Part 112.11 of the FSMA Produce Safety Rule states that "you must take measures reasonably necessary to prevent the introduction of known or reasonably foreseeable hazards into covered produce..."
Knowing how contamination can be spread is the first step.
Humans can spread pathogens through fecal material, saliva, mucous, or other bodily fluids, usually with improper handwashing & hygiene practices, illness, or injury.
Raw manure used as a soil amendment can pose a risk to fresh produce if not properly handled or applied, such as applying too close to harvest.
Both wild and domesticated animals can spread contamination through their feces.
Water can carry and spread pathogens, contaminating large amounts of produce through irrigation, crop sprays, washing & cleaning, flooding, etc.
Equipment, tools, and other food contact surfaces that aren't regularly cleaned and sanitized can harbor pathogens and pose a huge risk to fresh produce.
Source: Produce Safety Alliance Grower Training Module 1