Phytophthora cactorum is the fungus most often associated with collar rot of apple. These spores are capable of surviving extended periods of time in soil. Under moist conditions they germinate forming another type of structure called a sporangium. A sporangium is a minute reproductive structure produced by the fungus in response to free water. Sporangia germinate in the presence of free water releasing numerous one-celled motile spores called zospores.
Zoospores swim through water, contact roots of susceptible hosts, germinate, and infect.
P. cactorum and other Phytopthora species are present in most of our irrigation canals, where the biology of them is poorly understood. The latter three are usually confined to stone fruits. Most rootstock collar rot resistance ratings have been developed by evaulation for resistance to P. cactorum . Our experience with rootstocks and collar rot in Washington reveals that M.26, M.104, and M.106 are highly suceptible, M.2, M.7, and M. 111 are moderately resistanst, and M.4 and M.9 are highly resisant. However, even M.9 occasionally gets collar rot.