Agriculture: Apricot Pest Management Guidelines

Phytophthora Root and Crown Rot

  • Phytophthora spp.
  • 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 (unhealthy) 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 shorter, more frequent 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

    To effectively manage Phytophthora root and crown rot:

    • Select a good planting site.
    • Select an appropriate rootstock.
    • Plant trees on a slight mound or berm to promote drainage away from the crown.
    • 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 offer 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 per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    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. Do not apply more than 5 lbs/acre per application.
     
    B. PHOSPHOROUS ACID
      (ProPhyt, FungiPhite) 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 SL) Rate varies with method of application and size of tree 48 0
      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 (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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. 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, 7, 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.
    Text Updated: 10/14
    Treatment Table Updated: 10/14