Whether greenhouse produce is grown in soil, soilless media, or a liquid culture, the growing medium can be the source of pathogenic contamination and disease of the fruit or vegetable, as it can penetrate the roots of the plant. Some pathogens then can survive in the plant to become a potential human-disease trigger, such as certain strains of Escherichia coli (E. coli) and Salmonella, while others infect the plant itself causing root rot, wilt, or other adverse conditions.
Either way, the contaminated produce causes loss for the grower, with the “best-case” scenario being economic loss of the plants, and a much worse scenario being recall of a food product due to its potential to be the vector of food-borne disease in consumers.
While external pathogens often can be removed after harvest, there are issues with post-harvest wash of soft fruits such as strawberries, and limited options for chemical wash of organic produce. Additionally, pathogens contracted through the plant roots or otherwise internally absorbed cannot be simply “washed” away.
As such, preventing produce contamination and disease is vastly preferable to attempting to control or eliminate it after the fact.
Prevention. While soilless media and liquid cultures are most commonly used in greenhouse applications, soil can still be a disease vector, as it can be carried in on workers’ or visitors’ shoes; tools, machinery, and equipment; and crates, flats, and boxes that had been kept outdoors or even those brought in from suppliers. Once inside the greenhouse, the soil can contaminate potting mix and the produce itself if care is not taken to ensure tools, equipment — and hands — that contact the plants are cleaned and sanitized prior to contact.
One example given by Robert Hochmuth, a University of Florida, Institute of Food and Agricultural Science regional specialized agent, was that of a grower who was battling Fusarium wilt. He was doing a great job sanitizing the greenhouse between seasons, but had the disease occur early in the season. It turned out that the anchor, or stake, of the irrigation emitters was the source of the infection. When the grower put the emitter into a bag of media, it became contaminated. So, he had to sanitize the irrigation lines and anchors then, and from then on, between each season.
According to 2014 Iowa State University research on the survival of E. coli on strawberries grown under greenhouse conditions, showed that the highest survival was detected in soil that was contaminated at planting, with the E. coli surviving to later contaminate the leaves and fruit of the strawberry plants.
Disease pathogens can also infect plants from the greenhouse environment itself. For example, Hochmuth says, Fusarium wilt can linger in a greenhouse. “Even though you start out with fresh media each year, you can have Fusarium wilt because the infection can linger in other parts of the greenhouse in containers, materials and supplies,” Hochmuth says. “And once it infects the stem, it can proliferate into the root system.”
It is so persistent and proliferative, especially in tomatoes, that while sanitation, good cultural practices and/or herbicides can help, these are often not able to reduce the fungal disease to manageable levels. Instead, it often results in the grower turning to grafted transplants which would impart resistance to the crop, he says.
Other sources & solutions. Water also can be a culprit in soil, soilless, and hydroponic systems. For example, Pythium root rot is a common root-based disease which can be caused by overwatering, allowing soil to stay too wet for too long, and issues with hydroponic irrigation cycles, which may need to be modified.
Additionally, pathogens can be carried into the greenhouse in water. For example, runoff of contaminated surface water of lakes, ponds, rivers, and streams, can carry pathogens and fungi from the soil into wells and water supplies of the greenhouse irrigation systems. A Pew Charitable Trusts report also notes contaminated water as a likely cause of Cyclospora outbreaks associated with fresh fruits and vegetables.
While some contaminants are more difficult to prevent and eliminate than others, the overarching solution and/or control is sanitation. In addition to regular cleaning, sanitizing, and testing the environment, year-round control is critical. “The primary emphasis should be sanitation between seasons,” Hochmuth says, particularly for persistent pathogens, which can survive for months in plant debris. Plants that remain in the greenhouse year-round should be kept under strict control to keep them from harboring and passing along persistent pathogens.
Once the growing media has been treated to be or is purchased as free of pathogens, it should be handled as a food-grade substance — with all the safety measures given a food to keep it free of contaminants.