Killing temperatures for pathogenic organisms of plants

In general the effectiveness of thermal soil sterilization methods such as steaming with hot water vapor, depends on the applied energy respectively temperature and duration of exposure. This means that similar results can be achieved either by applying lower energy over a longer period of time or by applying higher energy over a shorter time.
General data on the required time of exposure for certain temperatures to kill specific phytopathogens is hard to provide, in particular since in accordance to their different stages of development they show different degrees of heat resistance. When hybernating these organisms are extremely resistant. Hence an exact killing temperature for many organisms can not be identified. The following list shows approximations of killing temperature at 30 minutes of steam exposure of soil borne organisms (G.B. Bollen):

  1. Up to 55°C:
    Parasitic nematodes (except Pratylenchus), saprophagous nematodes, Verticilium albo-atrum, Didymella lycopersici, Cylindrocarpon destructans, Thilaviopsis basicola, Phytiumarten, Phytophtora, Pratylenchus, Commor Ragwort, Chickweed
  2. Up to 65°C:
    Fusarium oxysporum, Fusarium redolens, Verticilium dahliae, Botytis, cinerea, Phialophora cinerescens, Rhizoctonia solani, most Penicillium- and Aspergillus- species, Ascomyceten, Algaes, Insects , Worms, Snails, Centipede, Mosaic virus
  3. Up to 75°C:
    plant pathogenic bacteria, Penicillium- and Aspergillus- Species, Potato X-Virus
  4. Up to 90°C:
    Tomato Mosaic Virus, Cucumber Virus, five mesophilic molds
  5. Over 90°C:
    Spore forming bacteria

In comparison pathogens harmful to plants are more sensitive than humus forming organisms. When using chemicals the same phenomenon occurs.  Higher organisms such as saprohagous or parasitic nematodes already die at temperatures higher than 55 degrees.

Certain types of Phytium, Rhizoctonia and Botrytis which cause molding die starting from temperatures up to 55°C after 30 minutes of exposure. It’s mentionable that fungi which act as counterparts to pathogenic types and are important to revitalize soil such as Aspergillus- and Penicillium-types are more heat resistant.

The same can be said for bacteria: Spore forming types, important to soil, are extremely resistant to high temperatures and are able to regenerate a new population even after having been exposed to more than 100°C. Amongst them in particular Bacillus subtilis is notable, which can control and even fight pathogenic fungi such as Rhizoctonia.

In contrast phytopagogenic bacteria are quite heat sensitive. It is unknown that these types survive temperatures higher than 70°C at over 30 minutes of exposure.

Research shows that the predominant majority of phytopathogenic oganisms die at temperatures up to 75°C as long as there are exposed to heat long enough. Therefore it is sufficient to heat soil to temperatures up to 98°C for sterilization in order to remove all diseases and preserve spore forming bacteria and cellulose decomposing fungi, which provide a certain natural protection against the resettling of phytopathogens.

Steaming with hot water vapor meets all general requirements. The high specific heat of water ensures high temperatures over a long period of time to achieve an effective and conservative sterilization. Other thermic methods such as the usage of hot air can only achieve similar results by applying extremely high temperatures of more than 2000°C or by longer time of exposure which always comes with the risk to dry out or even burn soil and negatively affect its fertility.

In a nutshell, steaming with hot water vapor is not sterilization in the traditional sense, since many beneficial organisms survive. A total sterilization is not desirable instead, a partial disinfection is achieved.

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