Fig. 1. This hydroponically grown lettuce is starting to show signs of powdery mildew, one of the most common fungal diseases in controlled-environment food crop production.
Photo: Christopher J. Currey

Diseases can be a less-than-exciting topic for some growers. However, when your crops start showing symptoms of a pathogen you will wish you had done something yesterday to prevent or stop the problem. The good news is that you can start taking steps today to reduce the chance of disease outbreaks in your crops. This article is going to focus on managing diseases for controlled-environment food crops.

Common pathogens

Like pests, the most common diseases in greenhouse food crop production may be familiar to you because they are some of the most familiar diseases in ornamental greenhouse crop production. The most common diseases of greenhouse crops include botrytis (or gray mold), powdery mildew, and pythium, to name a few. Botrytis and powdery mildew are foliar diseases (Fig. 1), while pythium is a root disease (Fig. 2) and, as such, can be particularly problematic in closed hydroponic systems where the nutrient solution is recirculating.

Viruses are similar to fungal or bacterial diseases in that they can cause economic damage to our crops, seriously so in some instances. However, unlike fungi and bacteria, there is no cure for a plant once it has been infected with a virus. Tomato spotted wilt, tobacco mosaic, and cucumber mosaic viruses are some of the more common viruses in food crop production. The vine or high-wire crops such as tomato and cucumber are more susceptible to viruses than leafy crops like lettuces and herbs. These diseases are also vectored by insects including thrips (tobacco mosaic and tomato spotted wilt virus) and aphids (cucumber mosaic virus).

Prevention

Regardless if it is a fungus, bacteria, or virus, preventing diseases from establishing and developing into an outbreak is key to long-term management. A pervasive culture of sanitation can reduce the likelihood of major disease outbreaks in the greenhouse. There are several steps that can be taken.

Having a footbath filled with a sanitizing solution placed at the entry to each facility so people are forced to walk through the solutions prior to entering a greenhouse can help reduce transmission of diseases. The use of coats or coveralls, gloves, and hair (including beard!) nets may also be used for further control. These measures are also positive steps for improving your food safety.

Fig. 2. The brown roots and dark root tips of lettuce are symptoms of a pythium infestation in the nutrient solution recirculating in this deep-flow hydroponic system.
Photo: Christopher J. Currey

Another essential step is keeping tools sanitized that are used for pruning and harvesting plants. The mechanical transmission of plant diseases on dirty tools is an easy way for diseases to spread rapidly across a crop. Tool sanitation is especially important for long-term crops, such as vining or high-wire crops (tomato, cucumber, pepper, etc.), due to: 1) long production cycles that require old growth to be pruned off; and 2) pruning requirements for training systems.

The degree of preventive sanitation measures is usually related to the susceptibility of a crop to diseases. For example, the degree of preventative sanitation is usually higher at a tomato production facility compared to a lettuce or herb production facility. While lettuce is susceptible to some of the same diseases as tomato (botrytis, powdery mildew, etc.), tomatoes are susceptible to a host of viruses that lettuces and leafy greens are not.

Many of the food crops grown hydroponically are grown from seed and, as such, much of the propagation is done in-house. This minimizes diseases coming into your facility from plant material that is brought in. Since most material is propagated relatively easily by seed, many producers grow their own seedlings in-house and minimize the risk of bringing in diseases with plant material. Seeds can sometimes transmit diseases, but this risk is minimized by purchasing high-quality seed from reputable suppliers and handling seed correctly once you have received it.

Control

Before we start talking about specific control measures, it helps to understand how diseases establish and thrive in greenhouse environments. One of the best ways to start thinking about diseases in the greenhouse is by thinking of the disease triangle. There are three components to the disease triangle - host, pathogens, and the environment. For diseases to develop, there must be susceptible hosts, pathogens present, and an environment conducive to pathogen growth and development.

First, there must be hosts for diseases. Unfortunately, the hosts are your crops. The good news is that just because the host is present doesn’t mean you will have disease problems. Plants that are lacking in vigor or declining in health are more susceptible to pathogens. Alternatively, healthy plants can be less susceptible to disease pressure. In addition to the health of the plant, cultivar genetics can also affect disease susceptibility. Some cultivars can be more susceptible to diseases, while other cultivars may be bred for increased disease resistances; take a close look at cultivar descriptions for more details.

Next, there must be pathogens present to infect crops. As previously described, there are a number of measures that can be taken to reduce the amount of diseases. Careful and consistent sanitation can help reduce pathogens. However, in addition to minimizing the presence and reproduction of fungal and bacterial disease inoculum, it is important to minimize vectors of plant viruses. Insect pests including aphids, thrips, and whitefly can serve as disease vectors; you should already be controlling these insect pests for the other economic damages they can cause.

Finally, the greenhouse environment needs to be conducive to pathogen development. Air temperature affects plant pathogens, but whether growth is promoted by cool or warm temperatures is specific to each disease. Humidity — specifically high humidity — is also conducive to disease development. To suppress diseases in the greenhouse, try to create conditions that are unfavorable for diseases, but still acceptable for plant growth. An excellent example of manipulating the greenhouse environment to reduce disease pressure is reducing humidity. A humid greenhouse is conducive to the development of plant diseases such as powdery mildew and botrytis, and reducing humidity can discourage the development of these diseases. One of the best strategies for reducing humidity is to open ridge vents at night while heating; the warm air from the heat will “push” the humid air out the open ridge vents. Since powdery mildew thrives in a moist environment, reducing the humidity suppresses development.

Even when we try our best to suppress diseases from starting out, chemical control may still be needed. Like chemical control options for insect and mite pests, there are both conventional and biorational pesticides available to fight diseases. Also, similar to insecticides, the number of conventional chemicals approved and labeled for use in food crop production in controlled environments is much smaller than the lists for ornamental and floriculture (i.e. non-edible) crops.

While there are several fungicides that can be used to control fungal diseases, there are no chemical controls available for any plant virus, making preventative measures paramount in preventing virus outbreaks. Strict sanitation measures and pest/vector management will help keep viruses from decimating a crop.

Take home message

Diseases can have a profound impact on the performance and profitability of hydroponic and controlled-environment food crops. The challenge of controlling diseases in food crops can be more challenging than ornamental crop production. Keeping the disease triangle (host, pathogen, and environment) in mind, you can take steps to manage diseases in your food crops before they become a problem.

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