April 2020

Although contamination of lettuce is relatively rare in the US, there have been a series of outbreaks associated with it in recent years. From 2010 to 2013 there were three outbreaks totaling more than 700 illnesses associated with just Romaine lettuce in the U.S. (Erickson, 2019). And in early April 2018, a multi-state outbreak of E. coli O157:H7 infections associated with head lettuce resulted in 210 reported illnesses, 96 hospitalizations, and 5 deaths. An analysis concluded that Romaine lettuce from Yuma, AZ was the cause of the outbreak (FDA, 2018). Three subsequent outbreaks in November and December 2019 were traced back to Romaine lettuce from Salinas Valley, CA (Kindy et al, 2019; FDA, 2020).

Guidance

The Produce Safety Rule offers comprehensive guidance for managing the production of lettuce. However, the science and understanding of effective practices continues to evolve as we learn more from outbreaks. The US Food and Drug Administration (FDA) just released a 2020 Action Plan to improve outbreak prevention and response practices, focusing on agricultural water, compost, trace back processes and data, and consideration of adjacent land uses (FDA, 2020). The Leafy Greens Marketing Agreement (LGMA) program – the world leader in produce safety practices for greens – also recently strengthened water quality standards well beyond what is contained in current federal regulations under the Produce Safety Rule. These new standards were laid out in response to 2018 and 2019 outbreaks and are currently being enforced by government agencies in both California and Arizona. The LGMA programs are continuing to further improve water standards and they are also looking at other practices concerning adjacent land use, compost and several other areas noted in the FDA 2020 Action Plan (LGMA, 2020).

Key Practices

Below are some key practices for safe head lettuce production, based on the guidance above and lessons learned from recent outbreaks.

  • Monitor and manage water quality: Test agricultural water quality, including water used for irrigation, mixing fertilizers or pesticides, and for washing produce (guidance on testing in Nevada can be found here). Ensure that water system inspection and water treatment practices are effective. Be aware of when and how water is applied to crops (overhead irrigation directly contacting lettuce is riskier than drip irrigation that does not). Guidance on die off rates for pathogens should be referenced in the event that contaminated water does come into contact with produce – these rates indicate when it will be safe to harvest.
  • Manage potential wind and water borne pathogens from adjacent land uses, including:
    1. Crop choice and placement – situate high risk crops like head lettuce so that they are protected and switch to lower risk crops if that is not possible
    2. Considering the direction of prevailing winds and installing wind breaks or hedgerows to act as a filter
    3. Evaluating and redirecting surface water runoff from areas above crop fields that could be carrying pathogens
    4. Establishing buffers between lettuce crops and livestock – from 30 to 1200 feet depending on the number of animals (Kindy et al, 2020; LGMA, 2019).
       
  • Consider adjusting harvest timing: Determine if nearby activities taking place shortly before or during the lettuce harvest could be a contamination risk, and adjust harvest timing as needed. For example, if raw manure is applied to adjacent fields at the end of the season and around the time of fall lettuce harvests, you may want to move to an earlier lettuce planting/harvest.
     
  • Monitor for freeze damage: Freeze damage to lettuce in the field is of concern during shoulder or cold season production. It can cause separation of the epidermis which weakens the leaves. Lettuce leaves will then appear discolored and translucent, peel or feather. Any openings on the leaf can attract pathogen growth, as well as causing quicker decay (Cantwell et al, 2002; LGMA, 2020). If lettuce has been freeze damaged, growers should closely consider the above practices and possibly abandon harvest.

There are also important general practices relevant to the safety of all vegetable and fruit crops. These include putting a broad food safety plan for the farm in place, including practices for worker health and hygiene, use of animal-based soil amendments, pre-harvest risk assessment (this entails following established procedures immediately before harvest, such as those laid in this LGMA decision tree), and post-harvest handling and storage.

Assistance:

The Desert Farming Initiative (DFI) provides free assistance with produce safety planning and visits farms statewide. DFI also works with the Nevada Department of Agriculture to offer ongoing Produce Safety Alliance trainings and focused workshops. Contact us with any questions.

Resources:

Cantwell, M. and T. Suslow. 2002. Lettuce, Crisphead: Recommendations for Maintaining Postharvest Quality. (Accessed November 5, 2019).

Erickson, Marilyn. October 14, 2019. Romaine Lettuce. (Accessed November 5, 2019).

Kindy, K. and Achenbach, J. 2019. Washington Post. Why romaine lettuce keeps getting recalled for E. coli contamination. (Accessed in December 2019).

Leafy Greens Marketing Agreement (LGMA). 2020. Leafy Greens Marketing Agreement Food Safety Program.  (accessed March 2020).

LGMA. 2019. Commodity Specific Food Safety Guidelines for the Production and Harvest of Lettuce and Leafy Greens. (Accessed March 2020).

US Food and Drug Administration (FDA). 2020. 2020 Action Plan to Help Advance the Safety of Leafy Greens. (Accessed March 2019).

US FDA. 2018. Environmental Assessment of Factors Potentially Contributing to the Contamination of Romaine Lettuce Implicated in a Multi-State Outbreak of E. coli O157:H7. (Accessed November 5, 2019).

Moe, J. 2020, Head Lettuce and Produce Safety, Desert Farming Initiative, University of Nevada, Reno, Research Report

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Associated Programs

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Food Safety Program

The Initiative's Food Safety Program provides services and resources for growers throughout the state of Nevada. The Initiative partners with the Nevada Department of Agriculture to demonstrate produce safety practices, share guidance and provide training.