Heat treatment is often referred to as sterilization, but temperatures routinely used to heat soil, containers, or other nursery items will not result in completely sterile soil or potting mix. The goal is to heat the potting mix to a point that kills the plant pathogens of concern. Phytophthora is not very heat-tolerant compared to other plant pathogens. However, heat treatment standards are generally selected to be high enough to kill most plant pathogenic fungi both because of the added benefit of eliminating these other pathogens and to provide a wider safety margin to ensure that Phytophthora propagules of all types are killed.

Heat can be used to sanitize both granular bulk materials (such as potting mix, soil, sand, etc.) and solid materials (e.g., containers, small tools). Because heat can penetrate roots and plant propagules, it can also be used to eliminate internal infections, as discussed under thermotherapy, if the plant material can tolerate the necessary heat treatment. 

Jump To

• The Basics
• Heating Potting Mix or Soil
• Heating Methods
• Heating Standards
• Measuring Heat Treatment Temperatures
• Effects of Heat Treatment on Beneficial Microorganisms
• FAQ: Heat Treatment of Soil Components
• Preventing Recontamination of Heat-Treated Materials
• Thermotherapy

The Basics:

Heat works best if the target pathogen propagules are moist before treatment.

Target organisms are killed more readily and at lower temperatures if they are hydrated. For 11-minute heat treatments with aerated steam, temperatures had to be increased by up to 36 °F (20 °C) to kill dry propagules of some plant pathogenic fungi compared to temperatures required for propagules premoistened for 16 hours (van Loenen et al. 2003). Etxeberria et al. (2011) found that substantially shorter times were required to kill oospores of P. capsici in water compared with moist soil, although both hydration level and heat exposure properties can vary between these methods. When wetting up dry materials before treatment, the wetting period begins when the driest part has been moistened. For example, dry aggregates of potting mix would need to be wetted to their centers before you start timing the wetting period. Based on data from van Loenen et al. (2003), treatment temperature and/or time should be increased above minimum standards to ensure efficacy if materials to be treated (e.g., potting mix, residues on used pots) have not been kept moist (close to field capacity) for at least 16 hours before treatment. 

Effective heating is a function of time and temperature. The time needed to achieve thermal kill of plant pathogens decreases as temperatures increase. The relationship between temperature and the necessary exposure time is logarithmic, not linear, so even small drops in treatment temperature can greatly increase the amount of time needed to kill target microorganisms. 

For instance, metal tools can be sterilized by direct exposure to flame from a butane or propane torch (about 2000-2500 °F [1100- 1400 °C]) for as little as a few seconds. Standard treatments for killing plant pathogens in hot water include 203 °F (95 °C) for 30 seconds and 185 °F (85 °C) for 3 minutes (Runia and Amsing 2001). However, longer treatment times at lower temperatures are more useful for treating large volumes and bulky materials (e.g., used pots and potting mix) because of the time required to uniformly heat the materials to the desired temperatures without overheating. Treatment temperatures need to be monitored to accurately assess the temperatures attained during heat treatment (see below).

All portions of the treated materials (e.g., containers, potting mix) need to reach the target temperature for the minimum treatment time. Begin timing when the coolest area of the treated material reaches the target temperature. Uniformity of heating can be affected by a variety of factors, including the method of heating, rate of heat loss at the edges of the treatment area, variation in compaction, moisture, clods, bin shape, and other factors. The coolest part of a heated mass of potting mix or stack or pile of containers may be at the center or near the edge, depending on these factors.

Heating Potting Mix or Soil 

Using pathogen-free potting mix is an essential starting point for producing nursery container stock that is free of soil-borne plant pathogens. In general, the efficiency of heating potting mix or soil can be increased by mixing it during the heating process. Hot and cold spots commonly develop during static heating of potting mix, due to variations in moisture and density and the specifics of heat flow in the system. Mixing during heating greatly improves the uniformity and efficiency of heating and reduces the time needed to reach target temperatures. However, additional equipment (such as a rotating drum) needed to accomplish mixing during heating makes the heat-treating apparatus more complex.

Soil and potting mix need to be moistened adequately before treatment to prevent excessive drying out during heating if a dry heat source is used for heating. This is not an issue if steam is used. However, condensation of water during steaming can make the soil or media excessively moist, which can reduce the rate and uniformity of heating. 

If a dry heat source is used, the potting mix needs to be moistened adequately before treatment to keep it from drying out. Drying is not an issue if steam is used because condensation of steam on potting mix particles increases its moisture content. If the potting mix is too moist before steaming, the additional moisture from condensing steam can make portions of the mix too wet, reducing the rate and uniformity of heating. Only dry steam (saturated with water vapor but no liquid water droplets) should be used for steaming potting mix to help keep it from becoming excessively wet.

Moist potting mix tends to cool slowly, especially if it is surrounded by an insulating layer to minimize heat loss from the edges. In such situations, application of heat can be discontinued once the material has been heated to a temperature that will not allow the coolest area of the potting mix to drop below the target temperature in less than the minimum treatment time. This can be accomplished by exceeding the minimum target temperature (but staying below 82° C [180 °F] as discussed below). Both mixing (if used) and further heating are stopped and the heated potting mix is kept in, or quickly transferred to, an insulated container. This approach reduces the amount of fuel or electricity needed to meet the necessary time x temperature treatment standard.

When choosing methods and equipment for heat treating potting mix in your nursery, an important point to consider is the scale that will be needed to handle the amount of material used in production. Some heating methods are better suited for treating large batches all at once, others are more efficient for small batches. Customized equipment that is specifically tailored to your specific needs and constraints may be the most economical over the long term.

Baker (1957) discusses the principles of heating potting mix in great detail in Chapter 9 and compares a variety of heating units in Chapter 10. Practical tips and relevant data are also provided. Although this reference is old, the physics of soil heating have not changed.

Avoid overheating potting mixes.  Heating potting mix to excessively high temperatures will use more energy than is required and can eliminate beneficial microorganisms (such as some soil bacteria) that may be present. Excessive soil heating (greater than about 82°C [180°F]) may also increase the chance of phytotoxicity due to exchangeable manganese, ammonium, soluble salts, and toxic organic compounds that are formed at high temperatures (Dawson et al. 1965). It is safe to treat a UC-type soil mix (fine sand and sphagnum peat moss or hypnum peat moss) to a temperature of 212°F (100°C) without developing soil toxicity to plants (Baker 1957). Soil mixtures high in readily decomposable organic matter such as manure, leaf mold, or compost (Baker 1957, p129) or high amounts of manganese are most likely to become phytotoxic when exposed to excessively high temperatures. 

Phytotoxicity due to exchangeable manganese will decrease over time as this element oxidizes. If treatment temperatures are higher than 82°C (180°F), soil should be aged before use to minimize the likelihood of manganese phytotoxicity. Reoxidation of manganese to insoluble forms is greatly accelerated by the activity of soil bacteria, whose populations are also reduced or eliminated at high temperatures (Sonneveld and Voogt 1975).

Heating Methods

The most common methods for applying heat treatments in nursery operations are:

· free-flowing steam (i.e., not under pressure) or aerated steam (steam/air mixtures) 

· hot water

· dry heat.

Heating can be accomplished for all these methods using electricity or fossil fuels. Diesel fuel is commonly used for steam generation; natural gas or propane are used to produce steam, hot air, or hot water. Solar energy can be used to generate dry heat (via solarization or in a solar oven) or hot water but has not typically been used to generate steam in nursery situations.

All three methods can be used to treat solid objects such as containers, trays, and racks. Only steam or dry heat are normally used to heat-treat potting mix. Hot water is the preferred way to heat treat plant propagules or plant roots to eliminate pathogens (thermotherapy, see below) because it is more effective at transferring heat than air and can be controlled more readily to achieve the precise temperatures needed for this application.

Steam. Steam is an efficient means for applying moist heat. Steam can be produced using a steam boiler or a steam generator. A search on "soil steam sterilization equipment" will primarily return information on large commercial grade equipment typically used in agriculture and nurseries. However, depending on the amount of soil you intend to treat at a time, smaller, less expensive equipment capable of producing dry steam may be feasible (e.g., steam generators, commonly heated using electricity). 

The use of dry steam (i.e., vaporized water, without liquid water droplets) is important for two reasons. First, steam transfers heat as it condenses, so steam in the vapor phase will heat materials more efficiently than droplets of hot water. Second, water has a high specific heat, much higher than potting mix or plastic containers. Most of the energy used to heat moist potting mix is actually needed to increase the temperature of the water in the mix. As the mix becomes progressively wetter due to condensation of water during steaming, more energy is required to attain a given temperature. Also, if the pore spaces in potting media or air gaps between containers become filled with water, circulation of steam will be impeded.

Aerated steam is made by blending steam and forced air from a blower in a plenum that supplies the steam-air mixture to the material being treated. A water trap is included to capture water that condenses when the steam mixes with the cooler air. Compared with steam, which is 100°C (212°F) at atmospheric pressure, steam-air mixtures can be adjusted to a range of lower temperatures. This allows for the use of target temperatures below 100°C (212°F) and avoids excessive heating of potting mix or other materials (e.g., containers made of some plastics). The publication by Greisbach et al. (2012) recommends the use of aerated steam and illustrates several different types of steaming units (p.52).

Steam and aerated steam can heat potting mixes efficiently because they diffuse through porous media. Denser, less porous materials, such as most soils other than sand, are much more difficult to heat uniformly using steam unless the material is agitated and separated into small fragments during heating. Static steaming (in bins or piles) of even porous potting mix can result in poor uniformity, requiring long steaming times to adequately heat the coolest portions of the pile. Steam will preferentially move through areas with low resistance, such as along the walls of the container or through fissures that develop in the media (known as blowout). This can result in cool spots in the pile or bin. Heating uniformity is affected by how and where the steam is introduced (Baker and Fuller 1976). Temperature nonuniformity, which increases the amount of time needed to treat potting media to a uniform minimum temperature, is minimal if soil depth is less than 1 ft (30 cm).

When producing aerated steam at temperatures typically used for nursery purposes (60-70°C [140-158°F]), the volume of air used will be greater than the volume of steam. Data from Table 1 in Aldrich and Nelson (1969) were converted to a volume ratio (air:steam) and plotted in the figure below. The characteristics of the steam, as well as air temperature and humidity, can affect how much air needs to be mixed in to attain target temperatures, so the actual temperature of the steam-air mixture needs to be monitored and flow rates adjusted as needed to obtain target temperatures. Manufacturers of steam equipment for nurseries typically sell matched aeration units, but these units are quite expensive. Aldrich and Nelson (1969) describe a user-built steam aeration unit, consisting of a blower with a particular flow rate, a mixing plenum, and appropriate controls for controlling air and steam flow.

Dry heat — static heating. Electric soil sterilizers are one option for heating potting mix, typically for relatively small volumes. Other available equipment may also be suitable for static heating using dry heat sources. Commercial moisture-proof heating equipment used in other industries (e.g., search "food warming equipment" and "towel warmer cabinet") may be low cost alternatives for heating small volumes. Asphalt kettles, kettle melters, or similar equipment with adequate temperature controls may provide another option for static heating of larger volumes. This type of equipment typically burns propane or other fuels to generate heat.

Dry heat — heating with mixing. Rotary kilns and rotary dryers are commonly used to heat granular materials using dry heat in a rotating drum that constantly mixes the material. These devices can be quite efficient. Most commercial equipment is designed for very large amounts of material. However, it is possible to scale down these devices to sizes that may be more appropriate for nursery use. We have constructed and tested a version of a rotary kiln for heating soil that uses an insulated cement mixer to tumble the soil and a portable propane forced air heater as a heat source (http://phytosphere.com/gear/drumheater.htm). A similar device can be made using a drum roller (made for mixing contents of large drums) instead of a cement mixer. Because heating occurs more quickly in this type of device compared to a static soil mass, the amount of fuel needed to attain target temperatures is minimized. Because of the agitation and airflow that occurs in this type of device, it may be necessary to add moisture (via a fine mist) during the heating to keep the potting mix from drying out excessively.

Dry heat — Solar energy. In areas with sufficient solar exposure, solar energy can be used to heat potting mix or containers to target temperatures. Where it can be used, direct solar heating is less expensive than using fuel or electricity. Conditions that reduce solar radiation, including shade, clouds, fog, smoke, and low solar angles, can limit the temperatures attainable with solar energy. Solar heating is most effective where direct sunlight is available for at least 5 hours centered around solar noon during the longer days of mid-spring through mid-fall and when daytime ambient temperatures are 21°C (70°F) or higher. Hence, it may be necessary to conduct solar heat treatment of potting mix and containers when conditions for solar heating are optimal and then store heat-treated materials under clean conditions for later use. Even if conditions are inadequate to reach target temperatures with solar heating, preheating soil using solar energy can reduce the amount of time and energy required to reach target temperatures using other methods.

Solar ovens. A solar oven can produce target temperatures needed for heat treatments in as little as one afternoon. Efficient solar ovens suitable for heating potting mix or containers can be built using a standard glass window (not low emissivity glass) that fits tightly over an insulated box lined with reflective material, such as aluminum flashing. If the oven is on wheels, its position and angle can be changed during the day to avoid shading and optimize the angle for intercepting solar radiation. External reflectors can be used to increase light intensity in the oven, but these are not needed if the oven is efficient. A metal rack should be placed on the bottom of the oven so that air can circulate completely around material in the oven.    

To maximize heating, soil or potting mix should be placed in dark-colored metal containers. Heating will occur faster if the distance between the wall of the container and the center of the heated soil is minimized, preferably no more than 15-17 cm (about 6-7 inches). Containers with soil should be tightly covered (preferably with clear heat-resistant plastic or glass) to prevent evaporation. Water that evaporates in a sealed oven will condense on the inside of the oven window and reduce solar radiation.The temperatures of materials heated in the oven should be monitored, preferably with temperature loggers, to ensure that an adequate time x temperature treatment threshold was attained. Solar ovens can attain temperatures that will soften or melt some plastics, so temperatures should be monitored when heat-treating plastic containers to avoid overheating. Some temperature loggers can be set to produce an audible alarm if a target temperature is exceeded. The internal oven temperature can be quickly lowered by opening or removing the glass top. See http://phytosphere.com/gear/solaroven.htm for additional information on constructing a solar oven for heating soil or other materials.

Non-vented greenhouses. In areas with high ambient daytime temperatures and full sun exposure, a properly constructed and oriented greenhouse may attain temperatures sufficient to heat-treat potting mix and containers. If enough space is available, it may be practical to prefill containers with potting mix and treat the mix and containers at the same time. The greenhouse should be covered with material that maximizes solar heating and minimizes heat loss and vents will need to be kept closed. Both greenhouse air temperature and temperatures of treated materials (e.g., mix-filled containers) placed in different portions of the greenhouse should be monitored to determine whether target temperature can be attained in all parts of the greenhouse. A circulating (not exhaust) fan may be helpful for achieving more uniform air temperatures within the greenhouse. Potting mix should be moistened before heat treatment and will need to be covered (e.g., with clear plastic film) to keep the soil from drying out and cooling via evaporation.

Solarization. Standard soil solarization methods can also be used to heat treat potting mix, containers, and surface soils. Soil solarization involves tightly covering materials with clear (not black) plastic to set up “greenhouse effect” heating. Note that clear plastic must be used. Although black plastic gets hot in the sun, it is very inefficient at transferring heat across the air gap between the plastic and the underlying material. Black plastic also shades and prevents direct solar heating of the covered material. Although various types of clear plastic can be used, the plastic should allow for good light transmission and needs to be strong enough to resist tearing, because heating will be poor if air leaks through or around the plastic. Clear thermal anti-condensate greenhouse film (6 mil thickness) has efficient thermal qualities and a long service life and is ideal for solarization. In cooler areas, using a double layer of plastic film separated by an air gap can reduce heat loss. 

A method for using solarization to heat treat soil in containers is described and illustrated in Stapleton et al. 2008. Another alternative is to place a layer of moistened container mix, no more than 20-25 cm (8-10 inches) depth, on an insulated platform (e.g., a table topped with foam insulation panels) and tightly covering it with 6 mil clear thermal anti-condensate greenhouse film. Insulation below the mix will reduce heat loss from the bottom. A similar method can be used to heat treat empty containers.

Soil solarization can be effective at heating surface soils. However, temperatures effective for killing Phytophthora may be difficult to attain at soil depths greater than about 20 cm (8 inches) because the heat is continuously lost to the cooler underlying soil. Heating is affected by the duration of continuous sunlight during peak hours and can be affected by slope, aspect, and shading. Because subsurface soil temperatures attained through solarization are typically no more than 43-52 °C (110-125 °F), treatments need to be conducted for an extended period to reach adequate time x temperature thresholds. Typical treatment duration for solarization of surface soil is 4 to 6 weeks during summer, but longer treatment periods may be needed in situations where solar heating conditions are more variable. A calculator for estimating the minimum solarization times and depth of heating in nursery beds is available at http://uspest.org/soil/solarize.

Heating Standards

Heating moist soil to 60°C (140°F) or higher for at least 30 minutes will kill propagules of Phytophthora and other water molds as well as most plant pathogenic fungi (Table below). If you are unsure whether you are measuring the temperature in the coolest part of the treated mix, you can increase the margin for error by extending the duration of heating (e.g., 60°C (140°F) for at least 1 hour) or by increasing the target temperature (e.g., 70°C (158°F) for 30 minutes). You can avoid the potential phytotoxicity problems that develop in some soils by using a maximum temperature of 82°C (180°F) or less. If you are treating plastic containers, you will need to make sure that the maximum temperature reached does not cause the plastic to soften or melt. In general, pots and trays made of thin, flexible plastics soften at lower temperatures than those made of heavier, more rigid plastics.

These target temperatures can be attained with steam-air mixtures and by various dry heat devices, including an efficient solar oven, where oven air temperature can exceed 110°C (230°F). However, it may be difficult to reach these temperatures using standard solarization, a solar-heated greenhouse, or a solar oven operating under suboptimal conditions. Alternative heating standards for killing Phytophthora at lower temperatures are 50°C (122°F) for at least 2 hours or 45°C (113°F) for at least 15 hours. For the 45°C standard, the total heating time would normally be accumulated over several successive days. At these lower temperatures, treatment times should be extended beyond these minimums to provide a greater safety margin.

Source: Baker, K.F., 1957.

Measuring Heat Treatment Temperatures

Place temperature probes in the coolest portions of the treated material (potting mix, stack of containers, etc.). You may need to use several probes to determine which areas are the coolest. The center of a mass of potting mix or pile of container may be the coolest, but if any edges are not insulated or sealed well, they may be cooler or may cool faster. Begin timing the heat treatment once the probes show you have reached the desired temperature at the coolest spot. Continue to monitor temperature to ensure that it does not drop below the target before the end of the treatment time.

For all temperature measurement and recording devices, check manufacturer specifications to ensure that the probe is rated for a temperature range that extends above your target temperature so the probe will not be damaged if the temperature exceeds your target. Some temperature measuring devices may also need to be protected from moisture.

Thermometers. Long-stem thermometers such as compost thermometers can be used if they can be mounted so that the gauge remains outside of any coverings used during soil heating. These thermometers use a straight rigid metal probe of various lengths that are inserted into soil or potting mix. Both analog and digital versions are available; digital thermometers typically respond faster and are more precise. Digital thermometers with wired or wireless external temperature probes can also be used. These provide more flexibility for placement of the probe in materials being heated than thermometers with long straight probes. They may also store maximum and minimum temperature readings.

Temperature loggers. Temperature loggers can both measure and store temperature data at specific time intervals in internal memory for downloading. Some loggers transmit temperature data wirelessly to another device. Temperature loggers with external wired probes are available at a wide variety of prices from multiple manufacturers. If using a wired external probe, the wire needs to be long enough to extend from the desired temperature monitoring location(s) to a point where the logger can be safely placed.

Button-style temperature loggers are buried directly in potting mix or placed in heated materials and need to be recovered for downloading and reuse. Most require specific readers to download the data.

Temperature indicator strips.  Non-reversible temperature indicator labels, stickers, or strips provide another option for monitoring temperature. These single-use temperature indicators are attached to items and show whether they have reached a specific temperature. Different versions of these products can indicate whether a specific temperature was exceeded or display a range of temperatures that allows the user to determine the maximum temperature attained. The indicators in these strips typically react within one to a few minutes, so they do not provide information on the length of time that the maximum temperature was maintained.

Effects of Heat Treatment on Beneficial Microorganisms 

As noted above, typical heat treatment temperatures (e.g., 60°C (140°F) for 30 min) are lethal to water molds including Phytophthora species and most plant pathogenic fungi but do not kill all microorganisms present. Most spore-forming soil bacteria and spores of various mycorrhizal fungi (Ellis et al. 2002, Hu et al. 2019, Sylvia and Schenck 1984) will not be killed by this treatment.

Spores of fungi known to form mycorrhizal associations with plant roots are ubiquitous in soil and are readily blown about. Studies have shown that container plants form mycorrhizal associations either in the nursery or after transplanting (e.g., Meyer et al. 2005). Plants growing in recently solarized soil were well colonized by arbuscular mycorrhizae (Stapleton and DeVay 1986). In general, saprophytic fungi can colonize heat-treated soil more readily than pathogenic water molds or fungi, many of which have limited saprophytic ability. Because colonization by airborne spores of various mycorrhizal fungi occurs readily in most areas, heat treatment of potting mix does not adversely affect plant establishment and growth or mycorrhizal colonization. In general, the elimination of pathogens by heat treatment improves the health and survival of container plants if pathogens are not reintroduced after heat treatment.

Some fungi that form mycorrhizae may be killed by heat treatment if very high temperatures (>80°C) are used (Ellis et al. 2002, Hu et al. 2019, Sylvia and Schenck 1984). However, in the soilless potting mixes normally used in nurseries, there is no guarantee that spores of suitable mycorrhizal fungi or other beneficial microorganisms are present to begin with. A typical soilless potting medium will have a rarefied soil microbial community that has little in common with a planting site in native habitat or elsewhere. The benefits of using heat treatment to eliminate Phytophthora and other pathogens from potting mix greatly outweighs the potential for short-term loss of any beneficial microorganisms that might be present in potting mix.

FAQ: Do all components of soil mix require heat treatment?

Some materials used in gemination and potting mixes, particularly vermiculite and perlite are manufactured using very high temperatures. These materials will be free of soil-borne pathogens if they are in factory-sealed bags and haven't been contaminated after manufacture.

Most other components, including organic materials and sand, are not handled in a way that guarantees that they will be free of Phytophthora and other plant pathogens. Sand, especially quarried from rivers, is commonly contaminated with Phytophthora and other soil-borne pathogens. Sand from potentially clean sources (e.g., upland mines) can also become contaminated in handling and transport. Clean soil mix components do not look any different from contaminated components. Components that are produced and moved in bulk can easily become mixed with contaminated material in storage and transport. Unless documented phytosanitary processes are followed throughout manufacture and handling, these other materials need to be treated as potentially contaminated and should be heat-treated before use in clean nursery production.

Commercial compost produced according to California standards should be heated to an extent that kills most plant pathogens in the composting process. However, depending on how the material is handled at the composting facility and afterward, it has a high risk of being recontaminated (e.g., by moving finished compost with the same equipment used for unfinished compost). California composting facilities, depending on their size, are subject to regulations concerning minimum temperatures x duration for the purpose of reducing human pathogens below levels that pose health threats, but composting standards do not account for plant pathogens. As a result, commercial compost should be heat treated before use in clean nursery plant production. 

Preventing Recontamination of Heat-reated materials

Heat-treated potting mix or containers can readily become recontaminated with soil-borne plant pathogens in numerous ways, including:

- placing heat-treated materials on the ground, on contaminated surfaces, or into contaminated equipment, vehicles, bins, or containers

- handling with contaminated tools or hands

- planting with pathogen-contaminated propagules or seed

- exposure to contaminated water via irrigation or splash from contaminated soil

- addition of organic amendments or fertilizers that are not heat treated and are contaminated due to their origin or handling.

The key to avoiding Phytophthora and other soil-borne plant pathogens in the nursery is to start clean and stay clean. If clean, heat-treated potting mix, containers or other items become contaminated after treatment through poor handling practices, the time and money spent for heat treatment will be wasted. Details about practices used to prevent recontamination of clean, heat-treated potting mix are discussed in the Best Management Practices (BMPs) for Producing Clean Nursery Stock.

Thermotherapy

Plant thermotherapy involves heating plant parts or entire plants to temperatures capable of killing or inactivating internal pathogens. Thermotherapy can work as a control measure if the plant can tolerate greater temperatures than the target pathogen. Phytophthora is a promising target organism for thermotherapy because it can be killed at lower temperatures than many other plant pathogens.

Hot water treatment, typically at about 49-52°C (120-125°F) for 30 minutes, can be effective for killing both surface contaminants and internal infections of some pathogens, including Phytophthora. Insufficient research is available to make specific recommendations for California native plant materials at present, because heat tolerance of plant material varies by species, propagule type, and preconditioning. See Baker (1957) (section 13, starting p. 223 https://archive.org/details/ucsystemforprodu23bake) for a detailed discussion of heat treatment of vegetative propagules.

In limited tests, Swiecki and Bernhardt (2022) found that thermotherapy was difficult to accomplish with container plants. Because there is a small margin between heat treatments that may be therapeutic and those that are lethal to plants, treatments need to be fairly precise with respect to both the target temperature attained and the treatment duration. However, they found that it is difficult to apply precise heat-treatments to container plants due to the difficulty of raising the temperature of roots and soil quickly and uniformly throughout the container. The problem increases as the container volume increases.

In pilot studies with Quercus agrifolia and Q. lobata grown in small (397 ml) containers, a 30-minute treatment at 47°C in a water bath appeared to eradicate infections of Phytophthora cactorum, P. kelmanii, and P. cambivora (Swiecki and Bernhardt 2022). These initial experiments indicate that thermotherapy may have some limited utility for treating Phytophthora-infected plants and is most likely to be successful for small plants and propagules. 

Credit: Tedmund J. Swiecki and Elizabeth Bernhardt, Phytosphere Research