Fig. 1. Is this tomato grower sloppy? While plant debris should normally be removed from the greenhouse, this grower uses beneficial insects, leaving healthy old foliage on the ground to minimize biocontrol losses.
Photo: Christopher J. Currey

“An ounce of prevention is worth a pound of cure.” Pests, diseases, food safety — all are important factors in hydroponic crop production. We can combat insects using biological controls, fight diseases with chemicals, and improve the cleanliness of produce by washing it. Reactionary approaches to problems cost money, take time and, in some cases, are not always going to remedy the problem; sometimes the damage is already done. Alternatively, one of the most effective ways to reduce the risks these factors pose in our greenhouse is through prevention.

One challenge for food crops is that the greenhouse is rarely completely empty; lettuce is often growing 365 days per year to keep the crop cycles going. Having a break to create a “clean slate” for production is difficult. Therefore, regular and routine cleaning and sanitation should be used to keep our crops healthy and safe.

Plants

First, start out with clean plant material, including young plants, cuttings and seeds. While many producers may grow their own plant materials for production, it is also common to bring in seedlings or young plants. This is especially true for vine crops such as tomato, where grafted plants are produced by specialists and shipped to growers. When plants arrive, it is a good idea to quarantine them until they can be inspected and you are sure they are free of pests and pathogens. If you are propagating your own plants, be sure to start with clean material. If you are harvesting your own cuttings, they should be harvested from stock plants that, like young plants, are free of pests and disease; otherwise, purchase them from a reputable supplier. While it may seem unlikely, some viruses can be transmitted by seed, so be sure you are purchasing high-quality seed that is also disease free.

Once plants are planted into systems, keep up the sanitation. As discussed in a previous article, it is a common practice to remove leaves as part of a regular pruning and training regime for vine crops. This is not only a good practice for balancing vegetative and generative (reproductive) growth, removing older leaves can help minimize disease incidence, as these leaves can be more susceptible to infection by diseases. Taking leaves that have been removed from plants can promote good sanitation, though you may want to consider leaving healthy foliage inside the greenhouse on the floor (Fig. 1). If you have seen this before inside tomato greenhouses, it is not because the growers are lazy or sloppy. Beneficial insects may be on the foliage you are removing, so you can diminish your populations there. Diseased foliage should always be removed from the growing area.

Aside from removing leaves from plants, you will want to remove any plant debris that is in the growing area such as old flowers or leaves and leaflets that have fallen off plants. This debris can reduce the effectiveness of chemicals that may be used to sanitize surfaces. Additionally, weeds and vegetation growing around the perimeter of your growing facility should also be controlled, as these plants can harbor pests and diseases that could enter production facilities.

Fig. 2. Stack of deep-flow technique rafts ready for sanitizing before being planted and placed back into production
Photo: Christopher J. Currey

Surfaces and equipment

Keeping surfaces and equipment clean is another key aspect of any sanitation program. Surfaces in growing areas, such as walls, benches and floors, are essential to keep clean for preventing pest and disease infestations, as well as for food safety. When sanitizing any surface, be sure they are clean first. As discussed above, all plant material should be removed, and any other debris should be vacuumed or swept up. Cleaning helps make any sanitizing agent more effective. When applying any sanitizing agent, be sure to apply from top to bottom as well as toward the drain; this way, runoff is never running over surfaces that were previously sanitized. There are several chemicals that can be used to sanitize surfaces and equipment, including quaternary ammonia products, hydrogen peroxide and chlorine dioxide. The nuances of using each of these different chemicals is beyond the scope of this article, so please pay close attention to the label’s instructions on how to safely and effectively use them.

In addition to surfaces, sanitize equipment. Additionally, frequently sanitize pruning shears. For plants that are highly susceptible to viruses, sanitize pruners between every plant. Tomatoes are extremely susceptible to some diseases, like tomato spotted wilt virus. Because of this, sanitize pruners between every individual plant to stop the mechanical transmission of this virus. Employees working with pruners can carry bottles of alcohol on their belts, and they can spray between each plant. Another approach is to keep a blowtorch handy and use heat to quickly sterilize tools. Other equipment, such as rafts from deep-flow technique (Fig. 2) and troughs in nutrient-film technique systems, should be sanitized as often as is feasible and practical.

Fig. 3. This sanitation station allows employees to wash their hands and clean their footwear prior to entering the greenhouse. Note the water is controlled by foot, allowing hands to stay clean after washing.
Photo: Christopher J. Currey

People

One of the largest sources of potential contamination, whether for diseases and insects or food-borne illnesses, is the people inside facilities. There are several ways that you can promote practices that will keep your facility and the produce you harvest sanitary and safe. The first is to promote good employee hygiene. Clean clothing, shoes and hands go a long way. To promote clean hands, consider putting sanitation stations near each entrance to the greenhouse (Fig. 3). These stations will make it easy for employees to wash their hands and use sanitizing agents prior to entering areas with plant materials. Plastic gloves can be made available at greenhouse entrances, as can other items like lab coats or disposable coveralls, hair and beard nets, and disposable shoe covers or booties. An alternative to disposable shoe covers is the use of a foot bath preceding entrances; by placing these baths filled with sanitizing solutions outside each entrance, people have to walk through them in order to access plants.

In addition to good personal hygiene and sanitation, you want to look at the direction of greenhouse traffic. Crops vary in their susceptibility to pests and pathogens, and your work should take this variation into account. If people are going to be moving between crops, always have them start out working with those crops that are most susceptible to pests and diseases, having them move towards the less-susceptible crops. For example, if a grower is producing lettuce and tomatoes and people are working with both crops, you would first work with the tomatoes (the more-susceptible crop) and then go work with lettuce (the less-susceptible crop).

Take-home message

Preventing outbreaks of pests, diseases, and food-borne illnesses is much easier than trying to fix them. To prevent these problems, employing good sanitation practices on a regular basis is essential. Put together comprehensive standard operating procedures for all the tasks in your facility. The only thing more important than putting a plan together is sticking to it.

Christopher is an assistant professors of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu