Susceptibility of Clematis Varieties and Species to Stem Infection by Phoma Clematidina as an Indicator for Resistance to Wilt

The susceptibility to stem rot and wilt caused by Phoma clematidina was tested for a range of clematis varieties and species widely grown in Europe, and compared with their susceptibility to wilt in practice as judged by amateur and commercial growers in two postal surveys. Most small flowering clematis species did not die back when unwounded stems were inoculated with spores of P. clematidina, and were also reported in the surveys as being very resistant to wilt. In contrast, large flowering hybrids were found to be highly susceptible both to stem infection by P. clematidina and to wilt in practice according to growers. A significant positive linear relationship existed between the susceptibility scores for disease in the stem inoculation test and those for wilt from the surveys. Fungal isolations were made from the stems of naturally wilted clematis plants from British nurseries and private gardens. Unlike the many other fungal species isolated, P. clematidina occurred very frequently, especially in stems of large flowering varieties. Together with the results from the grower surveys and stem inoculation trial, this indicates that this fungus is a regular cause of clematis wilt. The stem inoculation test with P. clematidina described in this study would be useful for European breeders and growers to determine the susceptibility of new clematis varieties to wilt before they are marketed.