Question: "Assuming due care is taken to thoroughly wash fruits and vegetables prior to eating them, do plants retain any chemical residue from fertilizers or herbicides, thus making them dangerous to eat?"
Answer per Dr. Ruth MacDonald (Iowa State University's professor and chair of the Department of Food Science and Human Nutrition):
No. The use of chemicals on edible plants is regulated by the United States Department of Agriculture (USDA) and producers must follow guidelines regarding how much and when they are applied. These chemicals have been tested by the Food and Drug Administration (FDA) to determine the safe limits allowed in foods and the food supply is regularly tested to evaluate the residual amounts that are being consumed. Trace amounts of chemical residues are likely to be present in foods, some of which will be from agricultural use, but others are from a variety of sources including air or water pollution. Washing of fruits and vegetables prior to eating will eliminate surface contamination. You can review the amount and types of chemical found in foods at the FDA website.