How to Manage Pests

UC Pest Management Guidelines

Walnut

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.

MANAGEMENT

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.

Common name   R.E.I.+ P.H.I.+
(trade name) Amount/Acre (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to environmental impact.
       
A. PHOSPHOROUS ACID    
  (Fosphite) 1–2 qt/100 gal water/acre 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
  COMMENTS: Most effective as a foliar spray; apply at 2- to 4-week intervals after trees become established. Early fall treatments, before leaves begin to senesce and trees are actively translocating from leaves to roots, are most effective. If treatments are applied earlier in season, best applied after foliage has fully emerged. 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. Thought to provide protection by either systemic acquired resistance or increasing ability of the plant to produce its own defense chemicals.
       
B. MEFENOXAM    
  (Ridomil Gold EC) see comments 48 0
  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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

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 County
  • G. T. Browne, USDA Crops Pathology and Genetics, UC Davis
  • W. D. Gubler, Plant Pathology, UC Davis
Acknowledgement for contributions to Diseases:
  • B. L. Teviotdale, Kearney Agricultural Center, Parlier

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