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How to Manage Pests
UC Pest Management Guidelines
Citrus
Phytophthora Gummosis
Pathogen: Phytophthora spp.
(Reviewed 9/08,
updated 9/08)
In this Guideline:
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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.
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 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 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 |
R.E.I.+ |
P.H.I.+ |
| (trade name) |
|
(hours) |
(days) |
|
| When choosing a pesticide, consider the general properties of the fungicide as well as information relating to environmental impact.
|
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| PREPLANT
FUMIGATION |
| A. |
METAM SODIUM* |
75–100 gal/acre |
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...or... |
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(Vapam, Metam Sodium) |
16 fl oz/tree (8 ft diameter canopy) |
48 |
0 |
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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 |
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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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| POSTPLANT |
| A. |
COPPER# |
Label rates |
24 |
0 |
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MODE OF ACTION GROUP
NAME (FRAC NUMBER1): Multi-site contact (M1) |
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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. |
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| B. |
FOSETYL-AL |
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(Aliette) 80WDG |
2.5–5 lb/5 gal water |
12 |
30 |
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MODE OF ACTION GROUP
NAME (FRAC NUMBER1): Phosphonate (33) |
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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/year. |
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| C. |
MEFENOXAM |
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(Ridomil Gold) SL |
1 qt/3 gal water |
48 |
0 |
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MODE OF ACTION GROUP
NAME (FRAC NUMBER1): Phenylamide (4) |
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COMMENTS: Use when
disease occurs. Spray the surface of trunks to cover lesions thoroughly. Can
be applied up to 3 times/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/year. |
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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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