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How to Manage Pests
UC Pest Management Guidelines
Citrus
Phytophthora Root Rot
Pathogens: Phytophthora citrophthora and P. parasitica
(Reviewed 07/03,
updated 07/03)
In this Guideline:
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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 overirrigating. 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 minisprinklers, 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 |
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| (trade name) |
Amount to Use
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PREPLANT |
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| A. |
METAM SODIUM* |
100 gal/acre |
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...or...
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(Vapam) |
16 fl oz/tree (8 ft diameter canopy) |
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COMMENTS: Apply with 6-12 inches of water. Do not plant
for at least 45 days. |
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| B. |
METHYL BROMIDE* |
350-450 lb/acre |
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...or...
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8 oz/tree (8 ft diameter canopy) |
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COMMENTS: Use low rate on sandy loam and high rate on
heavier soils with high clay content. Inject 7-8 inches, 12-18 inches
apart, and tarp immediately. Do not plant for at least 1 month. |
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| C. |
CHLOROPICRIN* |
400-500 lb/acre |
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...or...
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16 oz/tree (8 ft diameter canopy) |
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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. |
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NONBEARING TREES |
| A. |
MEFENOXAM |
1-1.5 fl oz/100 gal water for soil drench |
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...or...
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(Ridomil Gold) EC |
1-2 qt/acre for soil surface spray |
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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. |
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| B. |
FOSETYL-AL |
5 lb/100 gal/acre |
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(Aliette) 80WDG |
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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/year or 20 lb/acre/year. |
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BEARING TREES |
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| A. |
MEFENOXAM |
1-2 qt/acre |
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...or...
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(Ridomil Gold) EC |
0.75-1.5 fl oz/1000 sq ft |
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COMMENTS: Apply 2-3 times/year to coincide with flushes
of root growth. Apply in a banded surface spray under tree canopy.
Up to 3 applications may be made/year. |
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| B. |
MEFENOXAM |
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(Ridomil Gold) GR |
Label rates |
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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/year. Apply 0.5-1 inch water
after application. |
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| C. |
FOSETYL-AL |
5 lb/acre |
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(Aliette) 80WDG |
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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/year. Do not apply within 30 days of harvest. Do not
allow livestock to graze in treated citrus groves. |
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| * |
Permit required from county agricultural
commissioner for purchase or use. |
UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Diseases
H. D. Ohr, Plant Pathology, UC Riverside
J. A. Menge, Plant Pathology, UC Riverside
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