First Report of Stem Rot on Hydrangea Caused by Phytophthora hedraiandra in Japan

A stem rot disease appeared on hydrangea plants in Tochigi Prefecture, Japan. Four Phytophthora-like isolates were obtained, and a representative isolate was selected for analysis. Hydrangea plants inoculated with the isolate developed similar symptoms. Molecular phylogenetic analyses using the rDNA ITS region and cox1, EF1-α, and β-tubulin genes indicated that the isolate was Phytophthora hedraiandra. The sporangia were papillate, laterally sessile, and caducous. Antheridia were paragynous or amphygynous, and laterally sessile, and the oospores were aplerotic. Based on these characteristics, the isolate was identified as P. hedraiandra. We suggest naming this hydrangea disease “Ajisai kukieki byo”.