Risk assessment of Salmonella preharvest internalization in relation to irrigation water quality standards for melons and other cucurbits.

The production of melons, including cantaloupe, honeydew, and watermelon, and other cucurbits (specialty melons, cucumber, and squash) requires ample quantities of irrigation water of appropriate microbiological quality to ensure this essential input does not contribute to food safety risk to consumers. A great deal of uncertainty surrounds the setting of rationale and practical standards for growers to follow to meet these expectations. In open enVironments, it is unreasonable to expect that no pathogens of concern will ever be in surface water used for irrigation at some low level. Internalization of pathogens from soil and transfer to edible portions of fruits and vegetables has become a concern in recent years. The primary purpose of this research is to determine, by greenhouse and open field testing, the threshold level of Salmonella that would be required to represent a risk of fruit contamination by uptake of pathogen-contaminated irrigation water through the root system and subsequent transfer through the vine. We anticipate this threshold will be 10,000's times higher than levels of Salmonella reported in irrigation source water for domestic production of these crops. Food safety standards for melons and cucurbits will not need to remain preoccupied with the risk of internalization from roots.

Routine testing of irrigation water sources, conveyance networks, and delivery systems prior to and during crop production of fresh produce has become an increasing expectation for industry based Good Agricultural Practices implementation and food safety management audit compliance standards. One consequence of the scientific uncertainty surrounding the specific contribution of indicator testing for water quality microbiological limits on measureable gains in consumer protection is the concern for pathogen uptake through the root system and transport to the edible portions, including fruit. The recognized commodity:pathogen association of cantaloupe: Salmonella has raised a particular industry need for risk assessment research to address this specific aspect of preharvest water safety. This proposal seeks to extend and expand preliminary research on defining a threshold for root internalization, systemic vine transport, endophytic survival, and transference of Salmonella to melon fruit in greenhouse and field evaluations. A collection of diverse Salmonella serovars will be further tested at various doses and melon development stages and on multiple varieties for these demonstrated phenomena of, apparently transient, vine internal transport. In addition, we propose to extend this evaluation to determine a practical threshold for field‐grown cantaloupes, other melons, and cucurbits that will be useful in better establishing commodity –based prevention and audit standards and responsible irrigation water testing expectations. We further seek to establish a broader and richer data set upon which to justify future collaborative research on the genetic determinants of the internalization and transport interactions towards a host‐resistance contribution to redundant safety measures for unavoidable irrigation water and soil contamination in global regions of cucurbit production.

1. Conduct expanded assays of internalization with multiple Salmonella serovars to determine whether the observed absence of uptake is isolate or serovar specific. 

2. Verify preliminary data of systemic internalization of pathogenic bacteria into melon vines by repeating select greenhouse studies with larger populations of plants. 

3. Determine whether an internalization threshold can be observed for Salmonella into cantaloupe and honeydew fruit under greenhouse conditions. 

4. Determine the potential for root uptake of Salmonella to vines and fruit under worst case field conditions. 

5. Expand the delineation of irrigation water contamination thresholds for Salmonella to a broader group of melons and other cucurbits.

Uptake of enteric bacterial pathogens via roots, from mildly contaminated field soil or non-foliar contact irrigation should not be a preoccupying concern for the industry or public health regulators relative to other sources of risk to public health. While several highly intriguing research questions result from the investigation outcomes regarding the variable capacity of Salmonella to survive as an endophyte in melon vine tissue and the role for melon vine constituents to limit persistence of internalized cells, this does not seem a priority area for future funding by CPS or the melon industry.