Agriculture: Citrus Pest Management Guidelines

Phytophthora Root Rot

  • Phytophthora citrophthora and P. parasitica
  • Symptoms and Signs

    Phytophthora root rot causes a slow decline of the tree, especially in new plantings. The leaves turn light green or yellow and may drop, depending on the amount of infection. The disease destroys the feeder roots of susceptible rootstocks. The pathogen infects the root cortex, which turns soft and separates from the stele. If the destruction of feeder roots occurs faster than their regeneration, the uptake of water and nutrients will be severely limited. The tree will grow poorly, stored energy reserves will be depleted, and production will decline.

    Disease symptoms are often difficult to distinguish from nematode, salt, or flooding damage; only a laboratory analysis can provide positive identification.

    Comments on the Disease

    Phytophthora species are present in most citrus groves. They can survive adverse conditions as persistent spores in the soil. During moist conditions, large numbers of motile zoospores, which can swim in water for short distances, are produced. Zoospores are the infective agents that are carried in irrigation or rainwater to the roots.

    Phytophthora citrophthora is a winter and summer root rot that also causes fruit brown rot and gummosis. Phytophthora citrophthora is most damaging when citrus roots are inactive and their resistance to infection is low.

    Phytophthora parasitica is active during warm weather when roots are growing.

    Management

    Management of Phytophthora root rot involves the use of resistant rootstocks, irrigation management, fungicides, and fumigation.

    Cultural Control

    Provide adequate soil drainage and avoid over irrigation. If destruction of feeder roots is minimal, corrective action may include increasing irrigation intervals, switching to alternate middle row irrigation or a different irrigation system such as mini sprinklers, and installing subsoil tiles.

    Resistant Rootstocks

    When replanting or establishing new plantings, choose resistant rootstocks where possible, but also consider tolerance to other diseases, nematodes, and cold. The most tolerant rootstocks are trifoliate orange, swingle citrumelo, citrange, Alemow, and sour orange.

    Organically Acceptable Methods

    Use cultural controls and resistant rootstocks in an organically managed citrus grove.

    Monitoring and Treatment Decisions

    If a tree growing on susceptible rootstock looks stressed, dig up some soil and check the feeder roots. Sample for P. parasitica during July through September, and P. citrophthora throughout the year:

    • Randomly select 20 to 40 locations within a 10-acre orchard block with mild to moderate expected Phytophthora tree decline.
    • Sample from aroung the tree drip line or near irrigation emitter where roots are concentrated.
    • Put composite samples in a sealed plastic bag, but do not refrigerate or overheat.
    • Ship within 24 to 48 hours to a lab where propagule count per unit of soil and root infestation are determined.
    • Phytophthora populations of more than 15 to 20 propagules per gram of root zone soil may warrant a pesticide application. When planting or replanting in soil infested with Phytophthora, or when a susceptible rootstock has to be used, fumigation may be feasible if no other adverse conditions persist.
    Common name Amount to use 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.
    PREPLANT
    A. METAM SODIUM*§
      (Vapam) 75 gal/acre See label NA
        . . . or . . .
        16 fl oz/tree (8 ft diameter canopy) See label NA
      COMMENTS: Apply with 6 to 12 inches of water. Do not plant for at least 45 days. Fumigants such as metam sodium are a prime source of volatile organic compounds (VOCs), which are a major air quality issue.
     
    B. CHLOROPICRIN*§
      (Chloropicrin 100) 350 lb/acre See label NA
        . . . or . . .
        16 oz/tree (8 ft diameter canopy) See label NA
      COMMENTS: Use lower rate on sandy loam and high rate on heavier soils or high clay. Inject 8 to 10 inches deep, 12 to 18 inches apart, and tarp immediately. Do not plant for at least 3 months.
     
    NONBEARING TREES
    A. MEFENOXAM      
      (Ridomil Gold SL) 1–1.5 fl oz/100 gal water for soil drench See label See label
        . . . or . . .
        1–2 qt/acre for soil surface spray 48 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)
      COMMENTS: For citrus in nurseries: Apply at planting and at 3-month intervals during growing season. As a drench, apply 100 to 250 gal mixture per 1000 ft of row on an area wide enough to cover the root system. As a soil surface spray, apply as a broadcast or banded surface spray to seedbeds, liners, or bedded stock in sufficient water to obtain uniform coverage of the root system. For use on resets or new plantings: Apply at planting and up to three applications at 3-month intervals to coincide with root growth flushes during the growing season. As a drench, apply 5 gal mix around tree base within the watering ring. As a soil surface spray, apply in sufficient water to obtain coverage of the soil surface wetted by irrigation. Apply spray to the soil surface beneath the tree canopy. Follow immediately with an irrigation sufficient to wet the soil to 1 ft.
     
    B. FOSETYL-AL
      (Aliette WDG) 5 lb/100 gal per acre 12 365 (1 year)
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)
      COMMENTS: For use on trees in nurseries only. Apply in 100 gal water/acre to susceptible varieties as a foliar spray when conditions favor the disease. Trees should be sprayed to wet at the time of planting. Do not exceed four applications per year or 20 lb/acre per year.
     
    C. POTASSIUM PHOSPHITE
      (K-Phite 7LP) See label 4 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)
      COMMENTS: For use on all susceptible citrus. Apply in 100 to 250 gal/acre; spray to wetness when conditions favor disease development. Do not exceed four applications of this product per year.
     
    BEARING TREES
    A. MEFENOXAM
      (Ridomil Gold SL) 1–2 qt/acre 48 0
        . . . or . . .
        0.75–1.5 fl oz/1000 sq ft 48 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)
      COMMENTS: Apply two to three times per year to coincide with flushes of root growth. Apply in a banded surface spray under tree canopy. Up to three applications may be made per year.
     
    B. MEFENOXAM
      (Ridomil Gold GR) Label rates 48 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)
      COMMENTS: Apply in March to April followed by one or two applications at 3-month intervals to coincide with root flushes; rate depends on tree size and the number of applications per year. Apply 0.5 to 1 inch water after application.
     
    C. FOSETYL-AL
      (Aliette WDG) 5 lb/acre 12 30
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)
      COMMENTS: Apply to susceptible varieties as a foliar spray when conditions favor the disease. Spray to wet. Do not exceed four applications or 20 lb/acre per year. Do not allow livestock to graze in sprayed citrus groves.
     
    D. POTASSIUM PHOSPHITE
      (Prophyt) 4 pt 4 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)
      COMMENTS: For use on all susceptible citrus. Apply in 100 to 250 gal/acre; spray to wetness when conditions favor disease development. Do not exceed four applications of this product per year.
     
    E. OXATHIAPIPROLIN
      (Orondis) 2–9.6 fl oz/acre 4 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Oxy-sterol-binding protein inhibitor (49)
      COMMENTS: For use on all susceptible citrus. Apply in 100 to 400 gal/acre; spray to wetness when conditions favor disease development. Minimal re-application interval is 30 days. Do not make more than two applications of this product per year and do not use more than 19.2 fl oz/acre per year. Do not make more than two sequential applications before rotating to another mode of action. When three or more applications are needed for disease management, do not apply this product more than 33% of the total number of applications. May be applied as a soil or trunk spray or by chemigation.
    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.
    * Permit required from county agricultural commissioner for purchase or use.
    § Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.
    No information
    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. 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.
    Text Updated: 01/19
    Treatment Table Updated: 01/19