µ Ariake National College of Technology, Omuta 836 8585, Japan 1) Forestry Research and Instruction Station of Kumamoto Prefecture, Kumamoto 860 0862, Japan 2) Kumamoto University, Kumamoto 860 8555, Japan author’s e-mail: tsuka@ariake-nct.ac.jp ! " # et al. 5 10 15 20 25 30 35 30kV (mushroom) 45kV (mushroom) 90kV (mushroom) 0kV (mushroom) 30kV (unwanted bacteria) 45kV (unwanted bacteria) 90kV (un