Amendment of VS juice agar and soil leachate with grapefruit extract resulted in the inhibition of colony growth and sporulation of Phytophthora ramorum. Zoosporangia were more susceptible to the extract than pathogen hyphae and chlamydospores. Spraying of rhododendron inoculated with P. ramorum with grapefruit extract at cone. 165 μglcm' inhibited 2-3 times the spread of necrosis on stems and leaves.