Agriculture: Almond 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 on these 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. Surface water from irrigation districts is mostly contaminated with Phytophthora species. 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, plum rootstocks are more resistant than are peach or peach-almond hybrids. Of the plum rootstocks, Marianna 2624 is the most tolerant to Phytophthora.

    Management

    Proper water management is the most important aspect in controlling root and crown rot.

    • Do not allow water to accumulate or stand around crowns of trees.
    • Provide adequate drainage to
      • low spots in the orchard,
      • areas that flood frequently, and
      • places where water penetration is extremely poor.
    • Leave areas without adequate drainage unplanted.

    If you are replanting an area where Phytophthora is present, either plant trees on small mounds, as shallowly as possible, or on broad ridges with the upper roots near the soil level. Establish berms before planting; the ridges should be 8 to 10 inches (20–25 cm) high. Planting depth after settling should be no deeper than in the nursery, and the graft union should be well above the soil line.

    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. Foliar spray, 60-day interval.
     
    B. MEFENOXAM
      (Ridomil Gold SL) Label rates 48 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)
      COMMENTS: Rate varies with method of application and size of tree. Make applications in early spring and fall.
     
    C. PHOSPHOROUS ACID
      (Fosphite) 1–2 qt 4 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)
      COMMENTS: For use as a foliar or soil treatment. Note: Maximum residue levels (MRLs) may restrict the use of these fungicides.
    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 action. 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 a fungicide with a different mode-of-action group number.
    Text Updated: 08/17
    Treatment Table Updated: 08/17