Agriculture: Citrus Pest Management Guidelines

Phytophthora Gummosis

  • Phytophthora spp.
  • Symptoms and Signs

    An early symptom of Phytophthora gummosis is sap oozing from small cracks in the infected bark, giving the tree a bleeding appearance. The gumming may be washed off during heavy rain. The bark stays firm, dries, and eventually cracks and sloughs off. Lesions spread around the circumference of the trunk, slowly girdling the tree. Decline may occur rapidly within a year, especially under conditions favorable for disease development, or may occur over several years.

    Comments on the Disease

    Phytophthora fungi are present in almost all citrus orchards. Under moist conditions, the fungi produce large numbers of motile zoospores, which are splashed onto the tree trunks. The Phytophthora species causing gummosis develops rapidly under moist, cool conditions. Hot summer weather slows disease spread and helps drying and healing of the lesions.

    Secondary infections often occur through lesions created by Phytophthora.

    Management

    Management of Phytophthora gummosis focuses on preventing conditions favorable for infection and disease development. All scion cultivars are susceptible to infection under the right environmental conditions.

    Cultural Control

    Plant trees on a berm or high enough so that the first lateral roots are just covered with soil. Correcting any soil or water problems is essential for a recovery. In addition to improving the growing conditions, the following can halt disease spread

    1. Remove the dark, diseased bark and a buffer strip of healthy, light brown to greenish bark around the margins of the infection.
    2. Allow the exposed area to dry out. You can also scrape the diseased bark lightly to find the perimeter of the lesion and then use a propane torch to burn the lesion and a margin of 1 inch (2.5 cm) around it.
    3. Recheck frequently for a few months and repeat if necessary.

    Organically Acceptable Methods

    Use cultural controls and copper sprays on organically certified citrus.

    Monitoring and Treatment Decisions

    Late stages of Phytophthora gummosis are distinct, but early symptoms are often difficult to recognize. Yet early detection and prompt management actions are essential for saving a tree. If 50% or more of a trunk or crown region on a mature tree is girdled, it may be more economical to replace the tree than to try to control the infection.

    When establishing a new orchard, carefully check the lower trunk and rootstock of new trees for any symptoms of gummosis before you plant. When trees are wrapped in burlap, open and inspect a representative sample (at least 10% of the trees). When planting or replanting in soil infected with Phytophthora, or when a susceptible rootstock has to be used, fumigation may be helpful.

    Inspect your orchard several times a year for disease symptoms. Look for signs of gumming on the lower trunk and crown, and for soil buildup around the crown; do not allow bud unions to get buried. Wrappers on young trees should be lifted or removed for inspection. When you detect gum lesions, check soil and drainage conditions.

    Systemic fungicides can control Phytophthora gummosis and copper sprays can be used to protect against infection.

    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 FUMIGATION
    A. METAM SODIUM*§
      (Vapam HL) 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.
     
    POSTPLANT
    A. COPPER#
      (various products) Label rates 24 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
      COMMENTS: Use neutral, spray-dried, one package copper spray materials stirred into water to make consistency of house paint. Apply as paint or spray on trunk and crown right after excision of diseased bark; treat excised area and lower trunk. Can also be used as a protectant on trees where risk of gummosis is high. Not all copper compounds are approved for use in organic production; be sure to check individual products.
     
    B. FOSETYL-AL
      (Aliette WDG) 2.5–5 lb/acre in 5 gal water 12 30
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonate (33)
      COMMENTS: Spray or paint on trunk when disease occurs or conditions favor disease development. Use higher rate if trunk lesions are present. Thoroughly wet the lesion. If no lesion is present, wet the trunk from the ground up to a height of 2 feet. Do not exceed four applications of fosetyl-al per year.
     
    C. MEFENOXAM
      (Ridomil Gold SL) 1–2 qt/acre (soil application)
    1 qt/3 gal water (trunk spray)
    48 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4)
      COMMENTS: Use when disease occurs. Spray the surface of trunks to cover lesions thoroughly. Can be applied up to three times per year, but do not make soil and trunk applications of mefenoxam to the same tree during the same cropping season and do not apply more than 1.5 gal/treated acre per year.
     
    D. 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.
     
    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.
    # Acceptable for use on organically grown produce.
    § 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.
    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.
    NA Not applicable
    Text Updated: 01/19
    Treatment Table Updated: 01/19