Sick olive tree: identify disease by its symptoms

Diagnosis in one table

Turn over a leaf and examine a branch before treating anything. France Olive lists more than five viruses, three bacteria, at least twenty fungi and a dozen insects, nematodes or rodents capable of weakening an olive tree (Olea europaea), noting that their symptoms overlap with one another and with excess water, cold damage or compacted soil¹. Where the symptom occurs, on a leaf, olive, shoot or whole branch, already narrows the possibilities.

What you see	Likeliest cause	Section
Round dark brown spots ringed with yellow on upper leaf surfaces; leaves yellow then fall	Peacock spot	Peacock spot
Yellow upper surface, gray felt-like coating underneath, on leaves over a year old	Cercospora leaf spot	Cercospora
Small brown shells on shoots and beneath leaves; leaves sticky then black	Black scale, followed by sooty mold	Black scale
Olives with puncture marks, tunneled flesh, falling before harvest	Olive fruit fly	Fruit fly
Mined leaves in spring, chewed flower clusters	Olive moth	Olive moth
Rough brown growths on shoots	Olive knot (bacterial disease)	Olive knot
A vigorous branch suddenly dries in spring, leaves remaining attached	Verticillium wilt	Verticillium wilt
Scorched foliage and dry shoots high in the tree, without another explanation	Xylella fastidiosa, requiring identification	Xylella
Dying shoots, weak tree in waterlogged soil	Excess water, roots starved of oxygen	Root rot

The last case is not a disease, but looks just like one: France Olive sees it very often in olives planted in watered lawns on clay soil².

Peacock spot: ringed spots and falling leaves

Peacock spot is olive's most widespread disease. The fungus, Fusicladium oleagineum (formerly Spilocaea oleagina), produces round 2 to 10 mm (0.08 to 0.4 in) spots on upper leaf surfaces, blackish brown to orange-yellow³. The surrounding halo resembles the eye on a peacock feather, hence the name; the RHS notes that spots are visible year-round⁴.

The fungus needs free water on the leaf, and spores travel in rain splash. It operates between 8 and 24 °C (46 and 75 °F): around 16 °C (61 °F), six hours of leaf wetness suffice for infection, and spots appear two weeks later³. Above 25 °C (77 °F), rain no longer produces new spots⁵. In FREDON Corsica’s seasonal example, risk periods therefore run from early March to late June and late August to mid-November⁵.

Poorly controlled disease causes waves of leaf fall: less photosynthesis, fewer shoots, fewer olives. Grossane, Aglandau, Tanche, Cailletier, Lucques and Petit Ribier are more susceptible³.

Prevention begins with pruning shears. Annual pruning opens the canopy and shortens leaf wetness, which allows infection beyond 4 hours⁶; the RHS also recommends collecting fallen leaves and avoiding excessive pruning that triggers watershoots⁴. CopperA contact fungicide based on copper sulfate, hydroxide or oxide, such as Bordeaux mixture. It stays on the surface and prevents spore germination, without curing an already affected leaf. does not circulate in sap: it protects before rain, preventively in late August-early September in France Olive’s French seasonal calendar, then late February-early March, renewed after 40 mm (1.6 in) cumulative rain⁶. Two to three applications annually protect resistant varieties; four to five are sometimes needed for susceptible ones³.

Cercospora leaf spot: gray felt beneath leaves

A leaf yellow above and gray below points to Cercospora leaf spot, also called olive lead disease, caused by Pseudocercospora cladosporioides. Symptoms appear from fall to spring on leaves over a year old: yellow chlorosis with dead tips and edges, and a characteristic grayish felt underneath⁷. These leaves then fall from late spring to late summer, often adding to peacock-spot losses.

The cycle is deceptively slow: only leaves under a year old become infected, but incubationThe time between the fungus entering a leaf and visible symptoms appearing. may last 14 months. In France Olive’s French growing context, the spore coating generally forms from October to December, with the first fall rains and average temperatures of 10 to 20 °C (50 to 68 °F); infections follow and may continue until April⁷. Cayon, Aglandau, Picholine, Olivière, Cailletier and Frantoio are more susceptible, and coastal areas more affected than inland ones.

France Olive attributes much olive decline to poor management of these two leaf fungi, describing Cercospora as a newly developed disease¹. The response is the same as for peacock spot: prune for airflow, then apply copper a few days after the first fall rains⁷.

Black scale and its sooty mold

Dark brown, 3 to 4 mm (0.12 to 0.16 in) shells with a raised H-shaped ridge are female black scale insects (Saissetia oleae). They attach along veins beneath leaves and on young shoots⁸.

In France Olive’s French seasonal example, the insect has one generation a year. It overwinters as larvae, then females lay eggs from mid-June to mid-August; young larvae, the only mobile stage, travel on the wind⁸. Sap removal causes no direct damage. The real damage comes from sooty moldA black fungal coating growing on sugary honeydew from sap-sucking insects. It does not infect leaves but blocks their light. on honeydew: it blocks photosynthesis, weakens the tree and causes leaf fall⁸. It does not infect the plant and gradually decreases once insects are controlled; a water jet or soapy water removes it⁹.

In France Olive’s French seasonal calendar, count scales in February-March, beneath leaves: finding roughly one per minute corresponds to a low level⁸. Lacewing larvae, ladybugs and small parasitic wasps such as Metaphycus lounsburyi do the underlying work; harder pruning of infested branches complements them⁸. Ants protecting scales for honeydew can be kept away with a sticky band around the trunk⁹.

To recognize other species and choose a natural treatment, see getting rid of scale insects and armored scale insects.

Olive fruit fly: punctured olives

An olive with a brown puncture mark and a tunnel through its flesh has received an olive fruit fly egg (Bactrocera oleae). Adults measure 4 to 5 mm (0.16 to 0.2 in) and have a black wingtip spot. Females lay once olives reach 8 to 10 mm (0.3 to 0.4 in), one egg per fruit but 400 to 500 in total; a generation completes its cycle in a month, with three to five generations from June to October in France Olive’s French seasonal example¹⁰.

Damage is twofold: punctured olives fall before ripening, and oil from attacked fruit becomes more acidic and loses flavor. Mild winters, summers without heatwaves and irrigation favor this water-dependent insect; above 300 m (984 ft) altitude, it rarely proliferates. Lucques, Ascolana, Bouteillan and Grossane particularly attract it, Aglandau less so¹⁰.

Without insecticides, France Olive’s French handbook gives three responses, and timing matters more than the product. Its months below are seasonal examples; use local fruit development, insect activity and weather to time the measures:

Measure	When	How
Mass trapping	From late February-early March, or from early June in France Olive’s French calendar	One bottle trap per tree, ammonium phosphate at 40 g per liter (5.3 oz per US gallon), renewed every 3 weeks; moderate to low effectiveness6
Clay barrier	Once olives exceed 0.8 cm (0.3 in), late June-early July in France Olive’s French calendar	Calcined kaolinite clay at 3 to 5%, renewed every 3 to 4 weeks or after 20 mm (0.8 in) of rain, 3 to 5 applications6
Early harvest	No more than three weeks after the first exit holes, in September in France Olive’s French calendar	Harvesting before flies emerge preserves quality, at the cost of lower oil yield6

Clay also protects against Dalmatian diseaseOlive rot caused by Botryosphaeria dothidea, which enters fruit through fruit-fly egg-laying punctures., whose fungus enters through egg-laying punctures⁶.

Olive moth: mined leaves in spring

Olive moth attacks in two stages, leaves then flowers. Its first generation mines foliage; the second feeds on flower clusters⁶. In its French seasonal setting, France Olive recommends counting leaves with advanced mines in April: above 10% mined leaves, treatment with Bacillus thuringiensisA soil bacterium whose toxins kill caterpillars that eat it without affecting other insects. It can be used in organic farming. is warranted at white-bud stage, then 7 to 10 days later, as flowering starts⁶.

This product breaks down in light and washes off: treat in the evening, with no rain forecast for 24 hours, and do not keep an opened bottle beyond two years⁶.

Olive knot: galls on shoots

Rough brown growths on shoots are olive knot galls, called bacterial disease by France Olive, caused by Pseudomonas savastanoi. Severe cases reach main limbs and trunk; vigor and yield fall, and oil can develop an unpleasant taste¹¹.

The bacterium enters through wounds: hail, frost, leaf scars and pruning cuts. Infection occurs from fall to late spring, carried by rain and tools¹¹, and frost-split bark provides entry¹². Each gall exudes bacteria that infect neighboring shoots.

No product cures an affected tree: non-systemic copper only limits new infections. Cut gall-bearing shoots in hot, dry summer weather, pruning diseased trees last and disinfecting tools between trees¹¹. Avoid excess nitrogen, which increases bacterial virulence, and check a tree's health before buying: imports from Spain or Italy have introduced particularly virulent strains into France¹¹.

The oleander strain also forms olive galls¹³, and these terrace neighbors pass it between them: see oleander bacterial knot and plant galls.

Verticillium wilt: a branch suddenly drying

A vigorous branch drying abruptly, as if cut off, suggests Verticillium wilt, a soil fungus (Verticillium dahliae) blocking sap vessels. Usually the strongest branches high in the tree are affected; on a young olive, sometimes the whole tree dries, and basal shoots may then rebuild it¹⁴.

In France Olive’s French seasonal observations, drying appears in late winter, peaking in March and April. In the rapid form, leaves yellow then dry while remaining attached; in the slow form, foliage dulls and flower clusters dry. Some infected trees show only poor vigor and no flowers¹⁴.

The fungus survives up to 15 years in soil, reactivated each spring by host plants¹⁴. Hence the garden pitfalls:

• No tomatoes, peppers or potatoes beneath the olive, and leave no nightshade, datura, amaranth or goosefoot growing there;
• remove a branch as soon as it declines, prune affected trees last and disinfect tools;
• remove pruned wood from the plot;
• space out watering and moderate nitrogen, both of which favor the fungus¹⁴.

Xylella fastidiosa: seek identification, do not treat

Xylella fastidiosa is a bacterium transmitted by sap-sucking insects that blocks raw sap, and olive is a host: the RHS lists it as high risk¹⁵. It has caused rapid olive decline in Puglia, Italy, since 2013. Symptoms, scorched foliage and drying shoots high in the tree, are non-specific and resemble water stress or deficiency¹⁶.

If an olive dries without an obvious cause, do not treat: in France, report it to the regional DRAAF food authority or your region’s FREDON, which oversee surveillance¹⁷.

The monitoring calendar

In the sources’ French seasonal setting, two seasons concentrate risk: spring and fall rain for fungi, summer for insects. The calendar brings together published periods for each problem, each with its source. Follow local rain, temperatures and insect activity rather than applying these months everywhere.

In France Olive’s French seasonal calendar: February-March, count scales and apply the first copper; late June, clay on olives; late August, fall copper. Canopy-opening cuts are explained in pruning an olive tree, with complete care in olive tree care.

Quand le feuillage jaunit sans tache ni insecte : yellow olive leaves se lisent selon leur position sur l'arbre et la saison.

A spot you cannot name? Photograph the leaf: Floralens identifies the problem and tells you what to do. Diagnose with the app

Frequently asked questions

Which disease kills olive trees?

Xylella fastidiosa, an incurable bacterium causing olive decline in Puglia since 2013: the French source describes European Union rules requiring infected trees to be uprooted¹⁶. Verticillium wilt can also kill an entire young olive by blocking its vessels¹⁴. In gardens, the most common cause remains water: an olive can die from oxygen-starved roots in overwatered clay soil².

How do you treat a sick olive tree?

Identify the cause first, since the response differs completely. Against peacock spot and Cercospora, open the canopy by pruning and protect with copper before late-winter and late-summer rain⁶. Against fruit fly, traps and clay; against olive moth, Bacillus thuringiensis at white-bud stage⁶. A gall or Verticillium-affected branch cannot be cured: cut it off¹¹¹⁴.

What is the new disease affecting olive trees?

Xylella fastidiosa, responsible for rapid olive decline in Puglia since 2013 and now present in Italy, France, Spain and Portugal¹⁶. France Olive also describes Cercospora as newly developed, and fungi previously confined to other trees observed on olive in 2019¹.

How do you recognize olive knot?

By its galls: rough brown growths, first on shoots, then main limbs and trunk in severe cases¹¹. A gall does not disappear by itself and exudes bacteria that infect neighboring shoots: remove it in dry summer weather with a disinfected tool.

How do you treat olive moth?

In France Olive’s French seasonal calendar, count mined leaves in April; use local leaf and flower development to time the check. Above 10%, treat with Bacillus thuringiensis at white-bud stage, then again 7 to 10 days later, in the evening with no rain forecast for 24 hours⁶.

What are the most common olive diseases?

Peacock spot, the most widespread³, followed by Cercospora: two fungi causing leaf fall. Pests include black scale and the sooty mold following it, olive fruit fly and olive moth⁶. Olive knot and Verticillium wilt are less common in gardens but more serious.