Version 9/14/2020
This section discusses selected chemicals and treatments that are used to kill propagules of Phytophthora and other plant pathogens that are present in or on materials used nursery production. It is important to understand the appropriate uses and limitations of each treatment. Chemicals and heated materials can pose safety hazards to workers. Personnel should identify and observe all necessary safety precautions to protect themselves and others from injury. All chemicals should be handled, stored, and disposed of in accordance with applicable local, state, and federal regulations.
Several types of chemical sanitizing agents 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)
These materials can be used to sanitize a variety of materials, including containers, benches, and other surfaces. All of these materials are deactivated when they contact soil or organic matter. Therefore solutions become ineffective with use and over time. The following general procedures apply to all of these materials.
- 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 or rinsed to remove debris, soil, old potting media, etc., to the degree possible. Note that if items are rinsed with water first, they should be allowed to dry to the point that the sanitizing solution is not diluted excessively.
- 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.
Table 1. Dilutions of commonly available bleach products needed to obtain approximately 0.525% sodium hypochlorite concentrations (5000 ppm 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% |
| 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.
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 75°F = 24°C), in the presence of various metal ions. It is also 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 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. Concentrated bleach solutions are corrosive and can release toxic chlorine gas if mixed with ammonia or acids.
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 and 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 situation 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 efficacy. Furthermore, no data were provided for concentrations between 2000 and 10000 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. 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.
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.
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 the cells before acting to coagulate and denature proteins, whereas high concentrations cause immediate denaturation of surface proteins and may inhibit further absorption. Highly concentrated alcohol also evaporates more quickly, reducing contact time in spray or dip applications. Alcohol solutions are not corrosive and 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.
For most nursery operations, it is more economical to use hand sprayers filled with 70% isopropanol. Ethanol is most commonly available in the form of denatured alcohol (methylated spirits), which consists of 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. At present (2020), 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.
4/13/2020 - Modified description of quaternary ammonium products.
8/4/2020 - Added some details in alcohol section.
9/14/2020 - deleted link for quat reference which is no longer functional. Added reference instead. Updated name of MSDS to SDS.