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United States Department of Agriculture

Agricultural Research Service


item Linderman, Robert
item Davis, E Anne - Anne

Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2006
Publication Date: 7/1/2006
Citation: Linderman, R.G., Davis, E.A. 2006. Survival of Phytophthora ramorum compared to other species of Phytophthora in potting media components, compost, and soil. HortTechnology. 16(3):502-512.

Interpretive Summary: Ramorum blight, caused by Phytophthora ramorum, is an emerging disease in the nursery industry, occurring on a range of crops such as rhododendrons, viburnums, lilacs, and camellias. The disease is dissemminated geographically on infected plants. The method used to limit that spread is visual inspection of plants to be shipped, looking for infection lesions on leaves. However, sporangia produced on infected foliage could be deposited onto or into the container medium and the plant could remain symptomless and thus be shipped. In this study, we sought to determine if the pathogen could survive in potting medium, compost, or soil if introduced experimentally as sporangia or chlamydospores from culture, or as infected rhododendron leaf pieces. Using a trapping method and direct plating on selective medium, we determined that P. ramorum could survive from sporangium inoculum for nearly 6 months, and as chlamydospores for over 12 months. The pathogen could not be recovered from infected leaf pieces, however, presumably because the infected leaves were dried before incorporation. Nonetheless, these results indicate the high potential for P. ramorum to infest and survive in container media, with the potential of being disseminated to other regions.

Technical Abstract: Phytophthora ramorum, while thought to be primarily an above-ground pathogen, can be introduced into soilless potting media in the nursery industry as sporangia or chlamydospores and remain undetected while disseminated geographically. Inoculum of this pathogen, both North American (A-2 mating type) and European (A-1 mating type) isolates, was used to infest potting media components or soil, using either sporangia, chlamydospores produced in vermiculite culture, or dry infected rhododendron leaf pieces. Vermiculite chlamydospore/oospore inoculum of P. citricola, P. cactorum, and P. citrophthora were included for comparison. Survival was determined monthly by baiting (B) or direct plating (DP) on selective medium. Results indicated that P. ramorum survived in most media components or soil for up to 6 months when introduced as sporangia, or up to 12 months as chlamydospores. However, it was not detected at all from infected rhododendron leaf pieces by either detection method. These results show that P. ramorum can survive in potting media if introduced as sporangia or chlamydospores, and accordingly the pathogen could be disseminated geographically Without being visually detected.

Last Modified: 06/27/2017
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