A soilborne water mold that spreads through the movement of water in the soil or on the surface, affecting conifers. New infections can occur when the temperatures exceed 59°F. Resting spores can survive for many years in the soil or plant and form during cold and/or dry periods. When required temperature and moisture conditions are present, resting spores germinate and form another type of spore-producing structure called a sporangia.

When mature, numerous motile, infectious spores (Zoospores) are released. Zoospores are attracted to the plant roots by chemicals produced during growth. When they come in contact with susceptible tissue, they germinate, penetrate into the roots, form mycelium, and cause infection. As the mycelium continues to develop inside of the roots, the roots will die and turn brown.

The fungus spreads from the outer roots toward the larger roots, the root crown, and eventually the stem. The conductive tissue of the plants will decay, preventing the flow of water and nutrients to the upper portion of the tree. Needles will first turn chlorotic and then a reddish brown, while branches wilt. The infection leads to death of the tree.