How to Manage Pests

UC Pest Management Guidelines

Citrus

Phytophthora Gummosis

Pathogen: Phytophthora spp.

(Reviewed 9/08, updated 9/08)

In this Guideline:


Symptoms

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 develop 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. These infections kill and discolor the wood, in contrast to Phytophthora infections, which do not discolor wood.

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, you can halt disease spread by removing the dark, diseased bark and a buffer strip of healthy, light brown to greenish bark around the margins of the infection. 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. Recheck frequently for a few months and repeat if necessary.

Organically Acceptable Methods

Cultural controls and copper treatments are acceptable for use on organically certified citrus.

Monitoring and Treatment Decisions

Late stages of Phytophthoragummosis 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 R.E.I.‡ P.H.I.‡
(example 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. Not all registered pesticides are listed. Always read label of product being used.
 
PREPLANT FUMIGATION
 
A. METAM SODIUM* 75–100 gal/acre    
    . . . or . . .    
  (Vapam, Metam Sodium) 16 fl oz/tree (8 ft diameter canopy) 48 0
  COMMENTS: Apply with 6–12 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. Fumigate only as a last resort when other management strategies have not been successful or are not available.
 
B. CHLOROPICRIN* 400–500 lb/acre 48 0
    . . . or . . .    
    16 oz/tree (8 ft diameter canopy)    
  COMMENTS: Use low rate on sandy loam and high rate on heavier soils or high clay. Inject 7–9 inches, 12–18 inches apart, and tarp immediately. Do not plant for at least 3 months.
 
POSTPLANT
 
A. COPPER# Label rates 24 0
  MODE OF ACTION GROUP NAME (FRAC NUMBER1): Multi-site contact (M1)
  COMMENTS: Use neutral, spray-dried, 1 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) 80WDG 2.5–5 lb/5 gal water 12 30
  MODE OF ACTION GROUP NAME (FRAC 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 4 applications of fosetyl-al per year.
 
C. MEFENOXAM
  (Ridomil Gold) SL 1 qt/3 gal water 48 0
  MODE OF ACTION GROUP NAME (FRAC NUMBER1): Phenylamide (4)
  COMMENTS: Use when disease occurs. Spray the surface of trunks to cover lesions thoroughly. Can be applied up to 3 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.
 
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.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on organically grown produce.
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. 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441

Diseases

  • J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:
  • J. A. Menge, Plant Pathology, UC Riverside
  • H. D. Ohr, Plant Pathology, UC Riverside

Top of page


Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2014 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California

Accessibility   /PMG/r107100411.html revised: April 25, 2014. Contact webmaster.