The corn crop is in the worst condition since 1988 and the hot, dry weather is threatening the quality and size of the crop as it enters a critical phase, Rich Nelson, director of research at commodity advisory firm Allendale Inc., told CNBC.

On Monday, the USDA released a report that showed just 48 percent of the corn planted in 18 states is in good to excellent condition. This compares with 56 percent last week and 69 percent at this time last year. Analysts had expected a decrease to 51 percent.

In 1988, when there was a national drought, 23 percent of the crop was in good to excellent condition at this time of year.

The USDA also said 45 percent of the soybean crop is in good to excellent condition, down from 53 percent last week. However, CNBC reported the soybean crop isn't at such an important development stage.

“Corn should be seven to nine foot tall right now,” Nelson said. “All these reports of three to six foot corn is a little disappointing.”

The corn crop had been expected to be a record, as farmers planted the highest number of acres since 1937, according to CNBC. But hot, dry weather is affecting approximately two-thirds of the corn belt.

Nelson expects this year’s crop to yield 145.3 bushels per acre, which is below the 166 bushels per acre expected by the USDA. The trend is for 161 bushels per acre, according to Nelson.

Nelson expects the amount of acreage that gets replanted will be small.

Rain anytime soon would help late-planted and later pollinating corn, and give a bigger boost to soybeans which is poised to enter its key pod-setting stage of development later this summer, Reuters reported.

But both crops should stay under significant stress from the heat and dryness.

"We're still looking at a scenario providing below-average rainfall for at least the next 10 days," John Dee of Global Weather Monitoring told Reuters. A few storm systems over the weekend brought light and isolated heavy rainfall to parts of the Midwest, he said. But "a good chunk of the Midwest had no rain," he added.