In May 2011, severe floral abnormalities were observed
on three hydrangeas at commercial nurseries in Gunma
Prefecture. On these hydrangeas, several symptoms characteristic
of phytoplasma-infected plants were found: the
sepals developed into leaf-like structures (phyllody)
(Fig. 1a, b); the sepals were green although other floral
organs remained normal (virescence) (Fig. 1c); and a new
shoot formed from the center of the flower organ (proliferation)
(Fig. 1d). These symptoms have also been
observed on hydrangeas infected by ‘Ca. P. japonicum’
(Kanehira et al. 1996). Hydrangea growers in Gunma
Prefecture observed similar symptoms in 2010, but the
hydrangea was discarded without further analysis. In this
study, we performed a series of experiments to diagnose
the disease as described below.