Concern that hydroponic nutrient solution could be a vector for root rot pathogens has been around since the early days of hydroponics. Recirculating systems, as much as they conserve water and nutrients, have come under the spotlight as a means for transmission of the dreaded Pythium pathogen and a whole host of others. However, treatments and techniques for preventing and controlling such scourges have flourished within the hydroponic industry and growers both small and large have a range of options when it comes to solution disinfection methods.
While slapping on the latest gadget or dosing up the reservoir may seem like a great solution, many of the disinfection methods commonly used in hydroponics are not as straight forward as we would like to think.
The nutrient solution in a recirculating system, and within the crop root zone, is a carefully balanced environment or ecosystem made up of live plant roots, organic material, diverse microbial populations, nutrients, moisture, and gasses such as oxygen required for root respiration. Within a healthy root zone, a balance usually exists where beneficial microbes outnumber and out-compete any pathogens that may find their way into the nutrient solution or substrate. This balance is the ideal situation in a hydroponic system as many beneficial microbes not only have the potential to suppress disease but may have other growth-promoting effects as well. However, from time to time, root pathogen outbreaks may occur, and prevention of such issues is generally far more effective than any type of control measure.
Nutrient solution disinfection methods include a range of options — while most are effective at killing pathogens contained in the solution itself, they are not a guarantee of keeping a system clear of infection as pathogens contained inside infected root systems are protected from any treatment effects. Commercially, large-scale hydroponic growers who use solution disinfection methods select those that are the least risk to the crop, including UV, ozone, heat and filtration, as well as biological methods. Use of chemical disinfection agents that are dose responsive are less preferred as there is a risk of toxicity if rates applied are too high or the crop is at a sensitive stage.
UV — Ultra-Violet Radiation Disinfection
UV is a plant safe and effective water and nutrient solution disinfection method and smaller units are available through the aquarium/fish industry that can be used in hydroponic systems. Anti-microbial activity largely occurs within the UVc range of 200-280nm with plant pathogens becoming inactivated when UV affects the nucleic acid which strongly absorbs at or close to 260nm. During treatment, the UV equipment passes light from the UV lamp through a thin film of nutrient solution.
For the process to work effectively the solution must be clear and often pre filtration is used to remove any organic matter or other material first. The recommended UV dose rates for disinfection of recirculating systems is 100 mJ.cm-2 for control of pathogenic fungi and 250 mJ.cm-2 for control of all pathogens including viruses. When using smaller UV units it is important to obtain the correctly sized unit for the volume of solution to be disinfected and to replace the UV bulbs once the usable life of these has passed (UV bulbs will only be effective for a certain number of hours before they need replacement). The downsides of UV treatment are that these systems do not discriminate between pathogens and non-target or beneficial microorganisms, and UV can effect iron chelates in solution, requiring higher rates of iron application and use of the most stable chelate forms such as Fe-EDDHA.
Ozone treatment of nutrient solutions requires an ozone generator onsite that discharges ozone enriched air as small bubbles into the solution flowing through a venturi. Ozone dissolves from these bubbles into the solution over a certain contact time and reacts with organic matter including pathogens. One of the major benefits of ozone treatment for water or nutrient disinfection is that any ozone which has not reacted reverts to oxygen, which in turn increases the dissolved oxygen content of the solution. This has the potential to improve crop growth. Ozone generation systems must be installed correctly as off-gassing into the air surrounding the crop can cause crop damage. Ozone destroys pathogenic microorganisms and leaves no residue in the nutrient solution that might be toxic to plants, however, ozone treatment can break down iron chelates and may cause precipitation of manganese, so these trace elements need regular monitoring.
Membrane filtration can be used to remove pathogens once courser-grade filters have first cleared the solution of larger organic particles. Hydroponic water and solution filtration methods include micro filtration (pore size 100-1000nm), ultra filtration (10-100nm), nanofiltration (1-10nm), and reverse osmosis (<1nm). The most efficient membrane filtration systems use combinations of different filters to progressively remove smaller and smaller particles as the solution flows past.
Membrane filtration has been shown to be effective for several different pathogens, however, such systems require regular maintenance to keep the system working effectively.
Slow-Sand Filtration or Biological Disinfection
Slow-sand filtration or biofiltration is a method of water/nutrient disinfection that works by passing solution slowly through different layers of aggregate where large numbers of beneficial microbial populations reside. The sand/substrate screens have organic material in the upper layers, while the microbial species in the lower layers provide biological filtration that assists with pathogen removal. This system has the advantage that the nutrient solution discharged from the filter is enriched with beneficial microbial species that can further assist with the reduction of pathogens in the hydroponic system. To be effective the flow rates of solution through the biofiltration system need to be correct and slow enough for disinfection to occur.
Read also: The Importance of Growroom Hygiene
Heat disinfection (pasteurization) of nutrient solutions is one of the most reliable methods of eliminating all types of plant pathogens and is used commercially by greenhouse growers. Most heat treatment systems first pre-filter the solution to remove organic material, then heat to a temperature of 203°F for a holding time of 30 seconds which has been found effective in the control of root rot pathogens such as Phytophthora and Pythium. While heat treatment is effective and crop safe, the drawbacks are the high energy requirement to heat solutions up to temperature and the time required to cool the solution back to ambient before reintroduction to the hydroponic system.
The two main chemical disinfection agents that may be used to treat water supplies and hydroponic nutrient solutions are hydrogen peroxide (H2O2) and chlorine. While both are effective in killing pathogens if the correct rate is applied, both have the drawback of potentially damaging crops if overused. The effectiveness of chlorine as a disinfection agent is dependent on dose, temperature, organic loading, and microbial content of the solution being treated. Also, pathogen species and the life stage of the pathogen being controlled is also a factor in the effectiveness of chlorine treatment. Studies have reported that zoospores of Pythium were controlled by exposure to 2-2.5mg/l, however, a higher level of free chlorine of 14mg/l was required to control other pathogen species — this level was also phytotoxic to many common nursery species. Chlorination toxicity symptoms include necrotic mottling, stunting, and premature leaf drop.
Read also: Understanding Pythium Root Rot
Hydrogen peroxide is an unstable oxidizing agent that reacts to form H2O and an O-radical that reacts with any type of organic material including pathogens. The by-product of using hydrogen peroxide is the release of oxygen into the nutrient solution. The overuse of H2O2 in recirculating nutrient solution carries the risk of damaging root systems. Young sensitive plants are particularly prone to damage from H2O2 dosing with rates as low as 8-12 ppm found to reduce the growth of hydroponic lettuce, while concentrations of 50ppm were required to kill Pythium and 100ppm to control Fusarium. Both chlorine and hydrogen peroxide react with organic matter in the nutrient solution, thus rates need to be carefully considered and based on the organic loading of the system to prevent the risk of plant damage from occurring.
Application of surfactants to the recirculating nutrient solution is another method used for the control of pathogens. Non-ionic surfactants have the ability to rapidly lyse mobile zoospores of pathogens such as Pythium and Phytophthora within one minute of exposure. While surfactants can destroy large numbers of mobile zoospores in the nutrient solution, the application has no residual effect on plants already infected when the disease is already contained inside plant tissue. Thus, the use of non-ionic surfactants as a nutrient solution additive is more of a preventative action than a curative one.
The use of any nutrient solution disinfection method should be made carefully as many methods not only destroy plant pathogenic microbes, but also those species that are beneficial in hydroponic systems. Disinfection treatments can have potential downsides and incorrect use of some can even be phytotoxic to plants, particularly young sensitive crops, so knowing the pros and cons of each method is an important step before implementation.
Read next: Physiological Disorders of Indoor Gardens