Most plant protection decisions in an olive grove start from something seen, whether spots on leaves, fruit dropping, dead branches or a black film across the foliage. The same picture, however, can come from entirely different causes that call for different handling at different times. Peacock spot and cercospora both cause defoliation, the olive fly and the olive moth both cause fruit drop, and sooty mould is not treated as a disease at all. This guide opens with a table linking each symptom to its likely cause, then covers the groups of pathogens and insects, the calendar of interventions through the year, and the few measures that cover most problems.
Symptom table
| What you see | Likely cause | When it appears |
|---|---|---|
| Circular lesions with concentric rings on the upper leaf surfaces | Peacock spot | Autumn and spring, in wet weather |
| Lead-grey or dusty underside of the leaf | Cercospora leaf spot | Autumn and spring, often alongside peacock spots |
| Sunken reddish-brown lesions on fruit with orange spore masses | Anthracnose | Autumn, at ripening |
| Dieback on one side of the tree, with leaves staying attached | Verticillium wilt | Mainly spring |
| Galls and swellings on twigs, limbs and trunk | Olive knot | Year-round, after wounding |
| Galls on roots or at the soil line on young trees | Crown gall | Mainly in nurseries and young plantings |
| A hole in the fruit with a soft brown area around it | Olive fly | Summer and autumn |
| Leaf mines, flowers bound with silk, fruit dropping with the pedicel | Olive moth | Winter, spring and summer, by generation |
| Crater-shaped holes in young fruit | Olive weevils | Early summer, after fruit set |
| White cottony masses on the inflorescences | Olive psyllid | Spring, during bloom |
| Limpet-like bumps on twigs and a black film on leaves | Scale insects and the sooty mould that follows | From spring to autumn, it is worst in shaded trees |
| Galleries under the bark of weakened trees | Bark beetles | Mainly on stressed or injured trees |
Foliar diseases
Peacock spot is the most widespread fungal disease of the olive. It produces circular lesions with concentric rings on the upper leaf surface, starting on the lower inner leaves, and its damage results from premature defoliation that weakens the tree and reduces next year's flowering.
Cercospora leaf spot causes the same defoliation and, for decades, was credited to peacock spot. It is distinguished by the lead-grey appearance on the underside of the leaf, and unlike the peacock spot, it also attacks the fruit. The two frequently occur on the same tree and respond to the same copper-based programme.
Fruit diseases
Anthracnose is the most serious disease of the olive fruit and hits hardest in humid coastal regions. The fruit develops sunken reddish-brown lesions, is covered in humid weather by orange masses of conidia, and either drops or mummifies on the tree. Oil from infected fruit carries high acidity and a reddish colour.
Olive fly injury greatly eases the entry of the fungus, so fly control doubles as an anthracnose measure.
Wood and vascular diseases
Verticillium wilt is the most serious soilborne disease and the only one without a chemical cure because the fungus lives in the soil and within the tree's vessels. It is recognised by dieback on one side of the tree and by dead leaves that stay attached to the branches. Its microsclerotia survive in soil for at least thirty years, which makes management a question of site and variety choice.
Olive knot is bacterial, and forms galls on twigs, limbs and trunk. The bacterium enters exclusively through fresh wounds caused by frost, hail, pruning, and harvest, making operational hygiene the primary control tool.
Crown gall, caused by Agrobacterium tumefaciens, mostly concerns young trees and forms tumours on the roots and just above or below the soil line. Trees carrying many galls gradually lose the ability to transport water and nutrients upward, weaken, and eventually die. The bacterium survives for years in the soil, so certified planting material and careful handling of young trees are the practical defences.
Bark beetles belong in the same group, boring galleries under the bark. They attack mainly weakened, injured or neglected trees, so control rests on removing and destroying infested wood and on keeping trees vigorous.
Insect pests
The olive fly (Bactrocera oleae) remains the single most important pest, since the larva feeds on the flesh of the fruit and causes both drop and oil degradation. In heavy years, it destroys a large share of the production outright and downgrades much of what remains, driving acidity levels so high that the oil cannot be marketed for consumption. Correct identification against similar species is the first step, and infestation changes the composition of the oil directly.
The olive moth completes three generations a year, on leaves, flowers and fruit, with the fruit generation the damaging one. Olive weevils cut crater-shaped holes in young fruit and cause premature drop, usually in groves where the olive moth was not treated in time.
The olive psyllid covers the inflorescences with white waxy masses, though in most cases it needs no treatment, since populations collapse on their own with the first heat. Scale insects, led by black scale, excrete honeydew on which sooty mould develops, and sprays only reach them during the crawler stage.
The olive leaf moth attacks tender growth and locally the green fruit, with the worst damage in nurseries and young trees, where the loss of growth sets development back.
Why is sooty mould not a disease
The black film on leaves and fruit is produced by fungi growing on the honeydew of sap-sucking insects, and they do not attack plant tissue. Their damage is indirect, blocking light and reducing photosynthesis. A fungicide solves nothing here. Once scale insects or psyllids are brought under control, honeydew stops and the mould weathers off with rain and new growth.
The calendar of interventions
| Season | What to watch |
|---|---|
| Winter, after harvest | Copper spray immediately after picking and after pruning, covering peacock spot, cercospora, anthracnose and olive knot. Remove galls and infested wood. |
| Spring, new growth | Copper at 2 to 5 cm of growth for the fungal diseases, scale monitoring at 6 to 8 cm, first psyllid colonies |
| Flowering | Flower generation of the olive moth occurs only in groves with a light bloom. Avoid every bee-toxic spray |
| Fruit set, late May to June | Fruit generation of the olive moth at 3 to 5 mm of fruit, which also covers weevils and scale |
| Summer | Olive fly monitoring with traps, removal of olive knot galls in the dry season, and summer pruning where needed |
| Autumn, before the rains | The most critical spray of the year, copper against anthracnose, peacock spot and cercospora, repeated 3 weeks before harvest. |
Five measures that cover most problems
- Pruning for airflow: opening the centre of the canopy dries the foliage fast and simultaneously suppresses peacock spot, cercospora, anthracnose and scale
- Copper in autumn and spring: two sprays cover all the main fungal diseases and protect wounds against olive knot
- Hygiene of operations: prune and harvest in dry weather, disinfect tools between trees, burn infested wood and mummified fruit
- Olive fly control: managing the fly also cuts anthracnose, since fly punctures are the fungus's main entry route
- Protecting beneficials: needless broad-spectrum sprays wipe out parasitoids and lead to scale outbreaks larger than the original infestation
Alongside these, balanced nutrition keeps trees vigorous, and timely harvesting gets ahead of anthracnose before it spreads.
Frequently asked questions
What is the most serious olive tree disease?
Anthracnose is the worst for the fruit, and for oil quality, for foliage and tree vigour, it is peacock spot, and for the survival of the tree itself, it is Verticillium wilt, which has no cure.
Why are olive leaves turning yellow and dropping
The usual causes are peacock spot and cercospora, which cause mass defoliation; scale insects with sooty mould that blocks photosynthesis; and Verticillium wilt, though in that case the leaves usually stay attached to the branch.
When should copper be sprayed on olives?
The main spray goes on in autumn before the first rains, the second immediately after harvest and pruning, and the third in spring once new growth reaches 2 to 5 cm.
What is the most important olive pest?
The olive fly attacks the fruit directly and degrades the oil. The olive moth follows with its fruit generation, and locally, the olive weevil.
Can one spray cover multiple pests?
Yes, and that is the logic of the programme. Copper covers peacock spot, cercospora, anthracnose and olive knot at once, while the spray against the fruit generation of the olive moth also covers weevils and scale insects.
What should I do about galls on the branches?
Galls on the branches mean olive knot, and branches carrying many of them are cut several centimetres lower into healthy wood, in dry weather and preferably during summer, and the prunings are burned.
Important notes
Treatment dates vary by region, variety and season, so follow local plant protection guidance and work with an agronomist rather than a fixed calendar.
Use only products registered for olive in your country, follow the label and the maximum number of applications per season, and apply cultural preventive measures first.
References
- UC Statewide IPM Program, Olive pest management guidelines.
- Pacific Northwest Pest Management Handbooks, Olive leaf spot.
