How to Manage Pests
UC Pest Management Guidelines
Phytophthora Root and Crown Rot
Pathogen: Phytophthora spp.
(Reviewed 11/09, updated 11/09, pesticides updated 9/15)
In this Guideline:
Symptoms and Signs
Symptom expression depends upon how much of the root or crown tissues are affected and how quickly they are destroyed. Generally, crown rots advance rapidly and trees collapse and die soon after the first warm weather of spring. Leaves of such trees wilt, dry, and remain attached to the tree. Chronic infections, usually of the roots, cause reduction in growth and early senescence and leaf fall. These trees may be unthrifty for several years before succumbing to the disease. Phytophthora infections typically kill young trees because their root systems and crown areas are small compared to those of mature trees.
Comments on the Disease
Periods of 24 hours or more of saturated soil favor Phytophthora infections. Conversely, good soil drainage and more frequent but shorter irrigations reduce the risk of root and crown rot. Rootstocks vary in susceptibility to the different Phytophthora species; none are resistant to all pathogenic species of the fungus. Thus, the success of a rootstock may depend in part upon the species of Phytophthora present in the orchard. Mazzard and Colt rootstocks are more resistant than is Mahaleb.
Avoid locations with a history of Phytophthora root and crown rot, especially when planting susceptible rootstocks. Plant new orchards on berms to improve drainage at the crown area and design the irrigation system so that the trunk and crown of the tree is not wet by sprinklers. You can use various methods to reduce the soil population of the pathogen, but you cannot eliminate it. Be sure to verify that Phytophthora is the causal agent before treating a new planting with fungicides, because a number of factors or pest problems can cause poor growth and death of trees.
UC IPM Pest Management Guidelines:
J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:W. D. Gubler, Plant Pathology, UC Davis
B. L. Teviotdale, Kearney Agricultural Center, Parlier