How to Manage Pests

UC Pest Management Guidelines

Citrus

Phytophthora Root Rot

Pathogens: Phytophthora citrophthora and P. parasitica

(Reviewed 9/08, updated 9/08)

In this Guideline:


Symptoms

Phytophthora root rot causes a slow decline of the tree. 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 root rot that also causes brown fruit rot and gummosis. Phytophthora citrophthora is active during cool seasons 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

Cultural controls and the use of resistant rootstocks are acceptable management methods 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 P. parasitica during July through September, and P. citrophthora during January through March. Phytophthora populations of greater than 15 to 20 propagules per gram of root zone soil may warrant treatment. When planting or replanting in soil infested with Phytophthora, or when susceptible rootstock has to be used, fumigation may be feasible if no other adverse conditions persist.

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
 
A. METAM SODIUM* 75–100 gal/acre 48 0
    . . . or . . .  
  (Vapam, Metam Sodium) 16 fl oz/tree (8 ft diameter canopy)    
  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.
 
NONBEARING TREES
 
A. MEFENOXAM 1–1.5 fl oz/100 gal water for soil drench    
    . . . or . . .  
  (Ridomil Gold) SL 1–2 qt/acre for soil surface spray 48 0
  MODE OF ACTION GROUP NAME (FRAC NUMBER1): Phenylamide (4)
  COMMENTS: For citrus in nurseries: Apply at planting and at 3-month intervals during growing seasons. As a drench, apply 100–250 gal mixture/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 3 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) 80WDG 5 lb/100 gal per acre 12 365
  MODE OF ACTION GROUP NAME (FRAC 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 treated at time of planting. Spray to wet. Do not exceed 4 applications per year or 20 lb/acre per year.
 
BEARING TREES
 
A. MEFENOXAM 1–2 qt/acre    
    . . . or . . .  
  (Ridomil Gold) SL 0.75–1.5 fl oz/1000 sq ft 48 0
  MODE OF ACTION GROUP NAME (FRAC NUMBER1): Phenylamide (4)
  COMMENTS: Apply 2–3 times per year to coincide with flushes of root growth. Apply in a banded surface spray under tree canopy. Up to 3 applications may be made per year.
 
B. MEFENOXAM
  (Ridomil Gold) GR Label rates 48 0
  MODE OF ACTION GROUP NAME (FRAC NUMBER1): Phenylamide (4)
  COMMENTS: Treat in March-April followed by 1 or 2 applications at 3-month intervals to coincide with root flushes; rate depends on tree size and the number of applications per year. Apply 0.5–1 inch water after application.
 
C. FOSETYL-AL
  (Aliette) 80WDG 5 lb/acre 12 30
  MODE OF ACTION GROUP NAME (FRAC NUMBER1): Phosphonate (33)
  COMMENTS: Apply to susceptible varieties as a foliar spray when conditions favor the disease. Spray to wet. Do not exceed 4 applications or 20 lb/acre per year. Do not apply within 30 days of harvest. Do not allow livestock to graze in treated citrus groves.
 
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.
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

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