Susceptibility of Some Common Container Weeds to Phytophthora ramorum.

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 14 common weeds of containerized plant culture was inoculated with P. ramorum to determine susceptibility of aboveground parts. Three species were found to develop leaf lesions: northern willowherb (Epilobium ciliatum), fireweed (Chamerion angustifolium), and a fern (Pteris cretica). Weed roots from 11 species were inoculated to see if P. ramorum could persist on roots, and P. ramorum was isolated from most plant roots 1 month after inoculation when the washed roots were plated on selective medium; 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.