The discovery of the ramorum blight pathogen, Phytophthora ramorum, infecting many ornamental plants in nurseries and landscapes in several European countries, the United States, and Canada, underscores the threat that this pathogen poses to the nurseries (Osterbauer et al., 2004). An increasing number of plant species have been found to be hosts for P. ramorum, especially those in the Ericaceae, Fagaceae, and Caprifoliaceae (Davidson et al., 2005; Goheen et al., 2002; Linderman et al., 2006; Parke et al., 2004; Rizzo et al., 2002, 2005; Tooley et al., 2004; Werres et al., 2001). The host list is even greater considering all the plants with which P. ramorum has been associated or shown to be susceptible by artificial inoculation (Linderman et al., 2006). Furthermore, P. ramorum has been shown (Linderman and Davis, 2006; Shishkoff and Tooley, 2004) to survive in potting media infested by sporangia or chlamydospores (simulating inoculum that could be produced on infected aboveground tissue), thus underscoring the threat that the pathogen could infest media and remain undetected while being disseminated geographically. Growth media potentially infested with this or other soilborne pathogens, along with contaminated used containers, could be a source of inoculum to initiate infections on a wide range of susceptible plants (Jones and Benson, 2001). In addition, Jeffers (2005) demonstrated the survival of P. ramorum in medium around containers of infected plants in a nursery, thus requiring some treatment of the soil to eradicate this regulated, quarantined pathogen.
Growers currently attempt to decontaminate used containers by pressure washing or chemical sanitization. Many simply apply fungicides during the production cycle to prevent infections or to respond to occurrence of diseases. However, pathogens could be eradicated from soil or soil-less media by heat from steam, composting, or solarization, or by chemical fumigation (Baker, 1957; Jones and Benson, 2001). The use of aerated steam to pasteurize soil or potting mixes to eradicate soilborne pathogens, weed seeds, and insects was pioneered by K.F. Baker (1957). The principles of treating soil or potting media with heat at temperatures that would be lethal to pathogens without killing all microorganisms, some of which might be beneficial as antagonists remaining after treatment, were useful to nurseries where chemical eradication of soilborne pathogens was not feasible. Baker (1957) emphasized that air-steam pasteurization of soil or potting media in a range of 60 to 71 °C had the additional advantages (compared with using steam at 100 °C) of 1) reduced chance of destroying microorganisms antagonistic to plant pathogens and therefore leaving a biological buffer to block invasion of the medium by pathogens or the development of “weed” fungi that are activated by high temperatures; 2) reduced risk of developing soil toxins resulting from excessive heating; 3) reduced use of steam and therefore energy; 4) taking less time to treat and less time before the medium could be used; 5) less working hazard; 6) less moisture condensation in the medium; and 7) no damage to plastic containers.
The use of aerated steam mixtures at pasteurization temperatures has not been considered by most nurseries as a means of sanitizing contaminated containers. However, many nurseries have used hot-water dip tanks or even chemical baths in attempts to eliminate pathogens, often, if not usually, unsuccessfully. On the other hand, aerated steam is used at some nurseries to treat potting media, but usually not for decontaminating flats or other containers that might carry pathogens from previous crops. In recent years, we have noted that some growers have successfully used live steam injected into fabricated chambers, thereby diluting with air to create lower temperature air-steam mixtures that sanitize pots or flats.
It is well known that chemical fumigants can be used to treat soil as a pre-plant means of eliminating soilborne pathogens, insects, and weeds. Highly volatile fumigants such as methyl bromide or chloropicrin, however, must be injected into the soil and tarped to reduce gas escape. They are too toxic and volatile to be used easily to eradicate pathogens from soil-less media or contaminated containers unless they are confined in rooms such as would be used to fumigate fruit for insect control. Metam sodium, on the other hand, is more easily applied in a liquid form as a drench, thereafter releasing methyl isothiocyanate (MIT) as the toxic fumigant. Although it is known that MIT can kill most soilborne fungal pathogens, this has not been demonstrated for P. ramorum. However, it is conceivable that metam sodium could be applied efficiently and relatively inexpensively to soil-less media in a containment structure or groundbed to eliminate soilborne pathogens. Furthermore, it could be drench-applied to sanitize areas of nurseries from which containers with infected plants had been removed.
Thus, our primary objective was to determine the effectiveness of heat treatment via aerated steam mixtures, and fumigation with metam sodium, in eradicating P. ramorum (the European A1 and North American A2 mating types) and other soilborne pathogens including Pythium irregulare, Thielaviopsis basicola, and Cylindrocladium scoparium introduced into soil-less potting media or soil. A secondary objective with the heat treatment was to confirm that pathogens could be eliminated in plastic containers without melting the containers.
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