In this study, the disease
development of M. polygoni-cuspidati in the field and the optimum infection parameters under controlled
conditions were investigated to elucidate the pathogen’s potential biocontrol efficacy against Japanese
knotweed. Field observation in Japan showed that M. polygoni-cuspidati caused severe damage to its host
plant. When sentinel knotweed plants from the UK were placed amongst naturally-infected field populations of F. japonica, disease incidence and severity were highest in July when monthly precipitation
was also highest. In greenhouse inoculation tests, F. japonica was shown to be most susceptible at the
young leaf stage (7–12 days after opening). Disease severity was highest after an initial dew period of
42–48 h, and severe defoliation followed inoculation at a temperature range of 15–25 ?C. The optimum
post-inoculation temperature after dew treatment for disease severity was 20–25 ?C. In field inoculation
tests, high disease incidence and severity indicate that the pathogen has the potential to control the plant
effectively in the field. Humidity and temperature were shown to be the main factors influencing diseaseexpression and lesion development of M. polygoni-cuspidati in a field situation. These results provide
valuable information for any future use of M. polygoni-cuspidati as a CBC agent for management of
Japanese knotweed in the UK.