UC IPM: UC Management Guidelines for Phytophthora Root and Crown Rot on Walnut

UC Pest Management Guidelines

Aboveground symptoms of Phytophthora crown and root rot.

Walnut

Phytophthora Root and Crown Rot

Pathogen: Phytophthora spp.

(Reviewed 12/07, updated 12/07)

In this Guideline:

Symptoms
Comments on the disease
Management

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.

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. (For more information, see Phytophthora Crown and Root Rot of Walnut Trees, UC ANR Publication 21509, which may be purchased online.)

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.

Common name

Amount/Acre

R.E.I.+

P.H.I.+

(trade name)

(hours)

(days)

When choosing a pesticide, consider information relating to environmental impact.

PHOSPHOROUS ACID

(Fosphite)

1–2 qt/100 gal water/acre

MODE OF ACTION GROUP NAME (NUMBER¹): Phosphonate (33)

COMMENTS: A foliar spray; apply at 2- to 4-week intervals after trees become established. Do not apply more than 6 times per crop cycle or year. Do not apply with copper-based fungicides or fertilizers. Do not apply to trees that are heat or moisture stressed. Research done on almonds, but control appears promising for walnuts. Thought to provide protection by either systemic acquired resistance or increasing ability of the plant to produce its own defense chemicals.

MEFENOXAM

(Ridomil Gold EC)

see comments

MODE OF ACTION GROUP NAME (NUMBER¹): Phenylamide (4)

COMMENTS: Application rate varies with method of application and size of tree. Begin applications before trees begin to grow in the spring (early March), again in mid-summer (July), and before the second root growth flush in late August. Do not apply to trees within 90 days of planting. The effectiveness of this material may not warrant its use and expense.

¹	Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
R. P. Buchner, UC Cooperative Extension, Tehama Co.
G. T. Browne, USDA Crops Pathology and Genetics, UC Davis
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgement for contributions to the disease section:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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How to Manage Pests

UC Pest Management Guidelines

Walnut

Phytophthora Root and Crown Rot

Pathogen: Phytophthora spp.

SYMPTOMS

COMMENTS ON THE DISEASE

MANAGEMENT

PUBLICATION