Amendment of potato-dextrose agar with grapefruit extract at dose 40μg/cm3 inhibited linear growth of Phytophthora cryptogea about 50%. At such concentration of the product in soil leachate formation of zoosporangia was inhibited about 95%. Application of grapefruit extract as peat drench reduced population density of the pathogen about 80% and this high activity was observed at least 30 days.

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.