Phytophthora kernoviae has only been isolated from the United Kingdom and New Zealand. To understand what differences may exist between isolates from these two distinct geographical regions, virulence studies on three host plants and sporulation on host leaves were conducted on select isolates. Three host plant species (Rhododendron ponticum, Magnolia stellata, and Annona cherimola) were inoculated individually with sporangia of six different isolates from each geographical region. Results showed an overall higher virulence on all three hosts from isolates originating from the United Kingdom. After inoculation, P. kernoviae sporangia and oospore formation on different host leaves were observed and compared with P. cactorum and P. syringae. Results were host dependent, with P. kernoviae producing generally similar or higher amounts of both propagules compared with the other U.S. indigenous species. These results have implications for regulatory agencies and scientists who are interested in preventing its entrance into the United States and learning more about its potential spread.