There is considerable concern about the use of microbes as biological weapons against the human population of the U.S. and other countries. However, microbes also pose a serious threat to crops because of the vulnerability of agricultural systems to attack, the economic importance of agriculture, and the fact that it is known that some countries are developing biological weapons targeting crops. A formal risk assessment is needed to evaluate the magnitude of the threat and develop plans for preventing, deterring, detecting, and responding to attack. Disease spread through pathogen dispersal will be a key component of any risk assessement model.