Phytophthora is a natural soil-borne pathogen.
Drenth and Guest (2004) reported that rain splashes
can carry Phytophthora propagules up to 0.75 m
above the ground. Kuske and Benson (1983) reported
a 7% mortality of Rhododendron seedlings raised on
pine-bark beds 5–7 cm above the ground compared
with gravel beds. The gravel reduces the rain splashes
from the soil that carry inoculum of Phytophthora
parasitica.