Menu

 

THE SCIENCE OF DISINFECTION IN HYDROPONICS – ROOT DISEASE PREVENTION

 

Excerpt from ‘Integral Hydroponics Evolution’ (coming soon)

 

Overview

 

Disinfection is used to reduce the risk of plant disease caused by microbial load carried in hydroponic nutrient solutions from environmental contamination. Technologies available for disinfection include chemicals (monochloramine, chlorine, chlorine dioxide, iodine, copper ionization, copper salts, silver ionization, ozone, hydrogen peroxide, peroxyacetic acid, quaternary ammonium disinfectants, surfactants), non-chemical or physical treatments (filtration, heat, sonication and ultraviolet radiation) and ecological alternatives (biosurfactants, and slow sand filtration).

 

The success of controlling pathogens depends on the target organisms, and the dose of the disinfectant with respect to concentration or intensity and contact time.[1]

 

All disinfection methods—chemical and non-chemical—work in a similar manner; that is, they kill exposed pathogens (and organic plant matter), with the choice of disinfection method coming down to cost and ease of use.

 

Oxidising Agents (e.g. monochloramine, chlorine and hydrogen peroxide)

 

There are both arguments for and against the use of oxidising agents for pathogen control in hydroponics. For example, what many hydroponic experts assert is that oxidising agents should only be used for treating/sterilizing the source water supply prior to its use in the hydroponic growing system. Thereafter, oxidants shouldn’t be applied to the nutrient solution that comes into contact with the roots of the plants. This advice is based on the fact that oxidants can damage/burn the roots of plants and cause phytotoxicity. However, this advice also misses the point that many hydroponic growers apply oxidising agents to the nutrient solution which is supplied to the plants. Thus oxidising agents are in contact with the roots. Based on this, I will go into detail on the pros and cons of using oxidising agents in hydroponic solutions to ensure that growers have a sound understanding of the complexities of correctly applying oxidising agents to hydroponic solutions and the pitfalls that can be encountered.

 

It is important to note that in the following material I will be speaking about the phytotoxicity thresholds of various oxidising agents (e.g. chlorine and chloramine) with various crops. One of these crops is lettuce. Just keep in mind that lettuce is more sensitive to oxidising agents than other crops such as tomato. So, for example, lettuce may be harmed by chloramine in solution at 0.18ppm while tomato may be tolerant with up to 2.0 ppm of chloramine in solution. Just keep this in mind and apply applicable research re phytotoxicity thresholds to your crop of choice.

 

Let’s begin by looking at arguments against using oxidising agents and then look at arguments for using them.

 

The science…..

 

Against

 

Oxidising agents such as hydrogen peroxide (e.g. Oxy Plus, Liquid Oxygen) and monochloramine (e.g. Pythoff) are commonly sold through hydroponic stores as sterilisation/sanitation agents for root zone pathogen control. Other than this, chlorine is also commonly used by many hydroponic growers.

 

One serious issue presents when discussing the viability of oxidising agents in hydroponics and this is that while manufacturers and suppliers make various claims about their ability to control pathogens, where the science is concerned effective doses for chemical water treatment technologies for control of plant pathogens are in many cases above known phytotoxicity thresholds. Other than this, for many crops the phytotoxicity thresholds remain unknown. That is, a wide variation of publications are found between the different water treatment technologies with most research conducted on chlorine (20 articles) or copper ionization (12), but only 0 to 7 articles found on other technologies (e.g. hydrogen peroxide and monochloramine) currently in use.[25] Therefore, any marketing claims surrounding the effectiveness and the correct use of a given oxidising agent should be viewed with some caution.

 

The science…

 

Because oxidising agents such as hydrogen peroxide, chlorine and monochloramine are oxidants there are a few important things to consider. Firstly, oxidants are highly reactive with organic matter, decreasing their effectiveness. Secondly, because oxidants attack any organic matter they can damage or even kill the roots of plants. Lastly, oxidants may be uptaken by plants. As a result, when too much of a given oxidant is present in solution this can lead to phytoxicity. For example, hydrogen peroxide is uptaken by plants. Studies show that when hydrogen peroxide is applied to substrates, endogenous levels of hydrogen peroxide increase in plant tissue. This can lead to phytotoxicity when application rates are excessive.[26] These issues present as a serious problem when discussing the use of oxidants in hydroponics due to; 1) numerous studies have shown limited efficiency of oxidants to eradicate pathogens completely in hydroponic systems and; 2) where oxidants are used at levels high enough to effectively eradicate/control high levels of pathogens the amount required results in phytotoxicity and/or root damage. For example, chlorine applied at 2.4ppm[27], chlorine dioxide at 2.5ppm[28], ozone at 0.9ppm[29] and hydrogen peroxide at 8ppm[30] have resulted in phytotoxicity in different crops. Overlay this information with the fact that studies have shown that where some pathogens are concerned (e.g. Fusarium spp.) high rates of oxidants (above phytotoxicity levels) are required to successfully eliminate pathogens. For example, 100 ppm of hydrogen peroxide with 5 minutes contact time is required to kill condia of Fusarium oxysporum. Similarly, a very high dose of chlorine (8–10 ppm) is required to obtain high mortality of Fusarium oxysporum in water. These rates are phytotoxic to plants.

 

Based on this, there are significant implications where using oxidants in solutions that are in contact with the roots of plants. I.e. on the one hand, if pathogen control is inadequate (too low application of a given oxidant product to effectively control pathogens) this places the plants under stress and yield reductions will result because the plants may be compromised by pathogens. Conversely, if too high levels of oxidants are applied, while this may eliminate high degrees of pathogens it also places the plants under stress due to causing root burning and/or phytotoxicity.

 

Other than oxidants attacking organic matter, they are also shown to react with inorganic elements. For example, Paul Fisher (2011) found that when chlorine was applied at 2.6ppm to a broad spectrum nutrient solution containing 200ppm of N at pH 6.0 chlorine levels fell to almost zero within two minutes of application. See following image.

 

chlorine-fert-reaction-site

 

Source: Fisher, P. Presentation at International Symposium on Growing Media and Substrate Analysis. Barcelona 2011

 

 

For

 

On a more positive note where oxidising agents are used correctly a high degree of control can be gained over some of the pathogens that are commonly found in hydroponic systems. For example, it has been shown that Pythium spp. can be effectively controlled at doses of 2ppm of chlorine. This level of chlorine is not phytotoxic to many plants, nor should it damage/burn the plant’s roots.

 

Further, less than 100% eradication of a pathogen may be adequate if the pathogen levels in the treated nutrient solution do not reach a biological threshold where the pathogen is able to take hold. [31]

 

Studies have shown chlorine sensitivity varies with species, pathovar and type of propagules of the same pathogens. Erwinia spp. and zoospores of Phytophthora spp. and Pythium spp. are controlled at ≤2ppm. However, control of mycelia and sporangia of P. nicotianae required up to 4 and 8 ppm, respectively. [32] E. carotovora subsp. zeae Sabet has been shown to be extremely sensitive ; chlorine at 1ppm was an adequate to insure control of bacterial stalk rot of corn (Zea maysL.). Under most conditions. E.chrysanthemi and E. carotovora subsp. Carotovora Bergey were less sensitive, and survived at 10ppm. In another study chlorine at 3ppm reduced the total counts of bacteria by 80%. Two types of spores of Thielaviopsis basicola Ferraris had different sensitivities: chlorine at 1 to 3ppm resulted in 95% mortality of endoconidia.[33] A very high dose (above phytotoxicity thresholds) of chlorine (8–10 ppm) was required to obtain high mortality of Fusarium oxysporum in water. Doses as low as 4 ppm of chlorine for 0.5 min resulted in a mortality ≥50%.[34]

 

When looking at these research findings, oxidants can be used at below phytotoxic levels in hydroponic solutions to successfully control some of the commonly occurring pathogens such as Pythium spp. and Phytophthora spp. Conversely, the ppm in solution required to control other pathogens such as Fusarium spp. would result in phytotoxicity.

 

Issues/Considerations surrounding oxidant use in hydroponics:

 

  • There is limited efficiency data surrounding the use of sterilizing agents in general[35] and what data is available re oxidant use in hydroponics suggests that in many cases they are either phytotoxic or ineffective. [36]
  • Research into the use of oxidants shows that where bacteria are able to attach to surfaces this provides a primary means for bacteria to survive disinfection. This has implications in hydroponic growing systems where nutrient tanks/reservoirs, hoses, drippers etc provide tiny pits and crevices where pathogens can escape exposure to treatment.
  • Sterilisation can create an ecological vacuum in which pathogenic microflora can take hold and thrive.[37] For example, Pythium can spread rapidly in a sterilised environment, while its growth in a non-sterilised environment is much slower. This is also the case with Phytophthora. The conclusion is that certain micro-organisms are able to suppress diseases. If these microorganisms are killed, the nutrient solution loses its’ suppressiveness and an outbreak of a disease can rapidly spread.
  • Oxidants such as monochloramine, chlorine and hydrogen peroxide react with organic and inorganic compounds, as are typically found in hydroponic systems. The oxidation of organic and inorganic compounds not only reduces the effectiveness of the disinfestation process, but potentially reduces the effectiveness of organic additives. Additionally, because oxidants attack any organic matter they can damage or even kill the roots of plants.
  • Oxidants don’t cure disease – their purpose is to prevent disease outbreaks and to minimise plant damage by reducing pathogen spread. Once a disease has taken hold in a crop other options need to be looked at. Therefore, prevention and not cure is critical where the use of oxidants is concerned.

 

 

Chlorine

 

Chlorine is an extremely reactive and unstable element that can be applied to the nutrient solution as a gas (Cl2); as a liquid, mainly as sodium hypochlorite (NaOCl) or purified hypochlorous acid (HOCl); or as a solid, most commonly calcium hypochlorite (Ca(ClO)2).

 

The mode of action of chlorine for control of microorganisms is through both oxidation and chlorination. As an oxidizer chlorine removes electrons from a reactant (such as a pathogen cell membrane) and in the process chlorine becomes reduced to chloride (Cl-). As chlorine is an oxidant it attacks organic matter, including the roots of plants. At high enough levels this results in phytotoxicity, root burning and browning off. This damage is very difficult to diagnose because it looks similar to many root rot pathogens and growers aren’t necessarily aware of what is actually causing the problem

 

Water sanitation by chlorination was evaluated in growth chamber and greenhouse studies for management of Pythium in recycled effluent used for production of bell pepper (Capsicum annum L.) in pine bark, sand, and perlite. Results of growth chamber studies supported that chlorination rates as low as 2 ppm resulted in reduced plant growth in all media. In general, phytotoxicity from chlorine varies with plant species with 2.5ppm reported to cause phytotoxicity of certain crops. Because phytotoxicity is likely above the 2 ppm of free chlorine that is required to control certain pathogens chlorination alone is not likely to be effective in broad spectrum pathogen control.[38] For example, one study found Fusarium oxysporum and Rhizoctonia solani required 8ppm with 5 minutes contact time and 10ppm with 10 minutes contact time to achieve greater than 90% mortality, respectively.[39] Rick Donnan (2004) states that to disinfect a potentially pathogen contaminated water supply it is generally accepted that 10ppm of active chlorine would be sufficient to kill most pathogens; however, this concentration will cause phytotoxicity in plants.[40] Therefore, with resistant pathogens the required water treatment concentration would far exceed the threshold for crop phytotoxicity.

 

Research has shown that chlorine demand is increased by the presence of organic and inorganic compounds in solution.[41] Organic compounds in the solution are quickly oxidised by chlorine.[42] An initial chlorine concentration of 2ppm combined with 200 mg·L-1 of a 60% Canadian sphagnum peat substrate reduced free chlorine concentration to 0 (zero) ppm after 30 minutes contact time.[43] With many pathogens it is unknown whether the oxidation of organic matter in a solution will occur more quickly than oxidation of a target plant pathogen, thereby rendering chlorination ineffective.

 

Additionally, studies into the use of chlorine show that where bacteria are able to attach to surfaces (e.g. in nutrient tanks/reservoirs and inside nutrient lines) and form biofilms* this provides a primary means for bacteria to survive disinfection. For example, research with K. pneumoniae grown in a high-nutrient medium attached to glass microscope slides demonstrated a 150-fold increase in disinfection resistance.[44]

 

One other issue also presents and that is that in municipal water treatment facilities, a dose of one part of inorganic N to every three parts of free chlorine reportedly results in 99% conversion of free Cl to chloramines after 0.2 seconds at pH 7 or 147 seconds at pH 4.[45] In hydroponics, water-soluble fertilizer containing ammonium N, nitrate N, and/or urea N is often supplied at between 100 and 200 ppm total N and then treated with free Cl concentrations between 1 and 2 mg·L−1 Cl. With such a high N to Cl ratio, at least some of this free Cl would therefore be expected to convert to chloramines.

 

Date el al (2005) estimated that the critical chloramine amount at which lettuce plant growth was significantly inhibited was 0.18 mg (0.18ppm). This study found that exposure to chloramine for even 1 h by the addition of hypochlorous acid at 0.5 mg Cl/l to the nutrient solution containing 0.67 mM ammonium nitrogen significantly inhibited plant growth. [46]

 

Phytotoxicity from chlorine varies with plant species, with 2.5 ppm reported to cause phytotoxicity of certain crops. Because phytotoxicity is likely above the 2 ppm of free chlorine that is required to control certain pathogens, chlorination alone is not likely to be 100% effective for pathogen prevention.

 

In general, it is recommended to test different doses on a small group of plants for phytotoxicity and efficacy before applying any oxidising agent to the entire crop.

 

 

*Biofilms: Biofilms are a complex matrix of polymers and assorted microorganisms which can include pathogenic and non-pathogenic microorganisms.[47] Organic compounds on the inside surface of pipes[48], and nutrients provide energy/food for microorganisms and biofilm formation in irrigation pipes. Emitters are clogged when biofilm form a physical barrier, or indirectly by the formation of precipitates with minerals such as iron, manganese and sulfur dissolved in water.[49] Biofilms are highly resistant to oxidizing agents because of their complexity and variability in structure and composition.[50]

 

 

Monochloramine (e.g. Pythoff)

 

Inorganic chloramines such as monochloramine are formed when chlorine and ammonia are combined in water.

 

Monochloramine kills bacteria by penetration of the cell wall and blockage of the metabolism. Monochloramine is considered to have lower biocidal activity against bacteria than chlorine.[51] However, chloramines are probably as effective as chlorine for the deactivation of bacteria and other microorganisms, albeit the reaction mechanism is slower.

 

Monochloramine is shown to be more effective than chlorine in controlling bacteria attached to surfaces (biofilms).[52] Chloramines are much more persistent than chlorine and take a lot longer to dissipate from treated water which means they can build up in recycling hydroponic systems and cause root damage. As with chlorine, this damage is very difficult to diagnose because it looks similar to many root rot pathogens and growers aren’t necessarily aware of what is actually causing the problem. Studies have shown that chloramines cause phytotoxicity when used at sufficient levels to successfully eradicate/control more resistant pathogens.[53]

 

Date el al (2005) estimated that the critical chloramine amount at which lettuce plant growth was significantly inhibited was 0.18 mg (0.18ppm). This research found that exposure to chloramine for even 1 h by the addition of hypochlorous acid at 0.5 mg Cl/l to the nutrient solution containing 0.67 mM ammonium N significantly inhibited plant growth. [54]

 

 

Hydrogen Peroxide (H2O2)

 

Hydrogen peroxide can be used directly as an oxidizing water treatment or as an active ingredient in the class of “activated peroxygen” products where H2O2 is combined with organic acids such as acetic acid to form more stable and effective sanitizing molecules including peroxyacetic acid.[55]

 

Hydrogen peroxide-products can vary widely in composition, which may affect phytotoxicity thresholds and effectiveness. The required dose for water treatment varies between commercial activated peroxygen products because products vary in the ratio of hydrogen peroxide to peroxyacetic and other acids.[56] Due to this, it is important to examine technical information to determine if a product is right for your intended application.

 

Studies have shown that the effective dose to control microorganisms with hydrogen peroxide based products ranged from 12.3ppm hydrogen peroxide combined with 8ppm peroxyacetic acid for control of algae (contact time not specified)[57] to 185ppm hydrogen peroxide plus 120ppm peroxyacetic acid with 1 minute contact time to control Phytophthora spp..[58] Hydrogen peroxide needs to be used in relatively large amounts of 100 ppm for 5 minutes to kill condia of Fusarium oxysporum and these rates are phytotoxic to crops. Additionally, at a concentration of 0.5% hydrogen peroxide, microbial populations are shown to be reduced to near zero immediately after treatment but returned to pre-disinfection levels 2 days after treatment.[59] Like other oxidants, the presence of organic particles in the nutrient solution or substrate decreases the effectiveness of hydrogen peroxide.[60] For example, one study showed that 10 g·L-1 of peat reduced the amount of hydrogen peroxide and peroxyacetic acid from activated peroxygens by 33% and 50%, respectively, after 4 hours contact time.[61]

 

Another issue that presents is that pH has a strong effect on hydrogen peroxide effectiveness. One study showed that the optimum biocidal activity of hydrogen peroxide occurs at pH 3.0. Other studies support this finding. [62] Given that optimum pH in hydroponics is between 5-5 – 5.8 this reduces the biocidal activity of hydrogen peroxide.

Additionally, from a chemistry perspective, H2O2 (hydrogen peroxide) degrades to ‘nascent’ oxygen (oxygen that has been released from a chemical compound) and water and hence creates an oxidative environment which not only will drastically reduce the bioavailability of manganese but of most other micronutrients as well. For example, it has been shown that H2O2 in aqueous solutions degrades chelates. As iron in hydroponics is always supplied in chelated form, H2O2 in solution in all probability reduces the availability of Fe to plants. Other than this, iron and hydrogen peroxide don’t go well together at all. When they are combined in aqueous solutions they pair and form hydroxide and hydroxyl radicals setting off various oxidising reactions leading to insoluble iron.  Ultimately, the use of H2O2 in hydroponic working solutions may cause deficiencies and plant stress. For this reason, in my own view, and I expect the view of many others, the application of H2O2 to the roots of plants through the hydroponic solution is just a flat out bad idea all round.

Phytotoxicity thresholds for activated peroxygens and peroxyacetic acids remain to be established. Symptoms associated with hydrogen peroxide toxicity include leaf scorching, reduced plant growth and plant mortality. Symptoms associated with very high concentrations of activated peroxygens included necrosis and dehydration of leaf and flowers, and spots and blotches on the leaves.[63]

 

Oxidants and Organic Elements

 

As noted, oxidants react with organic particles and, as result, their effectiveness is reduced. This means that the use of oxidising agents with organic substrates (e.g. peat and coir) is best avoided.

 

Additionally, as many hydroponic growers also use organic based additives in solution (e.g. fulvic acid, amino acids, kelp or molasses based additives) this could exacerbate problems. That is, chemical reactions between oxidants and organic molecules can alter the organic molecules, rendering them ineffective and/or possibly toxic., i.e. it is important to note that, while the chemistry is extremely complex and involves multiple factors (e.g. pH, temperature), oxidizing agents such monochloramine, chlorine and hydrogen peroxide should not be used in conjunction with organic additives. Oxidants break down (hydrolyze or biodegrade) organic matter in various ways (dependent on the organic molecule and oxidant types/molecule). For example, it has been shown that humic substances (i.e. humic and fulvic acid) form a variety of halogenated disinfectant by-products (DPBs) when they are present in aqueous solutions with chlorine. Unsaturated carbon bonds readily undergo halogenation by chlorine of which humic substances contain many. Additionally, hydrophilic acids such as citric acid and amino acids will react with chlorine to produce chloroform and other products. Several studies have shown that ammonia, ammonium ions or other amino-nitrogen atoms (i.e. amino acids), when combined with chlorine, leads to rapid, and essentially quantitative, formation of inorganic and organic N-chloramines. The reactions that take place do not produce exclusively stable products that undergo no further change. On the contrary, especially in systems where a constant ‘residual’ of active chlorine is maintained, initial products react further and organic chlorine progressively becomes converted by further chlorination.

 

Similarly, studies have shown that monochloramine hydrolyzes or biodegrades organic matter. For example, chloramination of drinking water produces trihalomethanes (if chloramine is formed by chlorination followed by ammonia addition), haloacetic acids, chloral hydrate, hydrazine, cyanogen compounds, nitrate, nitrite, organic chloramines and 1,1-dichloropropanone (1,1-DCPN).Haloacetonitriles and non-halogenated acetonitriles are produced when chloramines are reacted with humic materials (humic and fulvic acid) and amino acids. The reaction pathway is complicated and very similar to that of chlorine, with many intermediates and by-products formed.

 

Hydrogen peroxide will also break down organic matter. In fact, one of its uses (‘humic acid oxidation with peroxide’) is in wastewater treatment for exactly this purpose, where hydrogen peroxide is used to biodegrade organic matter. Hydrogen peroxide is also shown to ‘modify’ (damage/destroy/change) amino acids through a process of oxidation, involving hydroxyl radicals.[64] Where organic substrates are concerned, the use of hydrogen peroxide with sphagnum peat would largely be ineffective for sterilization (disinfection) because the H2O2 would be rendered inert relatively quickly by the catalase enzymes in peat. H2O2 would still work partially as a sterilizer/disinfectant in peat but you would have to use high amounts, which could result in phytotoxicity if things were not handled correctly.

 

You may be thinking by now that I am not a great fan of the use of oxidising agents in hydroponics. On this point (cards on the table) you would be both correct and incorrect. That is, when compared to bennies, oxidants that are commonly sold through the hydroponics retail sector present with some issues. This is particularly relevant to novice hydroponic gardeners who typically apply oxidants at a given recommended dilution rate in various situations (media type, irrigation strategy, additives used) without testing the ppm of the oxidant at the irrigation point through the use of ORP meters, or liquid test kits, or test strips. In short, the correct use of oxidants requires careful application and monitoring. Typically, this level of caution/expertise isn’t applied by novice indoor hydroponic growers – nor often do novice growers understand the technicalities in using oxidants correctly.

 

Based on this, it is somewhat questionable whether that 20 or so dollar bottle of hydrogen peroxide or monochloramine purchased through hydroponic stores, when used as per manufacturer recommendations, is not either compromising yields through failing to effectively control sufficient levels of pathogens or, damaging the roots and/or causing phytotoxicity through overuse. Therefore, particularly where novice growers are concerned, particularly those who are growing in organic substrates and/or using organic additives, I recommend the use of bennies or biosurfactants over oxidants.

 

This said, you are probably beginning to understand that things are often far more complex than some would present. On this note, there is perhaps no perfect pathogen control agent (this includes bennies, which present with their own problems, if things are not handled correctly) and pathogen control needs to be handled holistically through maintaining ideal nutrient and substrate temperatures, applying grow room hygiene best practice, creating optimal environmental conditions and either creating a biologically devoid or biologically diverse growing system, or both (i.e. pre-sterilising water prior to use in the growing system, filtering it through activated carbon to remove e.g. chlorine residues, and then adding bennies).

 

 

Conclusions and Closing Remarks re Oxidising Agents

 

Hopefully, it is clear by now that the use of oxidising agents, for controlling pathogens, isn’t as cut and dry as some would present it. The fact is that monochloramine and hydrogen peroxide products are probably not the best choice when it comes to broad spectrum pathogen control, and better options exist. For example, a biologically diverse hydroponic system may be more desirable when compared to attempts to create a biologically devoid (disinfected) growing system through the use of products such as chlorine, monochloramine or hydrogen peroxide.

 

Problems present when considering disinfecting potential is greatly reduced due to products such as chlorine, monochloramine and hydrogen peroxide reacting with both organic and inorganic elements. For example, we have seen that 2.6ppm of chlorine falls to almost zero ppm within two minutes of application when applied to what would be about a standard hydroponic feed solution. This situation would also apply to hydrogen peroxide which is even more reactive than chlorine. Therefore, when using oxidising agents it is critical to monitor the ppm delivered to the plant/substrate at the feed outlet (dripper or outlet hose) as opposed to measuring the ppm in solution in the nutrient tank/reservoir.

 

Other than this, because oxidants react in unpredictable ways with organic compounds they ideally should never be used for pathogen control in growing systems that utilize organic substrates and/or organic additives.

 

We have seen that oxidising agents are effective at eradicating high levels of some pathogens when used below phytotoxic levels; however, it is also evident that in the case of other pathogens (e.g. Fusarium spp.) the ppm required of a given oxidising agent is above phytotoxicity thresholds.

 

To conclude, it is my belief, widely supported by expert opinion, that oxidants are most effective when used at high levels to completely sterilize the source water prior to it being introduced into the hydroponic system. Maintaining oxidants below phytotoxic/harmful levels in solution thereafter has limited treatment efficiency for broad spectrum pathogen control.

 

Using Chlorine to Sterilise a Source Water Supply and/or Disinfect lines etc Between Crops

 

I myself use chlorine (at 15ppm) in my grow room to sterilise the nutrient tank/reservoir, lines, pots and drippers between crops. After flushing the lines with chlorine treated water I then chase out the chlorine with a quick flush using RO (sterile) water. Thereafter, I use bennies in solution (my preferred method of pathogen control).

 

Other than this, when I work with suspect water supplies (e.g. rain water or stream water) I use chlorine to disinfect the water (at 15ppm) and then filter it through activated carbon (to remove residual chlorine) prior to adding bennies.

 

Products such as sodium hypochlorite (liquid typically 12.5% chlorine), calcium hypochlorite (bleaching powder/pool chlorine = approx 65 – 73% Cl), and chlorine dioxide are cheap sources of chlorine.

 

I use calcium hypochlorite because it is readily available through swimming pool specialists and it is as cheap as chips to purchase.

 

See following usage rates (ppm in solution) to treat 100 litres of water.

 

10ppm/100L with calcium hypochlorite (solid) at 65% chlorine = 1.54grams/100L

15ppm/100L with calcium hypochlorite (solid) at 65% chlorine = 2.31grams/100L

 

Because so little goes so far, and because of stability issues, buy calcium hypochlorite in small volumes. I.e. calcium hypochlorite is generally unstable and it loses potency over time. To ensure accurate ppm in solution it is, therefore, advisable to buy fresh stock from a pool supplier when it is required.

 

Always read manufacturer safety instructions before use.

 

If using chlorine to clean lines etc always run sterile, dechlorinated water (e.g. RO water) through the system after the chlorine treatment to flush out residual chlorine.

 

Use this online calculator to establish chlorine ppm in solution with any given product.

 

Using the calculator is dead easy! You simply enter in how many ppm you require in a given volume (e.g. 15 (ppm) in 100L) push enter and the calculator tells you how many grams (or mg as the case may be) of calcium hypochlorite (or any other chemical) you require.

 

By the way, mixing your own chlorine saves you a lot of money when using it as a cleaning agent between crops. For example, those bottles of household bleach that you purchase typically use calcium or sodium hypochlorite at about 5% (50,000ppm) chlorine as active ingredient. Using calcium hypochlorite at 65% chemical purity, this equates to roughly 77 g/L to mix a concentrated bleach solution.

 

OTHER DISINFECTION METHODS

 

Copper and Other Metal Ion/s (e.g. silver) for Pathogen Control

 

There are a few disinfection products/additives out there that utilize metal ions (e.g. silver and/or copper) as their mode of control for root pathogens.

 

One problem with these products is that plants uptake metal ions, and excess in solution will have an impact on growth and plant morphology For example, silver impacts on plant health/growth by inhibiting ethylene responses, reducing the number of staminate flowers, reducing petal abscission, and reducing plant height. Additionally, silver ionization has been reported to be ineffective in the control of various pathogens.[65]

 

Copper is an essential element for plant growth; however it is also a heavy metal and phytotoxic at even low levels. For example, copper phytotoxicity is shown to occur in tomato (Solanum lycopersicum) via the root zone at 0.74ppm;[66] in cucumber at below 1.05ppm,[67] and in pepper at 0.19ppm.[68] The phytotoxic levels listed here are for production in a hydroponic system using inorganic growing substrates. For plants grown in organic substrates (i.e. those containing peat or coir) larger amounts of copper will be required to produce a toxic effect (the phytotoxicity threshold will be higher) as copper will react with organic compounds and be removed from solution.[69]

 

Overlay the phytotoxicity thresholds (ppm in solution) to how much copper it takes to eliminate certain pathogens and you immediately can see a problem. I.e. Fusarium oxysporum (ineffective at 3ppm), Biofilm (1-2ppm), Phytophthora cactorum zoospores (0.8-1ppm) and Pythium aphanidermatum (1ppm at 1 hour contact time).[70]

 

As silver is more toxic to pathogens than copper, it also likely requires lower levels to be phytotoxic to plants.[71]

 

In general, studies show that copper and/or silver are efficient pathogen control agents for treating the source water, however, due to phytotoxicity issues, neither should be maintained in solution (where they come into contact with the roots of the plants) at levels high enough for effective, broad spectrum, pathogen control.[72] This presents with some issues because hobby hydroponic growers typically approach pathogen control by maintaining root disease preventatives in solution at all times.

 

Ozone and UVc

 

Okay, we’re just about done with pathogen control preventatives. The last two methods (UVc and Ozone) I’ll cover briefly.

 

Both are very good methods for disinfection; however, both present with one very serious issue and that is they knock chelates and micronutrients out of solution.

 

That is….

 

Manganese and iron and micronutrient chelates can be oxidized by ozone resulting in precipitation and deficiencies of vital nutrient elements. The major impact is on chelated iron (Fe) and possibly on manganese (Mn).  Both UV and ozone disinfection break down the chelates resulting in precipitation of Fe as Fe(PO4).2H2O (strengite). It is the breakdown products of the iron chelate which in turn can react with and precipitate the manganese ions. This would be the case when using manganese sulphate; however, it is uncertain whether it also applies with chelated manganese. The degree of breakdown depends upon the type of chelate and the concentration and exposure time of the ozone, but it is usually quite high. One study showed that 60% of Fe-EDDHA was broken down with each pass of irrigation water through an ozone injector.[73]

 

As with ozone, UVc disinfection of the nutrient solution knocks iron out of solution.

 

For this reason, both technologies are less than ideal for hobby hydroponic growers who aren’t regularly lab testing their nutrient and topping up elements that have dropped out of solution.

 

Basically, while both systems are good for pretreating (disinfecting) a source water supply, both systems shouldn’t be used for treating nutrient solutions unless these solutions are lab tested and topped up with elements that have dropped out of solution due to the use of UVc or ozone.

 

Root Disease – Cure

 

In Integral Hydroponics, Edition 1, I wrote about using Fongarid (active furalaxyl) as a cure for Pythium with:

 

[Quote]

 

“Once you have Pythium, control is not an easy matter. There are off the shelf fungicides that are available in Australia, but they need to be used with caution as they are systemic. I have found that Fongarid – a systemic fungicide that contains active furalaxyl – eradicates Pythium quite successfully. However, if Pythium is able to take hold in the crop this situation may change due to the reproductive cycle of the fungi (genetic mutations and more resistant spore types). For this reason prevention is a far smarter practice than cure.”

 

[End Quote]

 

My advice was based on two things. Firstly, research by the CSIRO conducted in 1998 demonstrated the ability of furalaxyl to eradicate pythium and, secondly, through my own experiences with the product in hydroponics I had found that drenching coco coir with a 20ppm solution of furalaxyl for four hours cured Pythium and regenerated healthy root growth within days. To date I haven’t found a better product, hence, still recommending furalaxyl years later.

 

Warnings: Furalaxyl is systemic fungicide and should never be used past week 3 of flower.

 

Use of Furalaxyl will also add sodium (Na) to nutrient solution. For this reason it is recommended that furalaxyl isn’t run constantly through the growing system. I.e. high levels of sodium are undesirable in solution.

 

Metalaxyl

 

Furalaxyl is unavailable in some countries (e.g. the U.S.). An alternative to Furalaxyl for controlling Pythium that is available in countries such as the U.S. is Metalaxyl. Similar to Furalaxyl, Metalaxyl is shown to control/eradicate Pythium when used at 20ppm in hydroponic solutions.[74] For further information about Metalxyl speak to an agricultural supplier.

 

There are undoubtedly other curative products/actives on the market. For this reason, if requiring a product that cures root disease, speak to a hydroponic and/or agricultural supplier for their recommendations.

If you have learnt from this material and believe others will gain from it please pass it on (spread the knowledge) through linking to this page/site via social media and/or forums.

 

Other articles on root disease preventatives:

Surfactants in Hydroponics

Beneficial Bacteria and/or Fungi in Hydroponics

Enzymes in Hydroponics

References

[1] Stewart-Wade, S.M., 2011. Plant pathogens in recycled irrigation water in commercial plant nurseries and greenhouses: their detection and management. Irrigation science 29(4), 267-297. AND Van Os, E.A., 2010. Disease management in soiless culture systems. Acta Horticulturae 883, 385-394.

[2] MacDowall, F.D.H. 1963. Effects of nonionic surfactants on tobacco roots. Can. J.Bot. 41:1281–1287.

[3] Stowe, B.B. 1958. Growth promotion in pea epicotyl sections by fatty acid esters.Sci. 128:421–423.

[4] Méndez, J., A. Vieitez, C. Mato, and A. Vázquez. 1967. Direct and synergistic influence of tweens on Avena coleoptile section elongation. Physiol. Plant.20:437–441.

[5] Stowe, B.B. 1959. Similar activating effects of lipids on cytochromes and on plant hormones. Biochem. Biophyl. Res. Comm. 1:86–90.

[6] Stowe, B.B. 1960. Growth promotion in pea stem sections. I. Stimulation of auxin and gibberellin action by alkyl lipids. Plant Physiol. 35:262.

[7] Stowe, B.B. and J.B. Obreiter. 1962. Growth promotion in pea-stem sections. II. By neutral oils and isoprenoid vitamins. Plant Physiol. 37:158–164.

[8] Stowe, B.B. and M.A. Dotts. 1971. Probing a membrane matrix regulating hormone action. I. The molecular length of effective lipids. Plant Physiol. 48:559–565.

[9] Stanghellini, M.E., Kim, D. H., Russmussen, S.L., and Rorabaugh, P. A. (1996) Control of Root Rot of Pepper Caused by Phytophthora capsici with a nonionic surfactant

[10] Xu Q., Nakajima M., Kiu Z., Shiina T. (2011). Biosurfactants for microbubble preparation and application. Int. J. Mol. Sci. 12, 462–475 10.3390/ijms12010462

[11] Fogliano, V. Ballio, A. Gallo, M. Woo, S. Scala, F. and Lorito, M (2002) Pseudomonas Lipodepsipeptides and Fungal Cell Wall-Degrading Enzymes Act Synergistically in Biological Control. Molecular Plant-Microbe Interactions. April 2002, Volume 15, Number 4, Pages 323-333

[12] Vatsa P, Sanchez L, Clement C, Baillieul F, Dorey S. Rhamnolipid biosurfactants as new players in animal and plant defense against microbes. Int J Mol Sci. 2010;11:5095–5108

[13] Sanchez, L. Courteaux, B. Hubert, J. Kauffmann, S. Renault, JH. Clément, C. Baillieul, F and Dorey, S. (2012) Rhamnolipids Elicit Defense Responses and Induce Disease Resistance against Biotrophic, Hemibiotrophic, and Necrotrophic Pathogens That Require Different Signaling Pathways in Arabidopsis and Highlight a Central Role for Salicylic Acid

[14] Yoo, D.S.; Lee, B.S.; Kim, E.K. Characteristics of microbial biosurfactant as an antifungal agent against plant pathogenic fungus. J. Microbiol. Biotechnol. 2005, 15, 1164–1169.

[15] Sharma, A.; Jansen, R.; Nimtz, M.; Johri, B.N.; Wray, V. Rhamnolipids from the rhizosphere bacterium Pseudomonas sp. GRP(3) that reduces damping-off disease in Chilli and tomato nurseries. J. Nat. Prod. 2007, 70, 941–947.

[16] Perneel, M.; D’Hondt, L.; De Maeyer, K.; Adiobo, A.; Rabaey, K.; Hofte, M. Phenazines and biosurfactants interact in the biological control of soil-borne diseases caused by Pythium spp. Environ. Microbiol. 2008, 10, 778–788.

[17] Takemoto, J.Y.; Bensaci, M.; De Lucca, A.J.; Cleveland, T.E.; Gandhi, N.R.; Skebba, V.P. Inhibition of fungi from diseased grapeby syringomycin E-rhamnolipid mixture. Am. J. Enol. Vitic. 2010, 61, 120–124.

[18] Krzyzanowska DM, Potrykus M, Golanowska M, Polonis K, GwizdekWisniewska A, Lojkowska E, Jafra S (2012) Rhizosphere bacteria as potential biocontrol agents against soft rot caused by various Pectobacterium and Dickeya spp. strains. J plant pathol 94

[19] Sha R, Jiang L, Meng Q, Zhang G, Song Z. Producing cell-free culture broth of rhamnolipids as a cost-effective fungicide against plant pathogens. J Basic Microbiol. 2011;52:458–466.

[20] Nielsen,C.J. Ferrin D.M. & Stanghellini, M.E. (2006) Efficacy of biosurfactants in the management of Phytophthora capsici on pepper in recirculating hydroponic systems. Canadian Journal of Plant Pathology Volume 28, Issue 3 2006

[21] Nielsen, C.J., Ferrin, D.M., Stanghellini, M.E., 2006. Efficacy of biosurfactants in the management of Phytophthora capsici on pepper in recirculating hydroponic systems. Canadian Journal of Plant Pathology 28, 450-460.

[22] Paggliaccia, D., Ferrin, D., Stanghellini, M.E., 2007. Chemo-biological suppression of root infecting zoosporic pathogens in recirculating hydroponic systems. Plant and Soil 299, 163- 179.

[23] Borah, S. N. Goswami, D. Lahkar, J. Sarma, H. K. Khan, M. R. and Deka, S (2014) Rhamnolipid produced by Pseudomonas aeruginosa SS14 causes complete suppression of wilt by Fusarium oxysporum f. sp. pisi in Pisum sativum. BioControlJune 2015, Volume 60, Issue 3, pp 375-385

[24] Nielsen, C.J., Ferrin, D.M., Stanghellini, M.E., 2006. Efficacy of biosurfactants in the management of Phytophthora capsici on pepper in recirculating hydroponic systems. Canadian Journal of Plant Pathology 28, 450-460.

[25] Raudales, R. E. (2014) CHARACTERIZATION OF WATER TREATMENT TECHNOLOGIES IN IRRIGATION, University of Florida

[26] Karajeh, M. R. (2008) Interaction of Root-Knot Nematode (Meloidogyn Javanica) and Tomato as Affected by Hydrogen Peroxide

[27] Cayanan, D.F., Dixon, M., Zheng, Y., Llewellyn, J., 2009. Response of container-grown nursery plants to chlorine used to disinfest irrigation water. HortScience 44(1), 164-167.

[28] Rens, L. R., 2011. Chlorine dioxide as sanitizing agent in recirculating irrigation for greenhouse hydroponic bell peppers. University of Florida, Master Thesis.

[29] Graham, T., Zhang, P., Zheng, Y., Dixon, M. A., 2009. Phytotoxicity of aqueous ozone on five container-grown nursery species. HortScience 44(3), 774-780.

[30] Vänninen, I., Koskula, H., 1998. Effect of hydrogen peroxide on algal growth, cucumber seedlings and the reproduction of shoreflies (Scatella stagnalis) in rockwool. Crop Protection 17 (6), 547- 553.

[31] Ehret, E. Alsanius, B. Wohanka, W. Menzies, J. and Utkhede, R (2001) Disinfestation of recirculating nutrient solutions in greenhouse horticulture

[32] Raudales, R. E. Fisher, P. R., Harmon, C. L. and MacKay, B.R. (2011) Review of Efficacy Tests for Chlorination of Irrigation Systems, Proc. Fla. State Hort. Soc. 124:285–288. 2011.

[33] Hong, C.X., Richardson, P.A., Kong, P., Bush, E. A., 2003. Efficacy of chlorine on multiple species of Phytophthora in recycled nursery irrigation water. Plant Disease 87(10), 1183-1189

[34] Cayanan, D.F., P. Zhang, L. Weizhong, M. Dixon, and Y. Zheng. 2009. Efficacy of chlorine in controlling five common plant pathogens. HortScience 44:157–163. And Cayanan, D.F., M. Dixon, Y. Zheng, and J. Llewellyn. 2009. Response of container-grown nursery plants to chlorine used to disinfest irrigation water. HortScience 44:164–167.

[35] Raudales, R. E. Parke, J. L. Guy, C. L. Fisher, P. R (2014) Control of waterborne microbes in irrigation: A review

[36] R. Amooaghaie (2011) Fungal Disinfection by Nanofiltration in Tomato Soilless Culture. World Academy of Science, Engineering and Technology Vol:5 2011-08-27

[37] Ehret, E. Alsanius, B. Wohanka, W. Menzies, J. and Utkhede, R (2001) Disinfestation of recirculating nutrient solutions in greenhouse horticulture

[38] Raudales, R. E. Fisher, P. R., Harmon, C. L. and MacKay, B.R. (2011) Review of Efficacy Tests for Chlorination of Irrigation Systems, Proc. Fla. State Hort. Soc. 124:285–288. 2011.

[39] Cayanan, D. F., Zhang, P., Liu, W., Dixon, M., Zheng, Y., 2009b. Efficacy of chlorine in controlling five common plant pathogens. HortScience 44(1), 157-163.

[40] Donnan, Rick—Reader Inquiries, Practical Hydroponics & Greenhouses,  Sept/Oct 2004, Issue 78

[41] Faust, S.D., Aly, O.M., 1983.Chemistry of water treatments. 723p, Butterworth Publishers, Woburn, MA

[42] Deborde, M., von Gunten, U., 2008. Reactions of chlorine with inorganic and organic compounds during water treatment-kinetics and mechanisms: A critical review. Water Research 42(1), 13-51.

[43] Huang, J., Fisher, P.R., Decio, D.B., Horner, W. E., Meador, D.P., 2011. Quantifying the effect of peat and other environmental factors on residual activity of sodium hypochlorite. Acta Horticulturae 891, 241-248.

[44] MARK W. LECHEVALLIER, CHERYL D. CAWTHON, AND RAMON G. LEE (1987) Factors Promoting Survival of Bacteria in Chlorinated Water Supplies

[45] White, G.C.(1992) Handbook of chlorination and alternative disinfectants. 3rd Ed. Van Nostrand Reinhold Co., New York, NY.

[46] Date, S., S. Terabayashi, Y. Kobayashi, and Y. Fujime. 2005. Effects of chloramines concentration in nutrient solution and exposure time on plant growth in hydroponically cultured lettuce. Scientia Hort. 103:257–265.

[47] Maier, R.M., Pepper, I.L., Gerba, C.P., 2009. Environmental Microbiology, second ed. 598p. Elsevier Academic Press, Burlington, MA.

[48] Maier, R.M., Pepper, I.L., Gerba, C.P., 2009. Environmental Microbiology, second ed. 598p. Elsevier Academic Press, Burlington, MA.

[49] Yan, D., Bai, Z., Rowan, M., GU, L., Shumei, R., Yang, P., 2009. Biofilm structure and its influence on clogging in drip irrigation emitters distributing reclaimed wastewater. Journal of Environmental Sciences 21(6), 834-841.

[50] Tachikawa, M., Yamanaka, K., Nakamuro, K., 2009. Studies on the disinfection and removal of biofilms by ozone water using an artificial microbial biofilm system. Ozone: Science & Engineering 31(1), 3-9. AND Berry, D., Xi, C., Raskin, L., 2006. Microbial ecology of drinking water distribution systems. Current Opinion in Biotechnology. 17(3), 297-302.

[51] American Society for Microbiology (2007). Monochloramine Treatment Not As Effective In Protecting Drinking Water

[52] Raudales, R. E. (2014) CHARACTERIZATION OF WATER TREATMENT TECHNOLOGIES IN IRRIGATION, University of Florida

[53] Date, S. Hataya, T. Namiki, T. (1997) Effects of Nutrient and Environmental Pre-Treatments on the Occurrence of Root Injury of Lettuce Caused by Chloramine AND Date, S., Terabayashi, S., Kobayashi, Y., Fujime, Y., 2005. Effects of chloramines concentration in nutrient solution and exposure time on plant growth in hydroponically cultured lettuce. Scientia Horticulturae 103(3), 257-265.

[54] Date, S., S. Terabayashi, Y. Kobayashi, and Y. Fujime. 2005. Effects of chloramines concentration in nutrient solution and exposure time on plant growth in hydroponically cultured lettuce. Scientia Hort. 103:257–265.

[55] Hopkins, D. L., Thompson, C.M., Lovic, B., 2009. Management of transplant house spread of Acidovorax avenae subsp. citrulli on cucurbits with bactericidal chemicals in irrigation water. Online Plant Health Progress doi:10.1094/PHP-2009-0129-01-RS. And Van Os, E.A., 2010. Disease management in soiless culture systems. Acta Horticulturae 883, 385-394.

[56] Raudales, R. E. (2014) CHARACTERIZATION OF WATER TREATMENT TECHNOLOGIES IN IRRIGATION, University of Florida

[57] Choppakatla, V. K., 2009. Evaluation of SaniDate® 12.0 as a bactericide, fungicide and algaecide for irrigation for irrigation water treatment. BioSafe Laboratory. Final Report 09-004

[58] Steddom, K., Pruett, J., 2012. Unpublished Research Report. Efficacy of sanitizers on water samples from greenhouse and nursery operations with natural populations of Pythiacious species.

[59] Barta, Daniel J. and Henderson, Keith. (2000) Use of Hydrogen Peroxide to Disinfect Hydroponic Plant Growth Systems, Conference Paper NASA Johnson Space Center, Houston, TX, United States

[60] Newman, S.E. 2004. Disinfecting irrigation water for disease management. 20th Annual Conference on Pest Management on Ornamentals. Society of American Florists: San Jose, CA. Website: http://ghex.colostate.edu/pdf_files/DisinfectingWater.pdf

[61] Huang, J., Meador, D.P. , Fisher, P.R. Decio, D.B., Horner, W.E., 2011b. Disinfestant chemicals to control waterborne pathogens are deactivated by peat particles in Irrigation water. Proceedings Florida State Horticultural Society. 124:289–293

[62] Raffellini, S. Guerrero, S. and Alzamora, S. M (2008) Effect of Hydrogen Peroxide Concentration and pH on Inactivation Kinetics of Escherichlacoli. Journal of Food Safety 28 (2008) 514–533.

[63] Raudales, R. E. (2014) CHARACTERIZATION OF WATER TREATMENT TECHNOLOGIES IN IRRIGATION, University of Florida

[64] Finnegan M, Linley E, Denyer SP et al. Mode of action of hydrogen peroxide and other oxidizing agents: differences between liquid and gas forms. J Antimicrob Chemother 2010; 65: 2108–15; and Dean RT, Fu S, Stocker R et al. Biochemistry and pathology of radical-mediated protein oxidation. Biochem J 1997; 324: 1–18: see also Kim JR, Yoon HW, Kwon KS et al. Identification of proteins containing cysteine residues that are sensitive to oxidation by hydrogen peroxide at neutral pH. Anal Biochem 2000; 283: 214–21.

[65] Raudales, R. E. (2014) CHARACTERIZATION OF WATER TREATMENT TECHNOLOGIES IN IRRIGATION. University of Florida. PhD thesis.

[66] Zheng, Y. and Dixon, M. 2008. Things You Should Know When Using Copper in Irrigation Water Treatment -Part IV: Can You Use Copper for Disease Control in Greenhouse Tomato Production? Ontario Greenhouse Vegetable Newsletter. 2(4): 5-6.

[67] Zheng, Y., Wang, L., Cayanan, D. F., & Dixon, M. (2010). Greenhouse cucumber growth and yield response to copper application. HortScience 45 (5): 771-774.

[68] Zheng, Y., Wang, L., & Dixon, M. (2005). Greenhouse pepper growth and yield response to copper application. HortScience 40 (7): 2132-2134.

[69] Zheng, Y., Wang, L., & Dixon, M. (2005). Greenhouse pepper growth and yield response to copper application. HortScience 40 (7): 2132-2134.

[70] Zheng, Y. Dunets, S. and Cayanan, D. Copper-Silver Ionization. University of Guelph, Guelph, Ontario, Canada. Retrieved 1/7/15 http://www.ces.uoguelph.ca/water/PATHOGEN/CopperIonization.pdf

[71] Zheng, Y. Dunets, S. and Cayanan, D. Copper-Silver Ionization. University of Guelph, Guelph, Ontario, Canada. Retrieved 1/7/15 http://www.ces.uoguelph.ca/water/PATHOGEN/CopperIonization.pdf

[72] Zheng, Y. Dunets, S. and Cayanan, D. Copper-Silver Ionization. University of Guelph, Guelph, Ontario, Canada. Retrieved 1/7/15 http://www.ces.uoguelph.ca/water/PATHOGEN/CopperIonization.pdf

[73] Fisher, P. Presentation at International Symposium on Growing Media, Composting and Substrate Analysis. Barcelona 2011

[74] Minuto, A. Grasso, V. Gullino, M.L.   Garibaldi, A. (2004) CHEMICAL, NON-CHEMICAL AND BIOLOGICAL CONTROL OF PHYTOPHTHORA CRYPTOGEA ON SOILLESS-GROWN GERBERA