Biologically based attacks on agriculture, or food produced from it, is called agro-terrorism. The effects could range from slight to apocalyptic, depending on the severity, scope, and number of attacks. Clearly, multiple attacks on multiple types of targets would compound the damage, but even a select few attacks could create big problems by sowing panic and destroying the public's confidence in food and its suppliers.
If a pathogen were introduced against a targeted crop, the resulting disease or damage might necessitate the destruction of huge amounts of food destined for humans or animals. If animals were infected or ate infected food, they might have to be slaughtered and their bodies safely disposed of. The economic effect on agribusiness, from giant corporations down to family farms, would be severe. Another problematic effect and related target is the water supply, as farm runoff would poison the water. Additionally, as U.S. farmers sell much of their product overseas, international commerce would suffer, and people overseas could also be sickened by tainted food.
The direct interference with crops already harvested is also possible, in storage facilities before processing. If the affected foods and beverages were consumed, resulting epidemic diseases could also overwhelm hospitals and, worst case scenario, the morgues.