Olive diseases and pests: identification from symptoms, with a diagnostic tool

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

In olive, most mistakes are not made at spraying but at identification. Peacock spot and cercospora leaf spot both defoliate, but one hides its symptoms on the underside of the leaf. Dalmatian disease never infects without an olive fly puncture. The jasmine moth feeds spectacularly and costs very little, while the psyllid is barely visible and can take half the flowering. This article is a symptom-based identification guide, with a tool that compares what is seen in the field against the profile of each pathogen and pest, and with a detailed description of each.

1. How a symptom is read on olive

The olive is evergreen, and that changes everything in diagnosis. A deciduous tree cleans out its inoculum every winter through leaf fall. The olive does not: peacock spot overwinters as mycelium inside leaves that remain on the tree, and fallen leaves on the ground are not a significant source of infection [1]. A four-year study in an orchard in southern Spain confirmed this quantitatively: infected leaves within the canopy were what mattered for pathogen survival and conidium production, with conidial density in the lesions highest from November to February and zero in summer [11]. The same holds for anthracnose, where the main inoculum source is the mummified fruit left in the canopy [3]. The experiment that shows it most clearly: mummies left in the canopy did not reduce their conidium production after six months, while those placed on or buried in the soil saw it fall by up to ten thousand fold [12]. Whatever is not removed by pruning stays.

The second particularity is that the olive reacts slowly. Weeks to months pass between infection and the visible symptom: the incubation period of peacock spot ranges from a few weeks to many months [1]. By the time the spot appears, the weather event that caused it is long gone. Identification in olive is therefore never only about what is visible now, but about what happened two months earlier.

The third is defoliation. Floral induction in olive usually occurs in late summer and early autumn. A five-year field study showed that premature shedding of diseased leaves reduces the rate at which axillary buds differentiate into flower-bearing shoots, and the earlier the leaves fall the greater the damage [2]. Autumn defoliation therefore does not cost this season's crop but the next one — and this is where plant protection meets alternate bearing, a subject covered in the article on olive nutrition.

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2. The identification tool

The tool compares the selected symptoms against the profiles of fourteen olive pathogens and pests. It does not work by simple counting: each symptom carries a different weight according to how distinctive it is. Grey-black discolouration of the wood counts for far more than a general chlorosis, because the latter fits ten causes. Some symptoms also count against a cause: leaves that dry and stay attached to the shoot argue for a vascular disease and against peacock spot, which sheds its leaves.

3. Peacock spot — the disease that costs through defoliation

Peacock spot, or olive scab, is caused by the fungus Venturia oleaginea, a name established in 2015 that replaced the earlier Spilocaea oleagina and Cycloconium oleaginum [3] [1]. It has been known since 1845 and occurs almost everywhere olive is grown. In lowland areas with high humidity the crop can be drastically reduced or lost entirely [1].

Symptoms

On the upper leaf surface, greyish-brown spots with indistinct margins appear first and are easy to miss. They then enlarge, become circular, deepen in colour, and during summer become surrounded by a yellow halo — the familiar peacock eye [1]. Typical spot size is 0.5 to 1.2 mm in diameter before enlargement [3].

There is also an atypical form in which the infection appears as a white circular spot. This occurs when the mycelium develops strongly in the outer cuticle layers and produces no conidiophores and conidia [1]. It is the form that misleads, because it looks nothing like the textbook image of the disease.

On flower pedicels and leaf petioles the spots are also greyish-brown but elongated rather than round, and the outcome is flower drop and fruit drop [1]. This symptom often goes unnoticed and is attributed to poor fruit set or to a nutritional cause.

Epidemiology

The fungus overwinters as mycelium in infected leaves retained on the tree. Infection occurs through conidia, which are slime spores: their release requires rain or free water on the foliage from dew or fog [1]. The mycelium develops between the cuticle and the epidermis, which is why systemic products do not reach it easily.

Optimal temperature for conidial germination is 9 to 25 °C [1], with an optimum around 20 °C and infections favoured by cool wet weather at 12 to 15 °C; high temperatures suppress the disease [3]. Extensive attacks occur in spring and autumn.

These values were confirmed with Greek isolates of the pathogen, from Chalkidiki: the conidial germination range was 5 to 25 °C with an optimum of 20 °C, and at least 12 hours of continuous leaf wetness at the optimum temperature were required for germination to begin; at 6 hours no conidium germinated [10]. That range means that in Northern Greece infection is mainly possible from September to June [10]. The practical conclusion is not about temperature but about wetness: rain that does not keep the foliage wet for twelve hours does not produce infection.

Management

Cultural measures are few but not negligible: pruning that allows the canopy to ventilate and shortens leaf wetness duration [1]. Control is mainly chemical and preventive, applied shortly before the periods when an outbreak is expected, that is in early spring and early autumn [1].

Two findings from comparative trials deserve attention. First, effective control of peacock spot increases yield, and the increase is especially evident under heavy attack [2]. Second, when disease incidence is already high, copper itself has a defoliating effect and can worsen the very problem it is meant to solve; in that case alternative active substances are preferred [2]. In greenhouse trials kresoxim-methyl showed both protective and curative activity, while tebuconazole did not reduce infections [2].

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4. Cercospora leaf spot — the one mistaken for peacock spot

Cercospora leaf spot is caused by Pseudocercospora cladosporioides, also known as Mycocentrospora cladosporioides [3] [2]. Like peacock spot it causes severe defoliation, and the two often coexist on the same tree. Telling them apart is simple, provided the leaf is turned over.

The distinguishing feature

On the lower leaf surface a characteristic grey to lead-coloured felting appears — a mass of conidia with a velvety texture. On the upper surface there is a matching yellow chlorosis that becomes necrotic, especially at the tip and along the sides of the blade [3]. Peacock spot presents its main picture on top; cercospora presents it underneath. The felting can disappear after rain washes the conidia away [3], so the inspection is done in dry weather.

A second difference: cercospora attacks mainly mature leaves on the lower branches, from autumn through spring and on leaves more than one year old [2] [3]. Peacock spot, by contrast, is more aggressive on young leaves: under controlled conditions on cultivar Picual a negative correlation was found between leaf age and disease severity — the younger the leaf, the higher the susceptibility [2].

Fruit infection and oil quality

Fruit symptoms are rarer and appear as more or less circular spots of 3 to 7 mm. On green olives they are ochre to brown, while at veraison they turn grey, sometimes with a pale or yellowish halo [3].

The finding that matters at the mill: in infected fruit the titratable acidity of the oil was not affected, but the peroxide value was twice as high as in oil from healthy fruit [2]. Acidity does not betray the problem; the oxidative state does.

Less is known about the epidemiology and control of cercospora than of peacock spot. Specific trials showed that four copper applications are required for substantial disease reduction [2]. Spread is over short distances, leaf by leaf, through conidia or mycelial fragments, favoured by wind and rain at 10 to 20 °C with high humidity [3].

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5. Anthracnose: one disease, several names

In Greece the disease has been studied since 1920 and occurs mainly on Corfu, in Preveza and in Chalkidiki, where it causes significant losses on both table and oil cultivars [1]. Internationally it is regarded as the most damaging disease of olive fruit [3]. In an outbreak in Apulia about 70% of the fruit rotted and extensive defoliation followed [2].

Symptoms

On the fruit, infection begins as a reddish-brown spot at one point of the surface that spreads rapidly. The infected tissue sinks and develops pronounced wrinkling, often in concentric rings, and within a few days is covered by the fruiting bodies of the fungus as black specks. Under high humidity the fruiting bodies release numerous spores as a slimy pink or orange mass [1] [3]. This pink slimy mass is the most reliable distinguishing feature of the disease.

Infected fruit drops easily even at early stages. Fruit that does not drop mummifies on the tree [1] — and that is precisely where the main inoculum source for the following year sits [3]. A single mummy can produce more than 21,600 conidia under optimal conditions, with maximum production at 20 to 25 °C and 96 hours of wetness [12]. Repeated washings reduced production, which became very low after the fifth [12] — meaning that autumn rains gradually empty the mummy, but only after they have first dispersed the inoculum.

On leaves the spots are brown, start at the tip and may cover up to half the blade, with fruiting bodies arranged in concentric rings [1]. Leaf infections on their own have no economic significance, but there is an indirect mechanism that does: infected fruit releases a toxin that causes chlorotic spots which expand to cover almost the whole leaf, ending in drying, leaf fall and significant defoliation, as well as drying of branches up to 5 cm in diameter [3]. The damage does not stop at the fruit.

Epidemiology and the olive fly connection

The fungus infects mainly ripe fruit. Green fruit shows resistance, probably because of inhibitory compounds in the sap [1]. Infection occurs either by direct penetration of the protective surface layers or through wounds, and requires free water or a relative humidity of 92 to 100% at 10 to 25 °C [1]; other work gives a range of 10 to 30 °C with humidity above 93% [3]. The incubation period is 6 to 15 days and is shorter when infection occurs through a wound [1]. The current review of the disease confirms that it is a complex of several Colletotrichum species differing in virulence, host preference and geographical distribution, and that the disease degrades yield and oil quality at the same time [13].

This is where the olive fly enters. Its oviposition puncture accelerates fruit ripening and facilitates rapid entry and development of the pathogen [1]. An orchard with an uncontrolled olive fly population is an orchard prepared for anthracnose.

What it costs in the oil

Table cultivars lose their commercial value entirely. Oil cultivars yield less oil and of degraded quality: cloudy, reddish and with high acidity [1]. This is the opposite of cercospora, where acidity does not move.

Control relies on copper products, applied mainly as a preventive measure before the autumn rains [1], and on removing mummified fruit from the canopy, which takes out next year's inoculum.

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6. Dalmatian disease — the one that does not exist without the olive fly

Dalmatian disease is caused by the fungus Camarosporium dalmaticum, assigned in modern taxonomy to Botryosphaeria dothidea [1] [3]. It is common in Greece and across the Mediterranean and appears in two forms, a dry rot and a soft rot, always on fruit [1].

• Dry form. A localised infection of unripe fruit in summer. A sunken brown, corky spot appears, on which the pycnidia of the fungus can be seen together with a slit made by an olive fly puncture [1].
• Soft form. In autumn and early winter, on ripe fruit, the infection becomes general: rot, shrivelling and dehydration of the fruit, again with the olive fly puncture present [1].

An infection may start as the dry form and develop into the soft form in autumn as the fruit ripens [1]. Both forms lead to fruit drop, sometimes massive, with losses reaching and exceeding half the harvest [3].

The pathogen develops at 20 to 30 °C with an optimum of 30 °C, and the lower the temperature the more its development is hindered [1]. Damage tends to intensify from late August [3]. The necrotic lesion makes the fruit unsuitable for table processing, and when infected fruit reaches the mill the oil quality is degraded [3].

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7. Verticillium wilt — the most serious soil-borne disease

Verticillium wilt is a vascular disease caused by Verticillium dahliae and is regarded as the most serious soil-borne fungal disease of olive [1], and indeed the most important disease of the crop overall [2]. In Greece it was first recorded and studied at Stylida, Fthiotida, in 1952, and has since been observed throughout the country [1].

The two forms — and why they resemble other things

Apoplexy, or sudden wilt. Usually seen in nurseries and young trees. Leaves on some branches roll lengthwise towards the lower surface, forming a tube, lose their deep green, turn dull and then brown, and dry rapidly without falling, remaining on the branches for a long time [1]. In spring the flowers and young fruit dry and also remain attached. Apoplexy develops from late winter to early spring and occurs more often on trees attacked the previous year [1].

Slow decline. This develops gradually and one-sidedly, from late spring to early summer. Leaves become chlorotic or yellow, dry and fall, resulting in defoliation and branch dieback [1]. Weak young shoots with small chlorotic leaves often appear on the dry branches, and these too soon dry out.

The characteristic internal symptom of both forms is grey-black discolouration of the central cylinder of the affected branch: in longitudinal section it appears as streaking, in cross section as distinct specks [1]. Corresponding browning of the wood is also seen in the central cylinder of the root system [3]. It is the only symptom that reliably separates verticillium wilt from dieback caused by root rot or water stress, and it requires a cut with the secateurs.

Why the soil does not clear

The fungus persists in the soil as microsclerotia that survive up to 14 years [1] — other sources give up to 20 years [3]. The most extensive review of the disease on olive, fifty pages long, reaches the same conclusion on management: no single method suffices and an integrated strategy is required, weighted towards preventive measures taken before planting [14]. Inoculum increases where susceptible crops such as potato and tomato are grown between the trees, and on weed hosts [1]. The fungus is favoured at 21 to 27 °C and stops growing above 30 °C [1], which is why the strongest symptoms are seen at flowering rather than at the height of summer.

Among Greek cultivars, Tsounati, Kalamon and Amfissis are considered susceptible, while Koroneiki is practically resistant — with the qualification that significant attacks on it are not rare when conditions favour infection and inoculum build-up [1]. Table cultivars and young trees are generally more susceptible.

One point matters for diagnosis: the fungus has defoliating and non-defoliating pathotypes, which can be distinguished by a molecular method [2]. Two orchards with the same pathogen can present very different pictures.

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8. Olive knot and root rots

Olive knot

Olive knot is caused by the bacterium Pseudomonas savastanoi and is the most common bacterial disease of olive. It is characterised by galls that usually develop on twigs and branches; leaves and fruit are affected only exceptionally [2]. The picture is almost pathognomonic and is rarely confused with anything else.

What matters for prevention is that the pathogen has been detected in symptomless stem tissue of naturally infected trees [2]. A cutting without galls is no proof of healthy material. Certified propagation material is the substantive measure, and transmission is favoured by wounds — frost, hail, mechanical harvesting and pruning itself with contaminated tools.

Root rots and Phytophthora

Root rot is an emerging problem in important olive-growing areas. Canopy symptoms are not specific: leaf chlorosis, stunting and dieback [2] — exactly the picture wrongly attributed to a nutrient deficiency or to verticillium wilt.

The distinguishing element is not in the canopy but in the soil and in the spatial pattern: Phytophthora infections are favoured by wet conditions, unusual rainfall or poor drainage, and under those conditions they spread rapidly [2]. They appear in patches, at low points in the field, along the rows above the drip line, wherever water stands. Verticillium wilt, by contrast, is observed sporadically and on isolated trees, rarely in a generalised form [1].

Several species have been found associated with root and crown rot of olive: the P. megasperma complex and P. palmivora in Spain, P. palmivora in southern Italy, P. citricola and P. dreschleri in California [2]. Preventive measures are soil-based: avoiding waterlogging and excessive irrigation, avoiding soil movement from infested to clean areas, avoiding planting on sites with a history of the disease, and using healthy planting material [3].

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9. Xylella fastidiosa — what quarantine means

The bacterium Xylella fastidiosa arrived in Europe in 2013 and the first outbreak of olive quick decline syndrome was recorded in Apulia, Italy, the same year [4]. Its name describes its biology: it grows very slowly on culture media (fastidiosa) and lives in the xylem of the host (Xylella).

The disease can be asymptomatic in its early stages, but usually causes leaf scorching, extensive dieback and eventually the death of the whole tree [4]. The scorching starts at the leaf margins and tips and resembles drought or salinity damage — and that is exactly where time is lost.

The vectors in the Mediterranean belong to the family Aphrophoridae: Philaenus spumarius, Philaenus italosignus and Neophilaenus campestris, with P. spumarius the main vector in Europe [4]. These are xylem sap feeders, ubiquitous and highly polyphagous, with nymphs developing on herbaceous vegetation — preferably plants with a basal leaf rosette [4]. The eggs overwinter in the soil. This explains why management of the orchard ground cover is not a detail.

The scale of what is at stake explains the severity: it has been estimated that full spread of the bacterium would cost the EU more than EUR 5.5 billion per year in production losses, with additional export losses of about EUR 0.7 billion per year. If it spread across the EU it could affect over 70% of the production value of olive trees more than 30 years old and 35% of younger trees [4].

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10. The pests: olive fly, olive moth, psyllid, fruit weevil, Pollinia, jasmine moth, black scale

Olive fly (Bactrocera oleae)

The olive fly is the most important olive pest worldwide [4] and is covered in detail in a separate article: life cycle, the symbiont Candidatus Erwinia dacicola, the role of temperature, action thresholds, bait sprays, mass trapping and the state of resistance in Greek populations. That article is currently available in Greek only.

The olive fly — full article, in Greek

For identification purposes two points suffice. The oviposition puncture is a small triangular depression on the fruit surface, visible with a hand lens. The larval gallery in the flesh and the exit hole, round and clean, appear later. Both have diagnostic value beyond the fly itself, because they open the way for dalmatian disease and anthracnose.

Olive moth (Prays oleae)

The olive moth is the second most economically important olive pest after the olive fly [4]. Its distinguishing characteristic is that it has three generations with entirely different targets, which is why it is so easily misread: the same pest produces three unrelated pictures within the same year.

• Phyllophagous generation — the longest, from autumn to spring. Larvae overwinter in mines in the leaf blade [4]. The symptom is transparent irregular areas where the mesophyll has been eaten out.
• Anthophagous generation — the shortest, around April. Larvae feed on the flower buds. One larva of this generation has been estimated to destroy 20 to 30 flowers [4].
• Carpophagous generation — the most damaging. Larvae enter the fruit during summer and feed on the seed, causing premature fruit drop [4]. The drop typically occurs as the larvae leave the fruit.

The scale of the damage is shown by long-term data: a 28-year study in Andalusia found that the olive moth can reduce production by 50 to 60%, with heavy attacks occurring approximately every three years and causing 40% premature fruit drop [4]. That periodicity is the useful conclusion: the olive moth is not an annual problem, and an annual treatment without monitoring is a waste.

Olive psyllid (Euphyllura olivina, E. phillyreae)

Greek literature and the national advisory bulletins refer to this pest as Euphyllura phillyreae [8], while the international literature mainly uses Euphyllura olivina [5]. These are closely related species with practically identical symptoms and management.

The psyllid is recognised by its own symptom: white cottony masses on new growth, on buds and inside the flower clusters. This is a waxy secretion of the nymphs, which are flat, green to tan, with five nymphal stages from 0.4 to 1.5 mm. Adults are light tan, 2.5 mm long, and strong jumpers [5].

There are three generations per year. Adults overwinter in sheltered areas of the trunk. The second generation develops on buds and flowers in May and June and is the one that causes the damage; the third, in September and October, usually goes unnoticed [5]. In terms of temperature, development is favoured at 20 to 25 °C, above 27 °C the insects are less active and above 32 °C mortality increases [5].

The damage is twofold: direct feeding on buds, flowers, tender shoots and small fruit, and indirect through honeydew that feeds sooty mould. During flowering and fruit set the waxy secretion causes flower drop and drop of small fruit. Under heavy attack, yield losses reach 30 to 60% [5].

Two things determine the success of a treatment. First, timing: it must be made before the nymphs form their waxy covering, which protects them from the products [5]. Second, the target: although the second generation does the damage, reducing the first generation is what keeps the second small [5]. In hot areas, pruning out centre limbs to improve air circulation reduces populations [5].

Olive fruit weevil (Rhynchites cribripennis)

This weevil leaves two distinct traces. On the leaves of new growth the adults make semicircular notches on the margins, clean and smooth as if cut with scissors — a picture often attributed to a caterpillar. On the fruit they drill feeding holes into the flesh, and that is where the economic damage lies.

The weevil is easily confused with the jasmine moth, since both feed on new growth. The shape separates them: the weevil leaves smooth semicircular notches on the margin, with no silk; the jasmine moth leaves irregular feeding damage, mines in the blade, and always silk at the shoot tip.

Pollinia scale (Pollinia pollini)

This grey scale does not settle at random on the tree. It establishes itself at the base of the buds, in the axils of shoot branchings, and in wounds and cracks in the bark of thin twigs [8]. The field picture is a dense colony of small scales in a line along the twig, not scattered individuals — and this separates it from the other olive scales.

The damage is not the sap removed. Because the insect sits at the base of the buds it prevents their normal development: it causes irregular and weak growth, reduced fruiting and necrosis of thin twigs [8]. The loss therefore falls on the following year's crop, just as with defoliation from peacock spot.

It is a serious pest that attacks mainly unpruned, neglected and weakened trees [8]. Scale control is achieved mainly through cultural practices that improve tree vigour — pruning and clean-up, balanced irrigation and fertilisation — and through removal and destruction of infested branches, which limits spread. Chemical control supplements the cultural work; it does not replace it [8].

For the other olive scales — oleander scale (Aspidiotus nerii) and olive scale (Parlatoria oleae) — where attacks are severe the official guidance recommends paraffin oils, so as to avoid destroying the beneficial insects [8].

Jasmine moth (Palpita unionalis)

The jasmine moth, or olive leaf moth, was traditionally a secondary pest and is today considered a primary one in nurseries and young irrigated plantings. Small larvae feed on the lower leaf surface and, as they grow, consume whole leaves and buds; at high populations they also attack fruit and their seeds [6]. A characteristic sign is the silken threads with which the larva binds leaves together before pupating.

The adult is white to off-white, with semi-transparent wings and a wingspan of about 30 mm, and is nocturnal. The larva reaches 18 to 20 mm. The female lays on average 385 eggs, singly on leaves and shoots. The insect overwinters as a larva. Under constant conditions at 27 °C the cycle is completed in 38 days, corresponding to up to nine generations a year; five are recorded in Italy and ten in Egypt [6].

Black scale and sooty mould

Black scale appears as hemispherical brown domes attached to shoots and leaf veins. Its cost is rarely direct; it comes through the honeydew that feeds sooty mould.

Sooty mould is not a parasite. It is fungi of the genera Capnodium, Fumago and Aureobasidium that develop on insect honeydew. The mycelium settles on the surface of wood, leaves and fruit, forming a black film that causes premature ageing by blocking photosynthesis and reducing gas exchange. Persistent sooty mould can cause defoliation [3]. The practical diagnostic sign is that the black coating washes or rubs off and reveals healthy tissue underneath — unlike the spots of peacock spot or cercospora, which are within the tissue.

Managing sooty mould necessarily goes through the insect that feeds it. The factors that favour scales, and therefore sooty mould, are clear: absence of pruning or pruning at too long intervals, insufficient spacing between canopies, excessive nitrogen fertilisation, repeated applications of poorly selective insecticides that reduce the beneficial fauna, mild winter temperatures and high summer humidity [3]. Three of the six factors are decisions made by the grower.

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11. The table of confusions

The table below gathers the pairs most often confused in the field, and the single feature that separates them.

12. Sources

The numbers in the text refer to the list below.

• [1] Michelakis, E. (2006). The main fungal diseases of the olive [in Greek]. Diploma thesis, Department of Plant Production, School of Agricultural Technology, Technological Educational Institute of Crete, Heraklion. Source for the Greek data: symptomatology and life cycle of peacock spot (spots, atypical white form, 9–25 °C, overwintering in leaves retained on the tree, the link between spring and autumn infections); dalmatian disease (dry and soft forms, role of Prolasioptera berlesiana, 20–30 °C); anthracnose (92–100% humidity, 6–15 day incubation, oil quality, occurrence on Corfu, Preveza and Chalkidiki); verticillium wilt (first record at Stylida in 1952, microsclerotia up to 14 years, 21–27 °C, susceptibility of Greek cultivars).
• [2] Martelli, G.P., Salerno, M., Savino, V. and Prota, U. (2002). An Appraisal of Diseases and Pathogens of Olive. Acta Horticulturae 586:701–708. Source for the effect of premature leaf fall on bud differentiation (five-year study 1994–1998), for the defoliating action of copper under heavy attack, for the findings on kresoxim-methyl and tebuconazole, for the four copper applications against cercospora and the doubled peroxide values in the oil, for the anthracnose outbreak with 70% fruit rot in Apulia, for the two pathotypes of Verticillium dahliae and the ineffectiveness of fungicides, and for the detection of Pseudomonas savastanoi in symptomless tissue.
• [3] El Kahkahi, R., Moustaine, M. and Zouhair, R. (2022). Symptomology of Major Fungal Diseases on Olive and Its Management. Austin Journal of Environmental Toxicology 8(1):1043. Source for the accepted name Venturia oleaginea and the spot size of 0.5–1.2 mm, for infection conditions of 12–15 °C with optimum germination at 20 °C, for cercospora (felting on the lower surface, fruit spots 3–7 mm, 10–20 °C), for anthracnose (toxin, branch dieback up to 5 cm, humidity above 93%, mummified fruit as the main inoculum source), for Botryosphaeria dothidea as the accepted name of dalmatian disease and losses above half the harvest, for survival of V. dahliae up to 20 years and the cultural measures, and for sooty mould (genera Capnodium, Fumago, Aureobasidium and the six predisposing factors).
• [4] Lantero, E., Matallanas, B. and Callejas, C. (2023). Current Status of the Main Olive Pests: Useful Integrated Pest Management Strategies and Genetic Tools. Applied Sciences 13(22):12078. Source for the three generations of Prays oleae and the damage of each, for the 20–30 flowers per larva of the anthophagous generation, for the 28-year study in Andalusia (50–60% production reduction, heavy attacks every three years, 40% premature fruit drop), and for Xylella fastidiosa (Apulia outbreak 2013, Aphrophoridae vectors led by Philaenus spumarius, eradication within a 100 m radius, cost above EUR 5.5 billion per year and 70% of the production value of trees over 30 years old).
• [5] Zalom, F.G., Vossen, P.M., Van Steenwyk, R.A. and Johnson, M.W. (2014). Olive Psyllid — Euphyllura olivina. UC IPM Pest Management Guidelines: Olive. UC ANR Publication 3452. Source for the morphology and five nymphal stages of the psyllid, for the three generations and overwintering of adults on the trunk, for the temperature limits, for the 30–60% losses under heavy attack, for the monitoring thresholds per flower cluster, and for the recommendation to treat before the waxy covering forms.
• [6] Khaghaninia, S. and Pourabad, R.F. (2009). Investigation on biology of olive leaf worm Palpita unionalis Hb. (Lepidoptera: Pyralidae) in constant laboratory conditions. Munis Entomology & Zoology 4(2):320–326. Source for the biology of the jasmine moth: cycle of 38 days at 27 °C and up to nine generations, five generations in Italy and ten in Egypt, mean fecundity of 385 eggs, overwintering as a larva, larval and adult size, damage mainly in nurseries and young plantings, the finding of Fodale that with 90% of branches attacked the loss does not exceed 20%, and the activity of Bacillus thuringiensis.
• [7] Database of approved plant protection products, from the Register of the Greek Ministry of Rural Development and Food. Source for the current registration status in Greece by crop, active substance, rate and pre-harvest interval. These approvals are valid in Greece only.
• [8] Regional Centre for Plant Protection, Quality and Phytosanitary Control of Heraklion (2017). Pests of the olive — Technical Bulletin of the Agricultural Warnings service, 9 May 2017 [in Greek]. Greek Ministry of Rural Development and Food. Source for the Greek thresholds and treatment timings: the 50% of normal fruiting threshold for the anthophagous generation of the olive moth and the treatment window at growth stages E and F with Bacillus thuringiensis, the 30% infested flowers threshold for the psyllid in a year of reduced flowering, the symptomatology and management of Pollinia scale, the recommendation of paraffin oils for oleander and olive scale, and the priority given to protecting young plantings and nurseries from the jasmine moth.
• [9] Perdikis, D. et al. (2009). Damage evaluation of Rhynchites cribripennis (Col., Attelabidae) in olive fruits. Journal of Applied Entomology. Source for the damage caused by the olive fruit weevil: fruit drop of 54% with four adults per cage, 38% with two and 16% in the control; damage most severe in the first month after fruit set; a single feeding hole sufficient to drop a small fruit; and the field infestation levels, 19% on 29 May peaking at 26% in mid-July.
• [10] Thomidis, T., Michos, K., Chatzipapadopoulos, F. and Tampaki, A. (2021). Evaluation of Two Predictive Models for Forecasting Olive Leaf Spot in Northern Greece. Plants 10(6):1200. A Greek study using isolates from Chalkidiki. Source for the conidial germination range of Venturia oleaginea of 5 to 25 °C with an optimum of 20 °C, for the requirement of at least 12 h of continuous leaf wetness at the optimum temperature and the absence of germination at 6 h, and for the conclusion that in Northern Greece infection is mainly possible from September to June.
• [11] Viruega, J.R., Moral, J., Roca, L.F., Navarro, N. and Trapero, A. (2013). Spilocaea oleagina in Olive Groves of Southern Spain: Survival, Inoculum Production, and Dispersal. Plant Disease 97(12):1549–1556. A four-year field study. Source for the evidence that infected leaves within the canopy are what matter for pathogen survival and conidium production, and for the seasonal variation in conidial density, peaking from November to February and falling to zero in summer.
• [12] Moral, J. and Trapero, A. (2012). Mummified Fruit as a Source of Inoculum and Disease Dynamics of Olive Anthracnose Caused by Colletotrichum spp. Phytopathology 102(10):982–989. Source for the production of more than 21,600 conidia per mummified fruit under optimal conditions, for maximum production at 20 to 25 °C with 96 h of wetness, for the reduction in production with repeated washings, and for the critical finding that mummies in the canopy retain their output after six months while those on or in the soil see it fall by up to ten thousand fold.
• [13] Talhinhas, P., Loureiro, A. and Oliveira, H. (2018). Olive anthracnose: a yield- and oil quality-degrading disease caused by several species of Colletotrichum that differ in virulence, host preference and geographical distribution. Molecular Plant Pathology 19(8):1797–1807. A review. Source for the identity of anthracnose as a complex of several Colletotrichum species differing in virulence, host preference and geographical distribution, and for the simultaneous degradation of yield and oil quality.
• [14] López-Escudero, F.J. and Mercado-Blanco, J. (2011). Verticillium wilt of olive: a case study to implement an integrated strategy to control a soil-borne pathogen. Plant and Soil 344:1–50. The most extensive review of verticillium wilt of olive. Source for the conclusion that no single method suffices and that an integrated strategy is required, weighted towards preventive measures taken before planting.
• [15] López-Escudero, F.J. et al. (2024). Olive Escudete (Dalmatian Disease) Caused by Botryosphaeria dothidea as a Result of Fly–Midge–Fungus Interaction. Horticulturae 10(4):321. Source for the mechanism of dalmatian disease: the mycangia in the abdomen of the female Prolasioptera berlesiana and the evidence for a mutualistic association with the fungus, the attraction of the midge to olive fly oviposition punctures regardless of the presence of an egg, the 48% of artificially punctured fruit containing a midge against none in shapeless wounds, and the appearance of Botryosphaeria dothidea only in fruit that contained a midge.