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United States Department of Agriculture

Agricultural Research Service

Title: Persistence of Phytophthora Ramorum in Soil Mix and Roots of Nursery Ornamentals

Author
item Shishkoff, Nina

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 16, 2007
Publication Date: April 17, 2007
Citation: Shishkoff, N. 2007. Persistence of phytophthora ramorum in soil mix and roots of nursery ornamentals. Plant Disease. 91:1245-1249

Interpretive Summary: Phytophthora ramorum causes stem cankers on oaks and foliar lesions and stem dieback on a number of host plants. It was first observed in the 1990s in California coastal forests and in Europe on nursery stock. Since then, it has been a major concern to the ornamental industry. Although the distribution of P. ramorum in the US is currently limited to parts of California and Oregon, there is potential for spread by the movement of water, soil, plants and plant products. Therefore, the life cycle of the organism needs to be clearly understood. P. ramorum is known to spread by rain and water splash onto above-ground plant parts. However, the soil phase of the organism is not well understood although the organism produces large chlamydospores, which are thick-walled persistent structures usually associated with long-term survival. This paper examines the persistence of chlamydospores in potting mix, and the persistence of the organism on living and dead roots. Chlamydospores remained alive in potting mix for a year or more, whether embedded in plant leaf tissue or alone. P. ramorum could persist in buried excised roots for at least 4-5 months. When living roots of known foliar hosts were drenched with a sporangial solution of P. ramorum and incubated for a month, all developed root infections except Lonicera hispidula. A nonhost, Buxus sempervirens, did not show root colonization. Infection was seen from both washed and surface-sterilized roots, suggesting that the roots, while symptomless, were internally infected. These facts, taken together, suggest a well-developed soil phase for P. ramorum.

Technical Abstract: Chlamydospores of Phytophthora ramorum appeared to persist in potting mix for many months, whether embedded in plant leaf tissue or alone. They survived up to 547-654 days albeit at very low levels (1.5%). P. ramorum could persist in buried dead root tissue for at least 4-5 months, although it was not clear in what form it was surviving: as mycelium or chlamydospores. When roots of 13 known foliar hosts were drenched with a sporangial solution of P. ramorum and incubated for a month, all developed root infections except Lonicera hispidula. A nonhost, Buxus sempervirens, did not show root colonization. Infection was seen from both washed and surface-sterilized roots, suggesting that the roots, while symptomless, were internally infected. These facts, taken together, suggest a well-developed soil phase for P. ramorum.

Last Modified: 4/21/2014
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