at $16.79 per bushel before settling back down to close at $16.65, up 45 cents.
The USDA reported that 27 percent of soybeans were in poor or very poor condition in the 18 states were most are grown. It was 22 percent a week earlier.
Only 40 percent of the soybean crop was in good or excellent condition, down from 45 percent a week earlier.
Corn is pollinating in many areas of the farm belt, and extreme heat during pollination can hurt formation of the ears and kernels, cutting into the amount of corn farmers will harvest.
Soybeans develop later and the most critical time for the plant is usually in early August when the bean pods begin to develop.