2018 ASHS Annual Conference

Organic fresh produce growers often use raw animal manure to improve and maintain soil quality, fertility and thus productivity. However, untreated, i.e., raw, animal manure used as soil amendment can result in contamination of fresh produce by pathogenic microorganisms that can cause foodborne illnesses. Contamination can occur at any stage throughout fresh produce production and processing. Concerns about potential produce contamination by foodborne illness pathogens, led the National Organic Program to require a 90-day wait time between soil amendment application and harvest for produce not in direct contact with soil, e.g., staked tomatoes and pepper. For produce in direct contact with raw manure-amended soils, a 120-day wait time before harvest is stipulated in the regulation. In this study, the survival of E. coli in organic soils amended with raw animal manure were inoculated with a three-strain cocktail of non-pathogenic, rifampicin-resistant E. coli (rif-EC) at 6 log CFU/ml (1-L per 2m² plot). A randomized complete block design with 4 replications for each of 4 treatments: horse manure (HM), dairy manure (DM), poultry litter (PL) and unamended (UnA), were established at the University of Maryland Eastern Shore Research Farm. Soils were analyzed on days 0, 7, 14, 28, 60, 90, and 120 pre- and post-inoculation. Transfer of EC to tomatoes, spinach, and radish was determined at 90 and 120 days post-application of manure and EC inoculum. The EC survival was higher and more persistent in the PL-amended soils, than in any other treatment. In all tomato plots, yellow-striped army worms invaded and caused major crop damage by day 90, and were suspected of contributing to the large number of EC-positive tomato fruits when assayed. Spinach did not mature before frost set, and radishes from all treatments, except UnA, had surviving EC in the initial wash water of the bulbs. This study provides useful information for advancing microbial food safety in fresh organic produce and paves the way for future evaluations of the microbiological and physico-chemical factors in the manure-amended soils that contribute to persistence and/or die-off of the inoculated EC strains.