The US EPA registers products that are classified as antimicrobial pesticides. The terms “sanitizer” and “disinfectant” are used by US EPA in connection with antimicrobial pesticide products used for public health situations (https://www.epa.gov/pesticide-registration/pesticide-registration-manual-chapter-4-additional-considerations). In this context, sanitizers are chemicals that substantially reduce bacterial populations on surfaces but do not destroy or eliminate all bacteria. In contrast, disinfectants irreversibly inactivate bacteria, fungi, and viruses, but not necessarily bacterial spores, on surfaces. We use the term sanitizer to describe chemicals used to eliminate pathogen contamination on surfaces even though some of these may be classified as disinfectants by the EPA. The Nursery Phytophthora BMPs are primarily focused on the exclusion of Phytophthora. Some “disinfectant” products that work well against viruses or bacteria may not be very effective against resistant Phytophthora spores.
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Basics
Chemical sanitizers are used in the nursery only for sanitizing surfaces. These include inert solid surfaces such as metal or plastic benches, tables, tools, shoes, gloves, hands, and clothing. Porous surfaces, such as wood (especially if aged, cracked, or deteriorating) or plant propagules with very uneven, partially layered surfaces are difficult to sanitize with chemicals because it is difficult to ensure that the chemical comes in full contact with the target. Furthermore, some chemicals such as bleach react with and are deactivated by contact with organic materials such as wood, so it can also be difficult to ensure that the sanitizer is at the effective strength when applied to surfaces such as wood.
Sanitizers work by penetrating cells. Chemicals used for sanitizing are generally biocidal and can disrupt and kill cells of any kind (including plant cells) that they penetrate. The amount of time required for these chemicals to penetrate into cells, mycelium, or spores of target plant pathogens poses the main limitation to their efficacy. Adequate treatment of solid surface materials with chemicals requires enough contact time between the chemical and the actual target pathogen propagule (spore or mycelium). As noted below, if propagules are embedded in organic debris or soil, chemical treatments may be ineffective due to inadequate exposure of the propagules to the chemical.
Sanitizers only kill microbes on external surfaces. The sanitizing chemicals discussed in this section can only slowly penetrate intact plant surfaces that have waxy or other protective layers. This allows some of these sanitizers to be used as surface disinfesting agents. Surface disinfesting treatments can kill microbes on the surface of objects, including plant propagules. Plant propagules such as seeds and cuttings can be treated with some sanitizers to kill spores or mycelium present on the outside of these items, but these surface disinfesting treatments do not eliminate internal infections.
Most “fungicides” are not sanitizers or eradicants. Systemic chemicals sold as “fungicides” for use against Phytophthora (which are water molds, not fungi) are absorbed by plants and may temporarily inhibit infection processes in infected plants, but systemic chemicals do not eliminate existing infections. An infected plant treated with a Phytophthora-labeled fungicide remains infected.
Most significant surface contamination is removed by cleaning
Contaminated solid surfaces pose a risk of introducing Phytophthora, other water molds, and soil-borne fungal plant pathogens into nursery stock. Spores and mycelium of these pathogens may be present directly on the surfaces, or are more commonly embedded in organic debris, soil, or roots adhering to these surfaces. Washing away all of this contaminating material will effectively remove these pathogens from the surface. Hence, for the purposes of sanitation in the nursery, removing all visible surface soil, organic debris, sap, etc., is the most important standard to meet. This includes the removal of biofilms that may form on surfaces that remain wet for extended periods. Thorough cleaning can be assisted by using detergents (wetting agents), using mechanical force (brushing), or a pressure washer (in situations where splash will not contaminate clean areas). All sanitizing agents work best if the surface is clean and free of debris to start with because many of these agents do not penetrate organic debris efficiently or may be deactivated by contact with organic materials.
Sanitizers, such as alcohol in a sprayer, can be used to clean and sanitize small items, such as gloves or small tools. When sanitizing agents are being used as a cleanser, the standard is still to use the agent to remove all visible contamination. A light misting spray of alcohol or another sanitizing agent over a surface with adhering soil and debris will be ineffective for cleaning or sanitizing.
Sanitizers provide greater biosafety
Use of a chemical sanitizer on a cleaned surface can provide a greater margin of protection against contamination than washing, at least in some situations. For instance, washing alone can reduce bacterial populations but generally will not eliminate all bacterial contamination. This is why sanitizing chemicals or other treatments (heat, UV) are used to reduce bacterial contamination to levels acceptable for critical health care and food processing applications. The most common sanitizing agents (see “Chemical disinfectants” at the CDC site https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html) have been developed and used primarily for their ability to kill bacteria and viruses. Wet conditions in plant nurseries can favor diseases caused by some plant pathogenic bacteria as well as other plant pathogens. The use of chemical sanitizing agents in connection with good phytosanitary practices can reduce the risk of introducing and spreading some of these pathogens.
When a sanitizer is used as a follow up treatment after cleaning, the surface should be allowed to dry first. Otherwise, the sanitizing agent will be diluted in the water remaining on the surface and will not be applied at the correct concentration. If the cleaned surface is not dry, enough of the sanitizing solution needs to be applied to completely displace the water film on the surface.
Aqueous Chemical Sanitizers
Several types of chemical sanitizing agents for nurseries are used in aqueous (water-based) solutions. These include:
- chlorine bleach (sodium hypochlorite, NaOCl)
- quaternary ammonium compounds
- hydrogen dioxide (=hydrogen peroxide, formulated with peroxyacetic acid)
Of these, chlorine bleach is the most effective material against Phytophthora and most fungal propagules and is the material of choice for critical applications such as containers and working surfaces.
These chemicals can be used to sanitize a variety of nonporous materials, including containers, benches, and other solid surfaces. All these sanitizers are deactivated when they contact soil or organic matter, so thorough precleaning of surfaces to be sanitized is necessary. Solutions can also become less effective with use and over time. The following general procedures apply to all of these aqueous sanitizers.
- Sanitizing solutions should be freshly made or tested before use to ensure target concentrations. Test strips are available to check the concentrations of these materials.
- Use only clean water free of organic debris or rust for diluting the chemicals. Water contaminants can deactivate sanitizing chemicals.
- Before treatment, surfaces should be brushed and/or rinsed off to remove debris, soil, residual potting media, etc., to the best degree possible. Pathogen propagules embedded within adhering debris or soil may not be killed by treatment with sanitizers, leading to an ineffective treatment. As noted above, allow rinse water to dry before using the sanitizers to avoid diluting the sanitizing solutions.
- A sufficient amount of the sanitizing solution needs to be in contact with all portions of the items being treated for at least the minimum specified time. In general, treated items should be fully immersed or flooded with a film of liquid for the duration of the treatment time.
- It may be necessary to dry or rinse treated items after treatment to remove chemical residues for some uses.
Bleach (sodium hypochlorite) solutions
Concentrations of sodium hypochlorite vary in available bleach products, so the concentration in any given product should be checked, and the dilution rate adjusted as necessary before preparing solutions. The following table provides dilutions for several common bleach concentrations. The final sodium hypochlorite concentrations in these diluted bleach solutions (about 0.525%) is equivalent to 5000 ppm (or 0.5%) available chlorine.
| Percent sodium hypochlorite in bleach | Parts bleach | Parts water | Diluted bleach percent sodium hypochlorite |
|---|---|---|---|
| 5.25% | 1 | 9 | 0.525% |
| 6.0% | 1 | 10.4 | 0.526% |
| 7.55% | 1 | 13.3 | 0.528% |
| 8.25% | 1 | 14.6 | 0.529% |
| 8.3% | 1 | 14.8 | 0.525% |
For example, adding 100 ml of 5.25% bleach to 900 ml of water will make 1000 ml of 0.525% NaOCl solution. If using 8.3% bleach, 100 ml of bleach would be added to 1480 ml of water to make 1580 ml of 0.525% NaOCl.
Bleach is corrosive to steel and some other metals, so is generally not appropriate for use on metal tools. Bleach will also degrade fabric and other materials. Damage to these types of materials can be reduced by rinsing off bleach residues as soon as possible after exposure.
The sodium hypochlorite in bleach solutions breaks down quickly in contact with soil or organic debris, and is also decomposed more quickly in light, at higher temperatures (above about 24°C [75°F ]), and in the presence of various metal ions. Sodium hypochlorite (NaOCl), breaks down to salt (NaCl) and water (H2O). It is less active at pH values less than 5 or higher than 7. Because it is difficult to monitor all of these factors, diluted bleach solutions should normally be made up freshly before use and replaced frequently. Chlorine test strips can be used to check chlorine concentrations, but commercial strips vary in the range of concentrations they detect, so check the range before you purchase. The most common and inexpensive strips detect up to 200 ppm chlorine. To use these strips to detect 5000 ppm chlorine requires further dilution of the solution you wish to test. For example, to check whether a diluted bleach solution contains 5000 ppm chlorine, you would need to add 1 part of the solution to 24 parts water and check that the test strip shows 200 ppm chlorine. Test strips that cover higher ranges of chlorine concentrations are available but are more expensive.
The minimum effective contact time for bleach solutions will vary somewhat based on the characteristics of the treated surface and how the treatment is conducted. A clean, solid, inert, non-hydrophobic surface can be effectively treated by complete immersion in 0.525% NaOCl (5000 ppm chlorine) for 1 minute and then allowing the bleach solution to dry. Bleach decomposes as it reacts with various materials being treated, so concentrations may become depleted near surfaces. Agitating (mixing) the bleach solutions helps maintain uniform chlorine concentrations around surfaces throughout the treatment period. If surfaces are rough or pitted, a longer contact time (up to about 5 minutes) is needed to ensure thorough exposure. If containers are being treated, it is important that the bleach solution circulates freely to all parts of the pots and there are no air bubbles preventing contact with the solution. Extended exposure times and periodic agitation may be needed to ensure that all containers are fully exposed to the bleach solution. If diluted bleach is sprayed on benches or other surfaces, enough spray should be used to completely saturate the surface. Allow at least 5 minutes of contact time and reapply if the bleach dries faster than this.
For applications such as bleach treatments for cleaning benches or equipment used in the bench leachate test, bleach residues should be rinsed off with clean water. When rinsing, be sure that sanitized items are not recontaminated by water splashing off the ground or other contaminated surface.
The sodium hypochlorite concentration in new, sealed bleach containers can also diminish over time, so it is better to use bleach that has been stored no longer than 3 to 5 months after purchase. This decomposition also occurs faster at high temperatures and in light, so store bleach in a cool, dark location. Concentrated bleach solutions are corrosive and can release toxic chlorine gas if mixed with ammonia or acids. Concentrated bleach is also hazardous to eyes. Read and follow all safety precautions and use appropriate personal protective equipment when handling full strength and diluted bleach.
Quaternary Ammonium Compounds
Quaternary ammonium compounds ("quats", "QAs"; search for “quaternary ammonium disinfectants” to find examples) are considered low-level disinfectants by the US Centers for Disease Control and Prevention. Quats are effective against most vegetative bacteria (not dormant spores), enveloped viruses, and some fungi. Before using a product, check the label and any supplemental materials to determine if the product is suitable for your use and whether activity against Phytophthora is actually claimed for particular uses. These products vary in composition and concentration and must be used at the concentration and exposure times described on the product label. Some labels may include a range of uses that may have different exposure times and concentrations. Many quaternary ammonium product labels require relatively long contact time (commonly 10 minutes) for disinfecting hard surfaces. Label recommendations for specific uses (e.g., hard surfaces, footbaths, etc.) may not be applicable to all target organisms on the label. One of the most important parameters for nursery use is whether the materials can kill thick-walled, resistant Phytophthora spores such as oospores or chlamydospores. Unfortunately, most products do not provide specific information on this parameter.
Another issue is that getting the correct dose is based on concentration and exposure time, but the relationship between these factors and efficacy is not necessarily linear. In one study (Smith and Clements no date, experiment 5 and figure 4) some survival of P. cinnamomi was detected in soil treated with the test quat at 0.2% (2000 ppm) for up to 10 min. No survival was detected in soil treated at 1% (10,000 ppm) for as little as 30 sec of contact time, but this concentration is about 5 times the highest labeled use rate for most quats. Beyond this, it is not clear whether the rinsing protocol in this study was sufficient to remove all of the quat from the soil. If it was not completely removed by rinsing, exposure times would have been longer than stated and the presence of residual chemical in the soil could have affected the test assay for pathogen survival. The overall effect would be to overestimate the efficacy of the quat in this study (i.e., 30 sec at 10,000 ppm might not be lethal to P. cinnamomi). Furthermore, no data were provided for concentrations between 2000 and 10,000 ppm or exposure times longer than 10 minutes, so it is not really possible to determine what other time x concentration doses might have been effective.
At present, few independent tests of products have been reported by researchers that provide good information about the activity of quats against various Phytophthora propagules, especially resistant spores. Given the lack of definitive data, quats are not generally recommended as primary disinfecting agents in critical applications. Efficacy may be adequate for treating hard surfaces that have already been cleaned thoroughly. To provide the best margin for safety, use the highest labeled use rate and use an exposure time that substantially exceeds the recommended minimum time. Reapplication may be necessary if surfaces dry before the desired contact time. Concentration of solutions should be tested before reuse (e.g., using commercial test strips) and solutions should be replenished or replaced in accordance with label specifications to maintain the required concentration. Make sure that test strips match the target range of your solution. If necessary, you can dilute the sanitizing solution as needed with water in a small container to adjust the concentration for test strips that cover a lower range of concentrations than your target.
Zoospores are relatively sensitive to quats and other surfactants, so small amounts of quat residues on benches or equipment have the potential to interfere with the bench leachate test. Because it can be difficult to be sure all these residues have been rinsed off, do not use quat solutions for cleaning benches or equipment used in bench leachate testing; use diluted bleach instead.
Hydrogen dioxide products
Several related products consisting of hydrogen dioxide (=hydrogen peroxide) and peroxyacetic acid are registered for sanitizing hard surfaces and for disinfesting irrigation systems. These materials also need to be made freshly before use. Some of these formulations are also labeled for post-harvest treatment of fruits and vegetables, and may be of use in surface sterilization for vegetative materials. However, few independent tests of these products are currently available, so more research is needed to assess their efficacy when used as sanitizers to kill resistant Phytophthora spores.
Alcohol-based sanitizers
Both ethyl alcohol (ethanol) and isopropyl alcohol (isopropanol) are effective sanitizers of tools, shoes, gloves, and hard surfaces. Alcohol-water mixes are available at concentrations of 70% to 99% alcohol. As noted on the CDC Chemical Disinfectants site, 60-90% isopropyl or ethyl alcohol is optimal for killing bacteria; alcohol also has virucidal and fungicidal activity. Some studies have shown better efficacy of 70% alcohol compared with concentrations of 90% or higher. This is related to the fact that lower alcohol concentrations are more likely to penetrate cells before acting to coagulate and denature proteins; high concentrations cause immediate denaturation of surface proteins and may inhibit further absorption. High percentage alcohol solutions also evaporate more quickly, reducing contact time in spray or dip applications.
Alcohol solutions are stable, although they can evaporate if not tightly sealed. Alcohol is the primary active ingredient in some formulated aerosol products, such as Lysol® Disinfectant Spray (79 percent ethyl alcohol). Note that if aerosol products are used, the treated surface still needs to be thoroughly wetted, not simply sprayed with a fine mist. Alcohol is not corrosive and is a good choice for cleaning and sanitizing steel tools such as pruners.
Contact times required to kill bacteria with alcohol are as short as 10 to 15 seconds (CDC, 2016). Contact time required to kill resistant Phytophthora propagules is likely to be longer, but specific data on this are lacking. When using alcohol to sanitize surfaces, the cleaned surface is completely saturated with alcohol and allowed to dry, which typically requires one to several minutes.
For most nursery operations, hand sprayers filled with 70% alcohol are useful for sanitizing small items such as gloved hands, small tools, irrigation wands, soles of shoes, and similar items. As with other sanitizers, surfaces should be precleaned. Alternatively, enough alcohol needs to be used to wash off soil and debris. If treated surfaces are wet, enough alcohol needs to be applied to completely displace the surface water film. As noted above, a light mist of alcohol is insufficient for sanitizing. Treated surfaces should be wetted with a continuous layer of alcohol.
Isopropanol is generally the least expensive and most widely available form of alcohol. Ethanol is most commonly available as denatured alcohol (methylated spirits), which is a mixture of ethanol and methanol and/or other solvents added to make the mixture poisonous if consumed. The additives and ratios of ethanol to methanol vary widely between manufacturers and products. Consult the product Safety Data Sheet (SDS, formerly known as Material Safety Data Sheets or MSDS) to determine actual composition, hazards, and precautions. Bioethanol denatured with isopropanol is marketed as an indoor fuel, and this 100% alcohol product can be diluted with distilled water to make a 70% ethanol/isopropanol solution that does not include methanol or other highly toxic organic solvents.