The Pathogen Path: Water-Soil-Plant

By Chris Blanchard, Flying Rutabaga Works

Chris's Field Day 044Food safety, as we’ve learned to talk about it in the wake of troubles like the 2006 E. coli 0157:H7 outbreak from spinach, or the 2011 outbreak of Listeria monocytogenes in cantaloupes, has everything to do with preventing the bacterial pathogens that make people sick from contaminating fresh fruits, vegetables, and herbs. Because these bacteria survive and spread in fecal material, the principles of microbiological food safety come down to three critical factors: keeping poop off food, keeping any poop that gets on food from spreading, and keeping anything in the poop from growing.

Up to the point the crop is harvested, the main emphasis is keeping the poop off of the food. This means managing the application of manures and manure-based composts, as well as paying attention to the source and quality of irrigation water. Sometimes the problems are obvious, as when you notice a pile of deer pellets in your field or when the sheep get into your green beans; sometimes it’s more subtle, such as wind-blown fecal material from a feedlot or run-off from an adjacent pasture.

We’ll never eliminate the presence of pathogens in our production fields, but we can take steps to reduce their presence, and to reduce the risk of them making people sick. While the desired degree of risk-reduction is up for debate because of potential environmental and economic costs, knowing more about the dynamics of food-borne human pathogens in the environment can help individual growers optimize decision-making.

Pathogens Can Thrive in Soil

Foodborne pathogens get into the soil through the application of irrigation water, animal-based fertilizers, and intentional or inadvertent animal incursion. Once there, E. coli 0157:H7 has been shown to survive for up to 260 days in the soil; it has been found on parsley leaves up to 177 days after the application of contaminated compost. Heat and moisture can extend the survival of E. coli 0157:H7 and Salmonella; cold and dry conditions likewise reduce the lifespan of these pathogens. Studies show that at 40 degrees F, E. coli 0157:H7 persisted for a third of the time that it did at 60 degrees.

Tillage and the incorporation of manure reduce survival times for foodborne pathogens. Several studies have shown that soils on organic farms, and biologically active soils in any farming system, accelerate the decline in E. coli 0157:H7 levels compared to conventionally managed soils.

Once in the field, pathogens don’t move freely in the soil. While irrigation and fertilizer applications spread contamination evenly around the field, spot applications of manure–such as those performed by deer or errant livestock–don’t get spread around by themselves. Workers, equipment, and overland water movement can spread bacteria, so manure should be removed from the field if it’s present, and a no-harvest buffer of five feet established around any manure incident.

The Path from Soil to Plant

Pathogens do move from the soil onto and into plants. Rain and heavy irrigation droplets can cause soil containing the pathogens to splash onto the above-ground portion of the plant, and roots are clearly susceptible to colonization by these bacteria. It is unclear whether E. coli 0157:H7 and Salmonella that have been internalized in the roots can move into other parts of the plant.

While one study showed that 90% of romaine lettuce leaves inoculated with E. coli 0157:H7 had fallen below normal detection limits in just 7 days, advanced lab work was able to detect its presence up to 35 days after inoculation. Because it only takes a few live cells of E. coli 0157:H7 to make a person deathly ill, any survival presents a potential health risk.

The surface of a leaf is a very active place biologically, and field-grown romaine lettuce has been shown to have bacterial populations that keep E. coli 0157:H7 from growing; presumably this would also be true of other pathogenic bacteria. On the other hand, plant diseases extend from the outside of the plant to the inside of the plant, making it easier for human pathogens to get inside the plant. For example, if a bird does its business on a chard leaf with cercospora leaf spot, the bacteria in the dropping has a much greater chance of getting to the inside of the leaf–which is full of stuff that can help the pathogens grow–than if that dropping had landed on a disease-free plant. Healthy plants, grown in a biologically rich environment, can be an effective food safety strategy.

Human pathogens can be transmitted to plants by more incidental means as well. Flies have been shown to harbor and transmit E. coli 0157:H7, and bacteria can be transferred up to 600 feet by wind from feedlots. For both of these means of transmission, air movement and proximity matters. Wind breaks can cut down air movement, reducing the movement of both flies and dust. Pollinator habitat may also reduce the incidence of contaminated flies contacting vegetable plants.

Water Moves Pathogens

Irrigation water has been implicated in foodborne illness outbreaks, which isn’t surprising when you consider the number of cows that spend hot days standing in rivers and streams. The Proposed Produce Rule for the Food Safety Modernization Act establishes a standard for fecal bacteria in irrigation water; testing would be required to demonstrate the water’s suitability for irrigation. The Proposed Rule would require weekly testing during the growing season for systems using water from streams, rivers, and lakes; and monthly testing for water transferred into a man-made on-farm reservoir.

Developing a testing program to evaluate the food safety implications of a given irrigation source requires an understanding of the risks inherent in that body of water. For example, water from a deep well tends to have a relatively steady water quality, while the bacterial load of a shallow stream changes frequently with changes in upstream livestock location and local weather events. A testing program could help establish whether rain events result in higher fecal bacteria counts, and sampling schedules could be adjusted accordingly.

Sunlight penetration and a lack of nutrients help to reduce the bacterial load in a body of water. Turbulence can cause bacteria to settle into the mud, as well as bringing bacteria previously in the mud up to the surface. E. coli can survive for months in stream sediments, and has been shown to overwinter in streambeds when embedded in the underwater sediments.

Fortunately, following a contamination event, the majority of the pathogen load dies off in a short time. E. colicounts increase in streams closer to feeding operations, so irrigation pumps should be located at a distance wherever possible; for example, you might consider locating an irrigation pump at the downstream end of your property (assuming you don’t have a feedlot contributing to the problem).

Holding water, as in a pond, can increase concentrations of foodborne illness organisms. Ponds also invite contamination by livestock and wildlife, unless they are fenced out; geese can be a particular concern because they are known to land in sewage treatment ponds and can transfer bacteria to clean sources via their feet or feathers.

Keeping livestock out of both ponds and streams can be extremely effective in reducing the potential impact of their manure on the microbiological quality of the water. In pastures, perennial grasses can filter up to 99.995% of leached E. coli in just four inches, and continue to reduce bacterial load with additional distance. Creating buffers of perennial grasses between grazing areas and water sources can dramatically reduce the potential pathogen load of run-off.

Irrigation application methods, such as drip or furrow irrigation, that don’t contact the edible portion of the plant can also reduce foodborne illness risk; however, research has demonstrated the potential for root crops to be contaminated when grown in contaminated soil.

Most of this is good news for organic and conservation-minded farmers. Biologically active soils and leaf zones combined with tillage and cover crops reduce pathogens in the soil. Good pasture management increases filtering of manure run-off. And good stream-bank management reduces sediment flow and disruption. All of these contribute to an improved farm as well as an improved food safety environment.

Chris Blanchard provides consulting and education for farming, food, and business through Flying Rutabaga Works. He has worked in farming for the past 24 years, managing farms and operations around the country. As the owner and operator of Rock Spring Farm since 1999, Chris raised 20 acres of vegetables, herbs, and greenhouse crops, marketed through a 200-member year-round CSA, food stores, and farmers markets.

September/October 2013

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