Let’s not be alarmists, but it is well past time for growers to recognize the potential food safety risks associated with the rapidly expanding use of biostimulants and biofertilizers in crop management.
In driving around the past several years, I easily see an increasing presence of these delivery tankers on the road and on-farm supply tanks and injection systems everywhere. While these products are and can be used safely, I have been directly involved in assessments of incidents of contamination of soil-applied and foliar applications that became contaminated with bacterial foodborne pathogens. This contaminated material resulted in destruction of the crop, occasionally involving large lots and including additional acres impacted by cross-contamination.
In addition, there have been several instances of contamination detection or outbreak traceback, which, during review, implicated the use of a biostimulant or biofertilizer on-farm or on an adjacent crop. Most of the time, the known or suspected source of contamination was a tank mix with contaminated surface water, but contamination at a supply or mixed biomaterials distribution facility has also been implicated.
Understanding Biostimulants
Focusing on biostimulants, this wording is not currently allowed on formulation labeling; however, a working USDA definition has been:
A plant biostimulant is a substance(s), microorganism(s), or mixtures thereof, that, when applied to seeds, plants, the rhizosphere, soil, or other growth media, act to support a plant’s natural nutrition processes independently of the biostimulant’s nutrient content. The plant biostimulant thereby improves nutrient availability, uptake, or use efficiency, tolerance to abiotic stress, and consequent growth, development, quality, or yield.
These formulations are produced from a variety of sources including seaweed, macroalgae, microalgae, fish, food waste protein hydrolysates and conversions, and diverse compost extracts. They may contain multiple bioactive compounds, including plant growth regulatory compounds, i.e., auxins, cytokinins, gibberellic acid, humic acids, polyamines, bioactive fatty acids, brassinosteroids, and chitosan. Many include proprietary cocktails of beneficial microbes and associated nutrient-boosts, such as molasses, whey, and distillery waste to stimulate activation and initial growth.
I am not calling out commercial formulation suppliers and distributors or organic growers specifically. I have encountered their use in organic, conventional, and other specialty crop production systems, globally. These diverse commercial and on-farm formulations have several well-documented or widely, empirically supported benefits in soil and foliar crop management applications including crop fertility (especially micronutrients), plant stress tolerance or avoidance, pest and disease management, plant growth and yield enhancement, improved fruit color development, and general soil health and remediation properties. From the academic side, there are numerous publications documenting the diversity of potential and scientifically validated benefits, but a lot of the information stimulating their use (no pun intended) is grower-to-grower anecdotal information.
Listen to the Science
I have touched on the “What” of the concern and will come to the “Why” for the concern below, but my key point here is that we do not have to guess if there are risks of growth of foodborne pathogens, especially Salmonella, in some lots or uses of these formulations. If contaminated, formulations that purposefully include a readily metabolized nutrient source, added to stimulate added or naturally occurring beneficial microbes, will support competitive and rapid growth of foodborne bacterial pathogens.
This has been demonstrated in our own studies and in on-farm assessments following a contamination detection event. Other research has shown that while some of these formulations, such as seaweed extracts, may inhibit Gram-positive bacteria such as Listeria, no such lethal effect has been observed with Gram-negative bacteria including Salmonella and E. coli O157:H7.
Compounding factors in cases resulting in large acreage crop destruction include elevated summer temperatures and the establishment of very difficult-to-eliminate contamination in sprayer tanks, lines, and around gaskets and O-rings. When applied through sprinklers or by drip line fertigation, Salmonella can establish residency in biofilms and persist long after application. This can lead to contamination of fields or orchards not planned to receive these applications. Once in this equipment, we have also published on growth potential of Salmonella in other foliar-applied pest management formulations used for row crops and tree fruit.
Finally, I need to point out one more point of concern I have encountered several times. These formulations are sometimes added into a crop management program by consultants as part of an emerging sustainability profile for the grower or, unplanned, in response to an adverse or unseasonable weather event. It seemed clear that the grower was not fully aware these materials were being rolled into the crop production plan or what the formulation consisted of, and how it was being diluted and applied.
It is in the grower’s best interest to be aware of the use of any inputs that have or can induce pathogen contamination risk and identify practical and effective preventive controls. Prohibiting the use of untreated surface water for tank mixes with formulations that contain food for pathogen growth, especially for foliar sprays, should be an easily implemented fix.
