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The tree on the right is dying from Phytophthora root rot.

Apricot

Phytophthora Root and Crown Rot

Pathogen: Phytophthora spp.

(Reviewed 11/07, updated 11/07)

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, but Marianna 2624 and Myrobalan 29C rootstocks are somewhat resistant. The success of a rootstock may depend in part upon the species of Phytophthora present in the orchard.

MANAGEMENT

The most effective ways to manage Phytophthora root and crown rot are to select a good planting site, select an appropriate rootstock, plant trees on a slight mound or berm to promote drainage away from the crown, and properly manage irrigation water. Avoid overirrigating in spring and fall when soil temperatures are most conducive to disease development and water use by the tree is low. Fungicides are available to treat soil around newly planted trees. Fall and/or spring foliar sprays with a phosphonate product offers suppression of Phytophthora during the critical fall, winter, and spring periods. If there is a history of Phytophthora root rot in the orchards and problems are anticipated, treatments may be warranted.

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

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider the general properties of the fungicide as well as information relating to environmental impact.
 
A. FOSETYL-AL
  (Aliette) WDG 5 lb/100 gal 12 365
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
  COMMENTS: For use on nonbearing trees only. Apply as a foliar spray, at 60-day intervals.
 
B. PHOSPHORUS ACID
  (Fosphite) Label rates 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
  COMMENTS: Can be used on bearing trees. Do not apply with copper-based products and allow 10 days before or 20 days after an application of copper before applying this product.
 
C. MEFENOXAM
  (Ridomil Gold) EC Varies with method of 48 0
    application and size of tree    
  MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
  COMMENTS: Applications made in early spring and fall. Do not apply within 90 days of planting trees.
 
 
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 two intervals is the minimum time that must elapse before harvest.
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. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Apricot
UC ANR Publication 3433
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
W. D. Gubler, Plant Pathology, UC Davis
W. W. Coates, UC Cooperative Extension, San Benito County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
J. L. Caprile, UC Cooperative Extension, Contra Costa County
B. A. Holtz, UC Cooperative Extension, Madera County
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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