How to Manage Pests
UC Pest Management Guidelines
Phytophthora Root and Crown Rot
Pathogen: Phytophthora spp.
(Reviewed 12/07, updated 4/09)
In this Guideline:
SYMPTOMS AND SIGNS
Symptom expression of Phytophthora root and crown rot 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. 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.
COMMENTS ON THE DISEASE
Periods of 24 hours or more of water-saturated soil favor Phytophthora infections. Conversely, good soil water drainage reduces 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. In general, Paradox rootstock is more tolerant of several Phytophthora species than is Northern California black walnut or English walnut.
The most effective management strategies for Phytophthora on walnut include careful management of soil water, prudent rootstock selection, and good general sanitation practices. Plant on berms. Avoid soil compaction. Do not allow irrigation water to stand for more than 24 hours. Pulse irrigation at short durations is best. If using sprinklers, do not allow water to splash on trees (use water guards). Use practices that promote good water infiltration and penetration.
Eradicating Phytophthora from orchard soil is generally not possible. Fumigating the soil after you have removed a diseased tree typically results in a beneficial though incomplete reduction in the population of Phytophthora in the soil. The population can rebuild quickly under conditions conducive to disease. Check with your local Cooperative Extension farm advisor concerning the effectiveness and availability of fumigants.
In soils infested with Phytophthora, use Paradox rootstock. Paradox is significantly more resistant to P. cactorum, P. citrophthora, P. drechsleri, and P. megasperma, and is somewhat more resistant to P. citricola and P. cinnamomi than Northern California Black rootstock or English rootstock. In orchards with a history of Phytophthora, treatment may be warranted.
UC IPM Pest Management Guidelines:
Acknowledgement for contributions to Diseases: