First report of poor growth and root rot caused by Phytopythium helicoides on shiso plants in a deep flow technique hydroponic system

In 2023 in Aichi Prefecture, shiso plant growth in a deep flow technique (DFT) hydroponic system was delayed from mid-March to late April with roots showing evidence of rotting. An oomycete species was isolated from the roots and identified as Phytopythium helicoides based on morphological characteristics and DNA sequence homology. Identical symptoms appeared on shiso plants after inoculating seedlings with zoospores from the strain, and the same oomycete species was reisolated from the diseased plants. This study is the first report in the world of root rot caused by P. helicoides in shiso in a DFT hydroponic system.