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United States Department of Agriculture

Agricultural Research Service


item Linderman, Robert
item Davis, E Anne - Anne

Submitted to: Sudden Oak Death Science Symposium
Publication Type: Proceedings
Publication Acceptance Date: 12/6/2005
Publication Date: N/A
Citation: N/A

Interpretive Summary: This paper is an oral presentation to be made at the Second Sudden Oak Death Science Symposium. The problem to be addressed is the potential survival of Phytophthora ramorum and other soilborne pathogens in potting media and contaminated containers and their eradication with aerated steam. The data indicate that P. ramorum and other species of Phytophthora can survive in potting mixes for up to 6 months, depending on the source of the inoculum. Plastic containers may also become contaminated with soilborne pathogens from a previous crop, and they need to be sanitized to eliminate the pathogens before use again. Aerated steam was used successfully to eradicate all the pathogens from infested potting mix and plastic containers. This information will be critical to growers in addressing the elimination of soilborne pathogens from potting mix and containers, especially with a quarantined pathogen like P. ramorum.

Technical Abstract: Phytophthora ramorum, thought to be largely a foliar pathogen on nursery crops, could be incorporated into container soilless potting media and thus become soilborne. Also, nursery containers from which infested media or infected plants were removed could be contaminated, as is true with other soilborne fungal pathogens. Thus our objectives were to: (1) determine the capacity of P. ramorum, compared to other Phytophthora species, to survive in potting media components using inocula as sporangia (P. ramorum only), infected rhododendron leaf pieces containing chlamydospores or oospores, or chlamydospore or oospore inoculum produced in culture on vermiculite; and (2) determine the efficacy of heat from aerated steam mixtures to eradicate P. ramorum and other pathogens (as colonized vermiculite) from potting medium in containers. Survival of Phytophthora species was determined monthly by baiting (B) or direct plating (DP) on selective agar medium. For objective (2) we used heat from aerated steam mixtures to sanitize plastic containers filled with infested potting medium. Varied temperatures were established, from 45° C to 70° C for 30 min at 5° C increments, by changing the air/steam ratio. P. ramorum was detected for 6 mo by B or DP from all substrates amended with sporangia or chlamydospores in vermiculite, but was not detected by either B or DF from infected leaf inoculum. All pathogens were killed by aerated steam treatments of infested medium at 60º C or higher. These results indicate (a) that P. ramorum can survive very well in potting mix components or soil and that aerated steam pasteurization is an effective means of eradicating P. ramorum as well as other pathogens from infested media and contaminated containers without destroying the containers.

Last Modified: 07/21/2017
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