Citrus gummosis (Phytophthora foot rot)

Phytophthora citrophthora and Phytophthora nicotianae P. citrophthora (R.E. Sm. & E.H. Sm.) Leonian, 1925; P. nicotianae Breda de Haan, 1896 (with P. palmivora in some regions) · formerly or also: Phytophthora parasitica (= P. nicotianae)

gomosis de los cítricos · Spanish, Spain and Latin America
gomose dos citros · Portuguese, Brazil
gommose à Phytophthora des agrumes · French, France and North Africa
柑橘脚腐病 (gānjú jiǎofǔ bìng) · Chinese, East Asia

Gummosis is a rot of the bark at the base of citrus trees caused by soil-borne Phytophthora water moulds. Dark, cracked bark oozes amber gum, the rot spreads around the trunk and into the roots, and trees yellow, drop leaves and decline - or die when the trunk is girdled. The same pathogens rot fibrous roots and cause brown rot of fruit near the ground. It is a disease of wet soil, so drainage, rootstock choice and keeping water off the trunk are the main defences.

Type: Oomycete disease · Oomycete (water mould), not a true fungus
Classification: Stramenopila (Chromista) › Oomycota › Peronosporales › Peronosporaceae
Main crops: Orange, Mandarin & tangerine, Lemon & lime, Grapefruit and pomelo
Found in: Mediterranean and Middle East, South and Southeast Asia, Americas, Africa and Oceania
Chemical groups: FRAC P07, FRAC 4, FRAC M01 (mode-of-action groups, not products)
Last reviewed: 2026-10-20

What causes citrus gummosis?

Citrus gummosis is caused by Phytophthora citrophthora and Phytophthora nicotianae P. citrophthora (R.E. Sm. & E.H. Sm.) Leonian, 1925; P. nicotianae Breda de Haan, 1896 (with P. palmivora in some regions), an oomycete (water mould), not a true fungus (order Peronosporales, family Peronosporaceae).

Older or alternative scientific names: Phytophthora parasitica (= P. nicotianae).

Where it occurs

Present in every citrus-growing region. P. nicotianae dominates in warm, subtropical and tropical areas; P. citrophthora in cooler Mediterranean-type climates and in winter; P. palmivora is important in parts of Florida and the tropics.

Region	Status
Mediterranean and Middle East	Widespread; P. citrophthora common
South and Southeast Asia	Widespread; a major cause of citrus decline
Americas	Widespread; also fibrous root rot in Florida and Brazil
Africa and Oceania	Widespread

Symptoms and identification of citrus gummosis

The organism

Phytophthora produces swimming zoospores in saturated soil and on wet bark, which spread in water and rain splash; it survives in soil as thick-walled chlamydospores and oospores. Laboratory isolation or soil propagule counts confirm it.

Signs in the crop, stage by stage

Crop stage	What you see
Nursery and young trees	Damping-off and collar rot of seedlings; bark lesions at or just above the soil line, especially where the bud union is low.
Bearing trees: trunk and crown	Water-soaked, dark brown bark at the trunk base that cracks and oozes amber gum; under the bark the wood is stained brown with a clear margin; lesions spread around the trunk and down into crown roots.
Canopy decline	Yellowing of leaves along the midrib, leaf drop, twig dieback and smaller fruit on the side above the lesion; trees die when girdled.
Fruit (brown rot)	Firm, leathery, light-brown rot of fruit hanging near the ground after rain, with a pungent smell and white mould in humid conditions.

Disease cycle of citrus gummosis

Life cycle of Citrus gummosis (Phytophthora foot rot) — diagram drawn by Laisha from the stages below. Durations depend on temperature.

Resting spores in soil germinate when soil is wet and warm, producing sporangia that release zoospores. Zoospores swim to fibrous roots and, splashed or carried in irrigation water, reach the trunk base, where they infect through bark, especially at wounds or where bark stays wet. Lesions expand during wet periods and stop in dry weather, leaving callused cankers.

Stage	Duration	Notes
Survival in soil	Years	Chlamydospores and oospores.
Sporangia and zoospores	Hours to days in saturated soil	Spread by water and splash.
Infection of bark and roots	Needs prolonged wetness	Wounds and low bud unions increase risk.
Lesion expansion	Weeks to months during wet periods	Gumming; may girdle the trunk.

Temperature

P. nicotianae is most active at about 25-32 °C; P. citrophthora at cooler temperatures (about 15-25 °C), causing winter infections in Mediterranean climates. Both need wet soil or bark.

Generations

Many cycles during wet seasons.

Conditions that favour it

Poorly drained, heavy or compacted soils; flood or high-volume irrigation that wets the trunk; susceptible rootstocks; bud unions planted at or below soil level; mulch, weeds or soil piled against the trunk; tree wraps that keep bark wet; and wounds from tools.

Economic threshold level (ETL)

For trunk gummosis, any active lesion justifies action. For fibrous root rot, soil propagule counts are used: the Florida citrus guide regards more than about 10-20 propagules of Phytophthora per cubic centimetre of soil as damaging, combined with soil type and rootstock; values for other regions may differ.

Crop	Threshold	Unit	Crop stage	Region	Source
Orange	More than about 10-20 propagules per cm³ of soil (P. nicotianae + P. palmivora)	propagules per cm³ of soil (laboratory count)	Bearing trees, soil sampled in the irrigated zone	North America (Florida)	UF/IFAS Florida Citrus Production Guide, Phytophthora foot rot, crown rot and root rot (PP-156/CG009)

Thresholds are local. They depend on crop price, control cost, variety and natural-enemy activity, so use the figure your local extension service publishes where one exists.

Damage and yield loss from citrus gummosis

Gummosis and root rot cause slow decline, lower yield and tree death, and are a leading cause of tree loss in many citrus regions with heavy soils or flood irrigation. Brown rot can destroy much of the low-hanging fruit after heavy rain.

Monitoring

Inspect trunk bases at least twice a year and after wet periods for gumming and cracked bark; scrape suspicious bark to see the brown margin.
Sample soil and fibrous roots under the canopy for laboratory propagule counts where this service exists.
Inspect fruit near the ground after heavy rain.

How to control citrus gummosis (integrated management)

Control works best as a sequence: prevent, monitor, then act only when a threshold or a risk forecast says so, using the least disruptive option that works.

Cultural control

Choose tolerant rootstocks (for example trifoliate orange and many of its hybrids are more tolerant than rough lemon or sweet orange) and plant with the bud union well above soil level.
Improve drainage, use drip or microsprinkler irrigation directed away from the trunk, and avoid flooding the trunk base.
Keep mulch, weeds and soil away from the trunk; avoid wounding the bark.
Plant only certified, disease-free nursery trees grown in clean media; prune low branches to keep fruit off the ground.

Mechanical and physical control

On small lesions, scrape away diseased bark to healthy tissue and let it dry, then treat the wound as locally recommended.

Biological control

Organic mulches kept away from the trunk and composts that suppress Phytophthora, and Trichoderma-based products, can reduce root infections in some soils.

Host resistance

Rootstock tolerance is the main resistance tool; scion varieties are generally susceptible at the bark if the bud union is close to the soil.

Chemical control of citrus gummosis — by mode-of-action group only

Laisha names chemical options only by their resistance-committee mode-of-action group (IRAC for insects, mites and nematodes, FRAC for fungi, oomycetes and bacteria, HRAC for weeds), never by active ingredient or product. What is registered differs by country and changes over time; the label and your local plant-protection authority decide what may be used, on which crop, at what rate and with what pre-harvest interval.

Fungicides are used after cultural problems are corrected, as trunk paints, sprays or soil applications where registered. What is registered differs by country and changes over time; follow the label and your local plant-protection authority.

Scheme	Group	Mode of action	Notes
FRAC	P07	Phosphonates (host defence and direct activity)	Foliar or trunk use; systemic.
FRAC	4	Phenylamides	Soil use; resistance documented.
FRAC	M01	Inorganic (copper) - multi-site	Trunk paints and fruit protection against brown rot.

Resistance management

Phytophthora populations with reduced sensitivity to FRAC Group 4 and to phosphonates have been reported in citrus after repeated use; alternate groups and rely on drainage and rootstocks.

Look-alikes and how to tell them apart

Diplodia and other bark gummoses — Gumming from cankers higher on the tree and branches, often after frost or sunburn; laboratory isolation separates them.
Citrus psorosis and other virus-related scaling — Bark scaling on scaffold branches without a soft, wet lesion at the trunk base.
Citrus greening (huanglongbing) — Blotchy, asymmetric leaf mottle and lopsided fruit, without trunk lesions; both can occur together.

Host range

All citrus species and hybrids, with susceptibility depending heavily on the rootstock. Phytophthora nicotianae and P. citrophthora also attack many other crops, including tomato, tobacco, papaya, pepper and ornamentals.

What is still uncertain

Damaging propagule levels were developed in Florida and need local validation.
The relative importance of each Phytophthora species differs by region and season.

Grow guides for its host crops

How to grow the crop well is the first line of defence. Laisha's free grow guides cover each crop from sowing to harvest:

How to grow Orange How to grow Mandarin & tangerine How to grow Lemon & lime How to grow Grapefruit and pomelo

Not sure this is what you are seeing?Ask Laisha: describe the symptoms or send a photo, and get a reasoned first read with what to check next — for your crop, your stage and your region.Ask Laisha

Sources

Phytophthora citrophthora (brown rot of citrus fruit) datasheet — CABI Compendium. https://doi.org/10.1079/cabicompendium.40958
Phytophthora nicotianae (black shank) datasheet — CABI Compendium. https://doi.org/10.1079/cabicompendium.40983
Phytophthora citrophthora (PHYTCO) — EPPO Global Database. https://gd.eppo.int/taxon/PHYTCO
Florida Citrus Production Guide: Phytophthora foot rot, crown rot and root rot — UF/IFAS EDIS. https://edis.ifas.ufl.edu/publication/CG009

Written by Laisha from the sources above, in its own words. This page is general reference, not a prescription: confirm identification locally and follow the label and your plant-protection authority.