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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGY AND EPIDEMIOLOGY OF EMERGING PLANT PATHOGENIC OOMYCETES

Location: Foreign Disease-Weed Science

Title: Susceptibility of some common container weeds to Phytophthora ramorum

Author
item Shishkoff, Nina

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 24, 2011
Publication Date: N/A

Interpretive Summary: Sudden oak death, caused by the pathogen Phytophthora ramorum, has killed thousands of oaks on the west coast of the United States. Although its distribution is primarily limited to parts of California and Oregon, there is potential for spread with the movement of water, soil, plants and plant products. An Emergency Federal Order requires that nurseries in regulated areas that grow host plants be inspected and found disease-free before plants can be shipped to other states. Nurseries may contain weeds that propagate in containerized systems. In this research, 13 common weeds in containerized plant culture were inoculated with Phytophthora ramorum to determine susceptibility. Three species were found to develop symptoms of sudden oak death: northern willowherb, fireweed, and the fern Pteris cretica. Weed roots were inoculated with P. ramorum to see if the organism could persist on roots. P. ramorum could be isolated from most plants one month after inoculation, but were isolated only to a minor extent from surface-sterilized roots of weeds. Additional experiments were done to collect and sample water runoff from pots containing inoculated plants, to see whether inoculum was produced on infected weed roots. In these experiments, little inoculum was found in runoff from root-inoculated weeds compared to Viburnum tinus. Root colonization was significantly greater in Viburnum compared to the weeds, and weeds that were hosts on aboveground plant parts had greater root colonization than weeds that were not. From these results, we can recommend that weed control be included as a Best Management Practice for nurseries to reduce the risk of transporting P. ramorum in containerized plants.

Technical Abstract: Phytophthora ramorum is known to infect a number of ornamental plants grown in containerized culture. However, pots may also contain weeds. In this research, the foliage of 13 common weeds of containerized plant culture was inoculated with Phytophthora ramorum to determine susceptibility of above-ground parts. Three species were found to develop leaf lesions: northern willowherb (Epilobium ciliatum), fireweed (Chamerion Angustifolium) and the fern Pteris cretica. Weed roots were inoculated to see if the organism could persist on roots, and P. ramorum was isolated from most plant roots one month after inoculation when the washed roots were plated on selective media; they were recovered only to a minor extent from surface-sterilized roots of weeds. Additional experiments were done to collect and sample runoff from pots containing inoculated plants to see if inoculum was produced on weed roots. In these experiments, little inoculum was found in runoff from root-inoculated weeds compared to Viburnum tinus. Percent root colonization recorded from washed roots was significantly greater in Viburnum compared to the weeds, and weeds that were foliar hosts had greater root colonization than weeds that were not.

Last Modified: 7/22/2014
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