Anthracnose

Anthracnose is the most critical fruit disease affecting the olive crop. Eighteen species of the genus Colletotrichum (mainly C. godetiae, C. nymphaeae, and C. acutatum) have been associated with this disease in most olive-growing regions, including non-Mediterranean areas (reviewed by Cacciola et al. 2012; Talhinhas et al., 2018; Moral et al., 2021; Moreira et al., 2021; Garcia-Lopez et al., 2023). The disease has two syndromes: fruit rot (also known as “soapy rot”) and the dieback of branches. In addition, oils coming from affected olive fruits display substantial chemical and organoleptic defects (Leoni et al., 2018; Moral et al., 2014)

Marked differences in cultivar resistance to the pathogen have been described under both controlled and field conditions (Moral and Trapero, 2009). Importantly, evaluating the fruit-rot incidence under field conditions during conducive seasons allows us to assess cultivar resistance (Moral et al., 2017), as there is a good correlation between disease severity and fruit-rot incidence (Licciardello et al., 2022). Here, we show the resistance/susceptibility of the olive cultivars presented in the International Olive Council´s catalogue according to their reaction to the pathogen (Table 1). Cultivars were grouped following a five-mean cluster analysis, where seven olive cultivars are considered highly resistant to the pathogen: Bosana, Crnica, Frantoio, Istarska Bjelica, Kato Drys, Koroneiki, and Ouslati. The use of resistant olive cultivars against Anthracnose is the most effective control method and can be combined with other measures, such as biological and chemical methods or cultural practices (Moral and Trapero, 2009; Moral et al., 2018; Preto et al., 2017).

Table 1. Reaction of olive cultivars selected for the catalogue to Anthracnose, caused by Colletotrichum spp.

Under field conditions, evaluations should be conducted during disease-conducive seasons in germplasm olive banks since Colleottrichum causes highly weather-driven epidemics (Moral and Trapero, 2012). In any case, it is recommended that evaluations only be carried out when well-known susceptible olive cultivars (e.g., Hojiblanca or Galega Vulgar) have a high incidence of fruit rot (Moral et al., 2017). To estimate the stability of the olive cultivar´s reaction to Anthracnose, researchers can calculate the proportion explained by the cultivar, the season and the interaction between cultivar and season (Figure 1). Because ANOVA is very sensitive to unequal classes on rating scales (i.e., scale class values do not hold the same quantitative significance), researchers should transform the classes values data into fruit-rot incidence before perfoming calculations of cultivar stability. For example, if they are using the 0-10, rating scale described by Moral et al. (2017), ANOVA should be performed on the percentage of affected fruits (Y) of the cultivars based on the sigmoidal equation:

In our evaluations, and according to η2 (relationship between the variability associated with an effect and the total variability of our analysis), the variety is the most important factor, explaining 56% of the total variance, followed by the interaction cultivar × season (19%) and the season (10%). Overall, interactions higher than 12% can be considered elevated, mainly due to moderately susceptible varieties, which can show marked changes in their reaction to the pathogen between seasons. Conversely, highly resistant and susceptible varieties are much more stable in their behaviour against the pathogen.

Figure 1. Proportions of variability explained for Anthracnose by the cultivar, year, and the interaction between cultivar and year. The analysis of variance (ANOVA) was performed on the percentage of affected fruits of the cultivars, and eta squared (η2) was calculated as the relationship between the variability associated with an effect and the total variability of our analysis.

References

1. Cacciola, S. O., Faedda, R., Sinatra, F., Agosteo, G. E., Schena, L., Frisullo, S., & di San Lio, G. M. (2012). Olive Anthracnose. Journal of Plant Pathology, 29–44.
2. Garcia-Lopez, M.T., Serrano, M.S., Camiletti, B., Gordon, A., Estudillo, C., Trapero, A., Diez, C. M., & Moral, J. (2023). Study of the competition between Colletotrichum godetiae and C. nymphaeae, two pathogenic species in olive. Scientific Reports, 13, 5344.
3. Leoni, C., Bruzzone, J., Villamil, J. J., Martínez, C., Montelongo, M. J., Bentancur, O., & Conde-Innamorato, P. (2018). Percentage of Anthracnose (Colletotrichum acutatum) acceptable in olives for the production of extra virgin olive oil. Crop Protection, 108, 47–53.
4. Licciardello, G., Moral, J., Strano, M. C., Caruso, P., Sciara, M., Bella, P., … & Di Silvestro, S. (2022). Characterization of Colletotrichum strains associated with olive anthracnose in Sicily. Phytopathologia Mediterranea, 61(1), 139–151.
5. Moral, J., & Trapero, A. (2009). Assessing the susceptibility of olive cultivars to Anthracnose caused by Colletotrichum acutatum. Plant Disease, 93(10), 1028–1036.
6. Moral, J., & Trapero, A. (2012). Mummified fruit as a source of inoculum and disease dynamics of olive Anthracnose caused by Colletotrichum spp. Phytopathology, 102, 982–989.
7. Moral, J., Xaviér, C., Roca, L. F., Romero, J., Moreda, W., & Trapero, A. (2014). La Antracnosis del olivo y su efecto en la calidad del aceite. Grasas y Aceites, 65(2).
8. Moral, J., Xaviér, C. J., Viruega, J. R., Roca, L. F., Caballero, J., & Trapero, A. (2017). Variability in susceptibility to Anthracnose in the world collection of olive cultivars of Cordoba (Spain). Frontiers in Plant Science, 8, 1892.
9. Moral, J., Agustí-Brisach, C., Agalliu, G., Oliveira, R., Pérez-Rodríguez, M., Roca, L., Romero, J., & Trapero, A. (2018). Preliminary selection and evaluation of fungicides and natural compounds to control olive anthracnose caused by Colletotrichum species. Crop Protection, 114, 167–176.
10. Moral, J., Agustí-Brisach, C., Raya, M.C., Jurado-Bello, J., López-Moral, A., Roca, L., Chattaoui, M., Rhouma, A., Nigro, F., Sergeeva, V., & Trapero, A. (2021). Diversity of Colletotrichum species associated with Olive Anthracnose Worldwide. Journal of Fungi, 7(9), 741.
11. Moreira, V., Mondino, P., & Alaniz, S. (2021). Olive Anthracnose caused by Colletotrichum Uruguay: Field symptoms, species diversity and pathogenicity on flowers and fruits. European Journal of Plant Pathology, 663–681.
12. Preto, G., Martins, F., Pereira, J. A., & Baptista, P. (2017). Fungal community in olive fruits of cultivars with different susceptibilities to Anthracnose and selection of isolates to be used as biocontrol agents. Biological Control, 110, 1–9.
13. Talhinhas, P., Loureiro, A., & 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.