Decisions made prior to planting the olive trees are essential to control the disease. The choice of cultivation site, plant density, and plantation system are very important since all conditions that reduce lightening and aeration of canopies and increase humidity and wetting period, greatly favour V. oleaginea infections. On hill areas the choice of sites with good exposures (south, south-west, south-east, west and east) is desirable. On the contrary, poorly ventilated or low-lying areas, low permeable soils, and land close to rivers, streams, or lakes should be avoided (Agosteo and Schena, 2011). In any case, correct soil drainage and other practices aimed at reducing soil moisture content can contribute to reducing disease incidence.

Traditional olive orchards in the majority of the Mediterranean Countries are planted at a density of about 100-300 trees per ha. During the last decades, intensive-planting- (300-400 trees/ha) and high-planting- (more than 1500 trees/ha) density orchards have been developed posing new challenges to the effective control of diseases and pests (Hall, 2011; López-Escudero and Mercado-Blanco, 2011; Connor et al., 2014). In these orchards a combination of planting design, pruning and controlled irrigation and fertilization is essential to reduce humidity and increase the efficiency of treatments by allowing the penetration of fungicides inside the canopy (Connor et al., 2014).

After planting, the cultural practices such as fertilization, soil management, pruning and irrigation have a significant influence on OLS severity. Excessive nitrogen fertilizations should be avoided since they have been associated to an increased plant susceptibility to OLS (Ouerghi et al., 2016; Roca et al., 2018) although, in a recent study, the incidence of the disease was not related to nitrogen fertilization (Rodrigues et al., 2019). In any case, a balanced fertilization is important to restrains excessive vegetative growth which can increase shading and relative humidity that favour infections. Potassium deficiency can favour OLS disease (Trapero and Blanco, 2008) and its supply was reported decrease the incidence of the disease (Ouerghi et al., 2016).

As far as soil management is concerned, the excessive growth of weeds which can sometimes reach the lower portions of canopies should be avoided. Soil tillage was associated to a reduced incidence of OLS since it prevented the excessive growth of weeds (Rhimini et al., 2020) but likely, similar results could also be obtained by mowing weeds.

Regular (annual) pruning which improves lighting and aeration is an important cultural practice for increasing lighting and reducing relative humidity and wetting periods inside the canopies (Issa et al., 2019). Pruning is also important to improve the penetration of fungicides inside the canopies during treatments.

Irrigation potentially affects the development of OLS disease by influencing the humidity of the orchard. However, no data are currently available on this practise. Nevertheless, it can be argued that management of irrigation by avoiding excessive use of water and the use of localized application (i.e., drip irrigation), along with the use of deficit irrigation, may help in reducing the incidence of OLS disease.

The use resistant germplasm is considered one of the most important strategies to control OLS. For each area in which olive is cultivated, the information available regarding the most resistant cultivars and genotypes should be considered to make the best choice of the cultivar, especially when eco-sustainable cultivation methods are applied, such as the organic one. However, the choice of the cultivar is not simple since the many studies conducted in open-field conditions to identify resistant cultivars have frequently produced contradictory results. Frequently, the same cultivar is classified as resistant in a study and susceptible in another (Sanei and Razavi, 2011; Ouerghi et al., 2016; Bernaschina et al., 2020). These contradictions are mainly caused by the incorrect identification of the plant material and by differences in behaviour of clones of the same cultivar. However, other factors such as variability in virulence of V. oleaginea strains, and different climatic conditions in the geographical areas under study may play an important role (Agosteo and Zappia, 2007; Trapero and Blanco, 2008; Bernaschina et al., 2020). In the last decade, molecular investigations, mainly based on the analysis of microsatellites, have contributed to better define olive cultivars and should greatly facilitate the future selection of cultivar resistances to OLS (Muzzalupo et al., 2014; Gómez-Rodríguez et al., 2021). For instance the OLEA databases is a portal established in 2007 by olive researchers in Europe and contains comprehensive information about molecular and biological traits of olive cultivars including valuable data about the resistance to biotic and abiotic stress factors (http://www.oleadb.it). In Table 1 are reported main olive cultivars with low foliar susceptibility to V. oleaginea.

Studies to select and produce new varieties resistant to OLS through breeding programs have been very limited. As a result, only very few resistant genotypes have been produced (Lavee et al., 1999; Lavee et al., 2004). Further genomic (Guerin et al., 2002; Mekuria et al., 2002) and transcriptomic (Benitez et al., 2005) studies will be fundamental to identify genetic traits useful to assist breeding programs for OLS resistance.

In a breeding program, it is of paramount importance to know the resistant/susceptibility mechanisms to a plant disease. Among the structural pre-existing mechanisms, a high thickness of the cuticular layer (Petri, 1913; Agosteo and Schena, 2011; Ouerghi et al., 2016) and thinner palisade cells (Morettini, 1954) were correlated with greater resistance to OLS. The comparison of different cultivars (Chetoui, Picholine, Arbequina, and Meski) suggested that a higher incidence of OLS disease tends to be associated with thinner cuticle and higher trichome density and diameter (Ouerghi et al., 2016). All that is consistent with the ontogenic resistance observed in olive leaves to OLS disease. In fact, young olive leaves are less cutinized respect to older ones and are more susceptible to OLS (Graniti, 1993; López-Villalta, 1999; Sergeeva et al., 2009; Viruega et al., 2011; Sanei and Razavi, 2011; Ouerghi et al., 2016). This observation is further strengthened by recent results obtained by Scibetta et al. (2020) through a molecular approach, who demonstrated that very young olive leaves are susceptible to OLS disease, while in the past they were considered immune or at least less susceptible to infections (Peglion, 1928).

As concern the chemical mechanisms, Iannotta et al. (2007) reported a negative relationship between the susceptibility to V. oleaginea infections and the level of oleuropein in the leaves. El Aabidine et al. (2010) demonstrated that tyrosol and its derivatives were associated with preexisting defenses, whereas oleuropein and rutin were correlated with induced defenses. Authors speculated that the biosynthetic pathways of latter phenolic compounds and/or the expression rate of the corresponding genes could be at the origin of the degree of resistance of olive tree/varieties to OLS disease. Rahioui et al. (2013) reported faster and more intense PAL (phenylalanine ammonia-lyase) activity and accumulation of soluble and parietal phenols in leaves of a resistant cultivar as compared to a susceptible one.

Chemical control of OLS disease is mainly carried out with contact copper compounds and to a lesser extend with cytotropic-translaminar (dodine and strobilurins) and systemic (triazoles) fungicides (Obanor et al., 2005; Roca et al., 2007). Copper compounds, such as bordeaux mixture, copper oxychloride, copper hydroxide, copper oxide and tribasic copper sulphate, have a double action: prevention of the pathogen attacks on healthy leaves and reduction of the inoculum for new infections (Iannotta et al., 2002; Obanor et al., 2008a; Belfiore et al., 2014). In fact, copper can penetrate leaves through wounds caused by erupting conidia during the evasion and being copper phytotoxic, causes the premature drop of leaves (Graniti, 1993). As a result, there is a reduction of the inoculum available for new infections as the masses of conidia on the fallen leaves are no longer available for further infections (Agosteo and Schena, 2011; Trapero and Blanco, 2008). Since the efficacy of copper is strictly related to the disease cycle of the pathogen (eruption of conidia), the use of predictive models to determine frequency and timing of fungicide applications would be desirable (Thomidis et al., 2021). However, the use of predictive model is still very limited and treatments are empirically made in autumn and/or spring in most olive-growing countries (Agosteo and Schena, 2011).

In recent years the use of copper compounds in agriculture have raised concerns for the high level of accumulation in the soil, the risk of surface water contamination, and the potential public health problems due to the entering of copper in the food chain (Lamichhane et al., 2018). As a consequence, the European Union has restricted the use of copper compounds prescribing a maximum of 28 kg/ha of metallic copper over a period of 7 years (on average 4 kg/ha/year) and further restrictions are expected in the near future. To optimize OLS control with copper fungicides, it is therefore advisable to use: i) formulations with high efficacy and lower copper amount; and ii) rate and spray volumes calculated on the basis of tree canopy- or tree-row-volumes (Lamichhane et al., 2018).

In comparative OLS disease control trials in which copper hydroxide, copper sulfate, copper oxide, or copper oxychloride were applied in autumn and spring (3-4 applications), the best efficacy was obtained with copper hydroxide, which provided a relatively low content of metallic copper (Pasqualini et al., 2020). Interesting results have been recently obtained with a new formulation containing copper complexed with gluconate and lignosulphonate (Disper Cu Max^®-Eden Modern Agriculture S.L-Spain), which, in spite of the lower content of metallic copper (e.g. five times lower than copper hydroxide), proved more effective than traditional formulations (Almadi et al., unpublished; Adawi et al., 2022). The higher efficacy was associated to a supposed cytotropic phytomobility of the formulation, which may also exert a curative action. Recently, nanoparticles appeared particularly promising to improve the efficacy and reduce the impact of control strategies against plant diseases (Ungureanu et al., 2021; Balestra and Fortunati, 2022). In particular, copper nanoparticles were more active in reducing V. oleaginea mycelial growth and OLS incidence compared with copper oxide and copper hydroxide (Ntasiou et al., 2021).

Among the curative fungicides, dodine has been reported to be as effective as copper compounds (Iannotta et al., 2002; Belfiore et al., 2014; Lahoz et al., 2014), or better when used in highly infected trees, as a result of its curative effect. In a recent study conducted on olive trees with a high latent infection incidence, dodine was more effective than copper in reducing new infections, the appearance of symptomatic leaves and the defoliation (Almadi et al., unpublished). In particular, the use of dodine against the autumnal attack of V. oleaginea, which causes heavy defoliation in the successive spring, allowed most of the old leaves to remain on the tree until the new ones had formed (Almadi et al., unpublished). Authors highlighted that this was important to supports the olive growth processes in the early part of the growing season and increased the growth of inflorescences (Almadi et al., unpublished). In Italy, protocols for integrated pest management specify only one treatment/year with dodine, however its application both in autumn and at the end of winter – beginning of spring may be advisable in the case of heavy autumnal infections (Almadi et al., unpublished). In this way dodine is used once per year, but with a strong synergic effect. Heavy treatment schedules should be aimed to reduce year-to-year the inoculum of V. oleaginea in order to progressively reduce the number of future treatments.

As concerns others curative fungicides, interesting results were obtained with strobilurins and triazoles. In particular, high protections were obtained with tebuconazole + trifloxystrobin (D'Ascenzo et al., 2014; Vizzarri et al., 2014; Najafi et al., 2021), azoxystrobin (Zuffa and Ricci, 2018), difenoconazole + azoxystrobin (Almadi et al., unpublished), and kresoxim-methyl (Viruega et al., 2002; Trapero et al., 2014), kresoxim-methyl + copper sulphate (Obanor et al., 2008a).

Replacing fungicides with other less harmful products for controlling OLS disease is a major challenge especially in organic agriculture. Some bacteria (Pseudomonas, Bacillus and Microbacterium) and fungi (Alternaria, Aureobasidium and Phoma) isolated from olive leaves were able to reduce the germination of conidia of V. oleaginea (Roca et al., 2009; Al-Khatib et al., 2011; Salman, 2017). Other antagonists such as Pseudomonas fluorescens (ORS3), Bacillus atrophaeus (BAT), and Bacillus amyloliquefaciens (FZB24) proved also effective in natural field conditions (Salman, 2020; Tziros et al., 2021). In recent years the investigation of epiphytic and endophytic microbial populations (microbiome) associated to olive trees has increasingly appeared as strategic to develop effective biological control strategies (Abdelfattah,et al, 2018; Abdelfattah et al., 2015; Malacrinò et al., 2022a). Indeed, the microbiome science is revealing the importance of complex network of interactions between microorganisms within each ecological niche, suggesting that a single microbial isolate might not be sufficient to contrast the development of plant pathogens. In this context, synthetic microbial communities (SynComs) defined as small consortia of microorganisms designed to mimic, at some scale, the observed function and structure of the microbiome in natural conditions might be the key to develop the next generation of effective and sustainable management strategies (Malacrinò et al., 2022b).

A number of natural substances have been assayed to control OLS disease. Rongai et al. (2012) reported the use of a Brassicaceae meal in vegetable oil to protect olive trees. Five applications of this formulation (2 in spring and 3 in autumn) provided a protection comparable with that exerted by dodine. Treatments with a pomegranate peel extract (PGE) few days before full boom significantly prevented latent infections over the summer period and reduced the premature defoliation of plants (Pangallo et al., 2022). Promising results were also obtained with aqueous leaf extracts of Ambrosia artimisiifolia (Kleef and Salman, 2021).

Another possibility for controlling plant diseases is the exploitation of plant induced systemic resistance (Buonaurio et al., 2009; Walters and Fountaine, 2009). In pot-cultivated olive plants, promising results were obtained with a number of plant resistance inducers, applied as foliar spray or soil drench: acibenzolar-S-methyl, 3-amino butyric acid, chitosan, harpin, laminarin, phosphates (FoliR-Fos 400^®), salicylic acid and silicon (Actisil^®) (Obanor et al., 2013; Nascimento-Silva et al., 2019; Tziros et al., 2021). Recently, in an open-field trial, a very good protection against V. oleaginea attacks, comparable to the dodine, was demonstrated using the self-defense inducer Disper Broton GS^® (Adawi et al., 2022). This formulation also contains complexed copper with an amount of metallic copper 10 times lower than copper hydroxide. It is important to underline that copper compounds and others fungicides in addition to the direct antifungal activity may also act by inducing resistance in the host (Hammerschmidt and Kuc, 1995; Gozzo and Faoro, 2013). Similarly, biological control agents such Bacillus amyloliquefaciens and plant extracts such as PGE are known to induce resistance in plants (Chowdhury et al., 2015; Belgacem et al., 2021).

Finally, worth of mentioning is also the use of biostimulants, which include microbials, botanicals and chemicals increasingly used in agriculture for their beneficial effects on plant growth and health (Corsi et al., 2022). Recently, they have been also used in oliviculture to promote the growth and to protect olive trees from Verticillium wilt (Almadi et al., 2020; Regni et al., 2021; Lopez-Moral et al., 2021).

Chemical control of OLS disease is traditionally performed according to a calendar of applications and spring and early autumn are indicated as critical periods for disease control in many Countries where olive is grown (Obanor et al., 2008a; Viruega et al., 2011). However, the rational control of plants pests should be based on integrated pest management (IPM), which is fundamental for ensuring agricultural productivity while maintaining economic and environmental sustainability (Rossi et al., 2019). IPM is based on eight principles: 1) prevention of pest occurrence and suppression of pest populations; 2) monitoring of harmful pests; 3) informed decision-making; 4) priority to non-chemical methods; 5) multi-criteria selection of pesticides; 6) pesticide use reduction; 7) avoidance of pests resistance to pesticides; and 8) evaluation (Barzman et al., 2015). Plant protection models and other decision tools have assumed a key role in supporting decision-making process.

Viruega et al. (2011; 2013) and Obanor et al. (2011) studies have significantly contributed to develop OLS disease epidemiological models, determining the influence of a number of host, pathogen and environmental factors on V. oleaginea infections. Roubal et al. (2013) developed empirical models essentially based on rain events, air temperature and duration of relative humidity. Thomidis et al. (2021) studied the effect of temperature and leaf wetness on conidial germination of Greek V. oleaginea isolates and verified the validity of the generic and polynomial models developed by Magarey et al. (2005) and Obanor et al. (2011), respectively. The generic model predicted lower severity, which fits well with the incidence of the disease symptoms on unsprayed trees. In contrast, the polynomial model predicted high severity levels of infection, but these did not fit well with the incidence of disease symptoms.

Respect to empiric models, mechanistic ones are highly reliable in investigating how biotic and abiotic drivers influence individual’s life-history and, in turn, the overall population dynamics/epidemics (Rossi et al., 2019). A weather-driven, eminently mechanistic and dynamic model, denominated SCABOE (scab of Olea europaea) has been constructed on the basis of the literature data to simulate the ecological behavior of OLS disease (Rodrı́guez, 2017; Romero et al., 2017).

SCABOE represents a good model for scheduling treatments with chemical or biological formulations in olive orchards, but it still requires further validation in different olive producing location and countries. In particular SCABOE should be improved through the acquisition of further knowledge on the effect of V. oleaginea on leaf fall (Romero et al., 2017), which could also contribute to the intervention threshold determination. In fact, in experiments aimed at evaluating the effect of artificial defoliation on photosynthetic activity, it has been demonstrated that a reduction in leaf surface area up to 20% can be compensated by an increase in the photosynthetic activity of the remaining leaves (Proietti et al., 2006). Hence, it can be argued that when the percentage of symptomatic + asymptomatic infected leaves exceeds this value, treatments should be performed in order to keep the disease under control.