This is the same organism that causes sprinkler rot on fruit. Several different species of phytophthora can cause crown rot, but the most common in Washington is Phytophthora cactorum. The fungus survives the winter as a thick-walled oospore in the soil. It is also found in the silt in canal bottoms and is carried to the orchard in irrigation water. In wet conditions, the oospores germinate to produce sporangia, which release zoospores into the water in the soil.
Apple trees are susceptible to infection between pink stage and shoot elongation.
Grove said when growers plant new orchards they should avoid poorly drained sites. The choice of rootstock is important, as some rootstocks are more susceptible than others. Malling 9 is resistant, M.7 and M.111 are moderately resistant, Mark may be susceptible, and M.26, M.104, and M.106 are susceptible. Grove noted that M.106 is also highly susceptible to fireblight. Grove cautioned growers against overwatering, which gives the zoospores more opportunity to swim up to the roots and infect the trees.