Peacock spot

Olive scab disease, also called peacock’s eye, peacock spot or leaf spot, is caused by the ascomycete fungus Venturia oleaginea and is the most important foliar disease of olive (Olea europaea) in many olive-growing regions, including southern Spain. The pathogen mainly attacks the adaxial surface of the olive leaf, causing typical scab lesions that are often surrounded by yellow haloes leading to premature leaf drop (Figure 1). Lesions can also occur along the main vein on the abaxial leaf surface and on leaf petioles, fruits, fruit peduncles and young shoots (Figure 2). The dark colour of the lesions is due to the production of asexual spores or conidia, which are the only inoculum causing infections, since the fungus lacks a sexual stage that produces ascospores. Heavy defoliation usually follows severe infection, and recurrent infections cause branch dieback and tree weakness. Most infected leaves and fruit fall to the ground (Figure 3). Olive oil obtained from fallen fruits is of poor quality because various saprophytic fungi colonise the fruit and change the acidity and organoleptic characteristics of the oil. The disease is particularly severe in nurseries and in orchards that are densely planted with susceptible olive cultivars when environmental conditions are favourable.

Venturia oleaginea survives the summers as mycelium in infected leaves that remain in the tree canopy. In autumn, the mycelia resume growth from latent infections caused during the previous spring or from old lesions, producing new conidia that are dispersed by rain splash and run-off. These conidia, formed in scab lesions, cause new infections as they spread via rain droplets. Because mature olive leaves have ontogenic resistance to infection by V. oleaginea, the main infection periods occur during periods of leaf growth. If these periods of foliar growth are rainy or accompanied by high relative humidity and mild temperatures (10-20ºC, optimum about 15ºC), severe infections occur. In a Mediterranean climate, these favourable conditions for infection occur in spring and, to a lesser extent, in autumn (Figure 4).

Olive scab is mainly controlled by a combination of sanitation and cultural measures (planting location and density, pruning and plant covers to reduce foliar wetting; avoiding excess nitrogen and potassium deficiency), and by applications of fungicides, mainly copper-based products. Correct timing of fungicides is essential for effective control. The application and effectiveness of these measures depends greatly on the susceptibility of the olive cultivars. Therefore, in areas that are favourable for olive scab, the use of resistant or moderately susceptible cultivars is a fundamental part of the integrated management of the disease.

Although it is well known that there are differences in susceptibility to scab between olive cultivars (Figure 5), very few studies have been conducted on the screening of olive cultivars or genotypes for their resistance or susceptibility to olive scab. The large number and variability of existing olive tree varieties mean that susceptibility or resistance to olive scab remains a controversial aspect. Although information is abundant, most studies are based on field observations, conducted in different environments and without common comparison criteria, leading to frequent contradictions. This has led, for example, to a variety like Manzanilla de Sevilla being considered very susceptible, moderately susceptible or resistant and, in available databases such as the OLEA databases (www.oleadb. it), there is a large amount of conflicting information.

In addition to the difficulties noted above, a factor that could contribute to explaining these differences is the existence of pathogenic variability in V. oleaginea populations. However, this aspect of the disease is still little known. Recent studies under artificial inoculation have demonstrated the occurrence of pathogenic variability between populations of V. oleaginea from different olive-growing regions. This variability could explain the differences in the behaviour of some olive cultivars that are very susceptible in their area of origin, such as Arbequina in Catalonia and Frantoio in Italy, but have been moderately or highly resistant to the pathogen populations present in Córdoba.

Field evaluations of olive scab carried out between 1994 and 2000 at the Olive Germplasm Bank in Córdoba allowed 323 varieties from 18 countries to be classified into five susceptibility categories. Table 1 summarises this classification for all varieties evaluated, showing 44 of the 56 olive cultivars included in the IOC World Catalogue of commercially propagated olive varieties, which are grouped by the susceptibility category to which they belong. In each category, the representative variety is highlighted in bold.

Table 1. Susceptibility of olive cultivars to scab caused by Venturia oleaginea

¹ Cultivars included in the IOC World Catalogue of commercially propagated olive varieties. The representative cultivar of each category is indicated in bold.

² This cultivar is not included in the IOC World Catalogue, but is representative of this category. 

³ Marked differences between repetitions (olive trees) or observations (years) that did not allow the classification of the cultivars.

A among the advanced selections from the Córdoba olive tree breeding programme, there are new genotypes with resistance or tolerance to scab, as resistant cultivars have been included among the parents used for crosses. Although the mechanism or mechanisms responsible for the resistance of olive varieties to V. oleaginea, have been the subject of various studies, they are still not well understood. Structural characteristics related to the thickness and composition of the cuticle and, especially, biochemical mechanisms related to the formation and accumulation of phenolic compounds in the infection zone - mainly derived from oleuropein, a common component of olive leaves and fruits - have been suggested. The genetic basis of resistance to olive scab is also unknown, although there is evidence that points towards a recessive allele and recent molecular studies have identified several genes involved in disease resistance.

Legend of the Figures

Figure 1. Characteristic leaf symptoms of olive scab: A) dark circular lesions without yellow halo (winter symptoms); B) dark circular lesions with yellow halo (spring symptoms).

Figure 2. Variability of symptoms of olive scab: A-B) lesions on the upper side of the leaves; C) lesions on the underside of the leaves and on the peduncle of the fruit; D) lesions on fruits and shoots.

Figure 3. Olive trees severely defoliated by Venturia oleaginea.

Figure 4. Life cycle of olive scab caused by Venturia oleaginea.

Figure 5. Two severely affected olive trees of a cultivar highly susceptible to olive scab (center) and two lightly affected olive trees of a resistant cultivar (left and right) at the World Olive Germplasm Bank in Córdoba.