The US Food and Drug Administration Food Safety Modernization Act (FSMA) Produce Safety Rule (PSR) prohibits the harvest of dropped fruits and vegetables due to potential microbial contamination. Under the FSMA PSR dropped produce includes (i) produce that has detached from the parent plant and unintentionally contacts the ground and (ii) produce that is attached to the parent plant and unintentionally contacts the ground. Due to the benefits of plant growth and pest management, agricultural ground covers are a common horticultural practice implemented in the fresh produce production environment and produce may come into contact with these ground covers. Thus, this thesis aims to (i) quantify the survival of bacteria on different agricultural ground cover types and in different production environments and (ii) evaluate the cross-contamination risk of mulch to fresh produce from different drop heights and contact times. A seven-strain Salmonella cocktail was spot inoculated on coupons of biodegradable mulch, landscape fabric, and plastic mulch, and held in a growth chamber (23°C, 55% RH). At 0, 0.06, 0.17, 1, 2, 3, 5, 7, 30, 60, 90, and 140 days post-inoculation (dpi), coupons were enumerated for Salmonella. Coupons of plastic mulch were also spot inoculated with a green-fluorescent protein-tagged generic Escherichia coli and held in a growth chamber, greenhouse, and field environment for enumeration at 0, 0.06, 0.17, 0.41, 1, 2, 3, 5, and 7 dpi. Fresh cucumber, jalapeño, and tomato were dropped from 0, 1, 2, 4, and 6 ft using height-modified PVC (polyvinyl chloride) pipes onto generic E. coli inoculated plastic mulch, as well as tomato onto inoculated biodegradable mulch. Produce samples were enumerated after 3 s of mulch contact. Fresh cucumber, jalapeño, and tomato were also grown in contact with generic E. coli inoculated plastic mulch for 0, 1, 3, 5, and 7 days post-placement in the field. Salmonella survived on all ground covers for up to 140 dpi in the growth chamber. From 0 to 30 dpi, biodegradable mulch had the lowest Salmonella reduction, followed by landscape fabric and then plastic mulch (P < 0.05). No significant differences in ground cover type and Salmonella reduction were observed at 90 dpi (P > 0.05). Plastic mulch had the highest reduction of generic E. coli in the field followed by the greenhouse and growth chamber over 7 dpi (P < 0.05) with field and greenhouse coupons achieving approximately a 6-log reduction by 0.17 and 7 dpi, respectively. Ground cover type and environment impacted bacterial survival and highlighted the importance of growing location in risk management. Cucumber and tomato samples dropped from 4 (33%; 17%) and 6 (100%; 43%) ft were damaged, respectively. In general, generic E. coli transferred to the tested commodities regardless of drop height or contact time. These findings support that dropped produce should not be harvested due to potential damage and when surfaces were contaminated, transfer was likely to occur. Similarly, if surfaces were contaminated, regardless of contact time (0, 1, 3, 5, and 7 d), transfer was likely to occur indicating cross-contamination poses a food safety risk despite unintentional or intentional ground contact. Food safety efforts should focus on minimizing visible contamination, as outlined in the FSMA PSR, that may contaminate fresh produce in the production environment. Growing produce in contact with the ground alone may not be the sole factor in the contamination of fresh produce, as a contamination event is needed. / Master of Science in Life Sciences / The US Food and Drug Administration Food Safety Modernization Act (FSMA) Produce Safety Rule (PSR) is a federal regulation that sets science-based standards for the safe production, harvest, and handling of fresh fruits and vegetables. Within the harvesting requirements of the PSR, produce that drops to the ground cannot be harvested because of potential microbial contamination. Dropped produce includes (i) detached from the plant and touching the ground and (ii) attached to the plant and touching the ground. The ground can include different surfaces including agricultural ground covers, a common horticultural practice that increases plant growth and decreases pests. This thesis aims to evaluate the food safety risk of (i) bacteria on different ground covers and in different growing environments and (ii) fresh produce contacting the ground while detached (i.e., drop) and attached to the plant. Three ground cover types, biodegradable mulch, landscape fabric, and plastic mulch, were inoculated with Salmonella and held in a growth chamber with moderate conditions (23°C, 55% RH). At 0, 0.06, 0.17, 1, 2, 3, 5, 7, 30, 60, 90, and 140 days, sample coupons were evaluated for Salmonella counts. Plastic mulch was inoculated with green-fluorescent protein-tagged generic Escherichia coli (E. coli) and placed in a growth chamber, greenhouse, and field for 0, 0.06, 0.17, 0.41, 1, 2, 3, 5, and 7 days to investigate survival. At the selected time-points, generic E. coli counts were evaluated on plastic mulch. Fresh cucumber, jalapeño, and tomato were either dropped through height-modified PVC (polyvinyl chloride) pipes at 0, 1, 2, 4, and 6 ft onto generic E. coli inoculated plastic mulch and only tomato onto generic E. coli inoculated biodegradable mulch (detached produce), or grown in contact with generic E. coli inoculated plastic mulch in the field (attached produce) for 0, 1, 3, 5, and 7 days. Salmonella was present on all ground covers for up to 140 d in the growth chamber. From 0 to 30 d, biodegradable mulch had the highest concentration of Salmonella, followed by landscape fabric and then plastic mulch; however, no differences in material were observed at 90 d. Plastic mulch had the lowest concentration of generic E. coli in the field followed by the greenhouse and growth chamber over 7 d. Ground cover types (biodegradable mulch, landscape fabric, and plastic mulch) and growing environment (field, greenhouse, and growth chamber) influenced bacterial survival and should be considered in food safety management and assessment of preharvest contamination risks. Cucumber and tomato samples dropped from 4 and 6 ft were often damaged, compared to lower drop heights. Bacteria transferred to cucumber, jalapeño, and tomato regardless of drop height and contact time. Dropped produce should not be harvested due to damage and likelihood of cross-contamination. Contamination was not influenced by contact time confirming fresh produce in contact with a contaminated source is likely to become contaminated despite unintentional or intentional ground contact. Thus, food safety efforts should focus on minimizing visible contamination on surfaces that may contact fresh produce in the growing environment.

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