Verticillium wilt of olive caused by Verticillium dahliae

Verticillium wilt of olive, caused by the soil-borne fungus Verticillium dahliae Kleb., is globally the most important disease affecting this crop. Over recent years, this fungus has led to the demise of thousands of olive trees across the world. The situation is exacerbated by factors such as susceptible cultivars, irrigation practices and planting on soil that was previously planted with host crops like cotton or tomato (Montes-Osuna and Mercado-Blanco, 2020).

Given the absence of efficient chemical solutions, the vulnerability of popular olive cultivars, and the persistence of the fungus in the soil, there has been a significant effort to identify resistant cultivars. This is seen as a cornerstone in a holistic approach to managing the disease (López-Escudero and Blanco-López, 2007).

In this sense, numerous olive cultivars and related germplasm have been assessed for resistance in recent times (García-Ruiz et al., 2014, 2015; Markakis et al., 2022; Godena et al., 2022). This evaluation process is ongoing and hundreds of cultivars have been studied globally (Table 1). Regrettably, the majority have shown vulnerability to V. dahliae infections (López-Escudero and Mercado-Blanco, 2011). A mere trio of cultivars, namely Empeltre, Frantoio and Changlot Real, have exhibited strong resistance to Verticillium wilt, but these cultivars show some agronomical challenges, making them less suitable for use in impacted areas (López-Escudero et al., 2004, 2007; Martos-Moreno et al., 2006; Trapero et al., 2013; García-Ruiz et al., 2014; 2015; Trapero et al., 2015). In addition, work is also being carried out to evaluate wild genotypes and develop new cultivars resistant to the pathogen (Valverde et al., 2023). On the other hand, there is promising work in the search for biological control products that help control the disease (López-Moral et al., 2021; Mulero-Aparicio et al., 2023; Montes-Osuna et al., 2021b).

Because the pathogen is found in the soil and there are some cultivars with a high level of resistance, some work has focused on grafting susceptible cultivars onto a resistant rootstock. In this sense, Bubici et al., (2013) concluded, under controlled conditions, that grafting could be a solution. However, in a later study conducted by Valverde et al. (2021), they evaluated symptoms in a highly infested field over 5 years and concluded that the pathogen is capable of passing through the resistant rootstock and reaching the grafted (susceptible) cultivar, causing the death of the plants.

Currently, the main research topics aimed at controlling the disease are:

-Continue screening olive cultivars, including those from germplasm banks and wild genotypes, as well as breeding olives for the development of new cultivars.

-Develop and fine-tune of molecular technology for the identification of resistant cultivars.

-Develop control products, either through chemical control or  biological means.

-Optimise integrated control strategies, including fertilisation, irrigation management and overall disease management in the field. 

Table 1. Level of resistance against Verticillium dahliae in the 55 cultivars selected for the catalogue. Resistance is assessed both under controlled conditions (laboratory/greenhouse) and at field level, due to differences observed depending on the type of evaluation.

References

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