Phytophthora of Tree Fruit

Wet weather continues to be a problem, not just for foliar disease, but root and crown rot, as well. Phytophthora root and crown rot is observed in wet spots: low-lying areas of orchards with heavy, poorly-drained soils. These pathogens can infect all fruit producing plants. Occurrence of this disease is sporadic and tied to wet weather, but unfortunately, is likely to occur wherever susceptible rootstock is planted on a poorly drained site. The disease is caused by members of the genus Phytophthora, a type of ‘water mold’ and not a ‘true fungus’(Fig. 1). Phytophthora species prefer cool, wet weather, like what we have been experiencing now. Phytophthoras can infect the roots, the crown and the graft union. Of these, collar rot is more lethal and can cause rapid tree death, especially if it is a small tree. In apples, Phytophthora crown rot is commonly observed on 3- to 8-year-old trees grown M.7, MM 104, MM.106, and even MM.111 rootstocks; B.9 is considered resistant, but do not equate resistance with immunity! Excess nitrogen can make resistant rootstocks susceptible, and susceptible rootstocks really susceptible.