Andalucía is the main producing region of olive oil (Olea europaea L.) and table olives in Spain—the worldwide leading producer country (Barranco et al., 2010; IOC, 2013). Thus, any factor that affects olive tree production has a significant effect on the economy of the country. Verticillium wilt of Olive (VWO), caused by Verticillium dahliae Kleb., is currently the most destructive and threatening disease of olive in the Mediterranean region (Bubici and Cirulli, 2011; Hiemstra and Harris, 1998; López-Escudero and Mercado-Blanco, 2011; Tsror, 2011). This disease is the major problem in soils that are infested with highly virulent defoliating isolates of the pathogen, which kills a substantial number of trees annually (López-Escudero and Mercado-Blanco, 2011). The extension of VWO has been associated with the establishment of new olive orchards in infested soils, the use of infected plant material, and the spread of highly virulent isolates (defoliating pathotype). Additionally, a number of effective means of dispersal has efficiently contributed to a wide disease distribution (García-Cabello et al., 2012; López-Escudero and Mercado-Blanco, 2011; Trapero et al., 2011). Therefore, the use of a combination of all of the available measures and strategies has been reiteratively encouraged to control VWO (López-Escudero and Mercado-Blanco, 2011; Tjamos, 1993), with the use of resistant cultivars being the most effective method (Hiemstra and Harris, 1998; López-Escudero and Mercado-Blanco, 2011; Mercado-Blanco and López-Escudero, 2012; Tsror, 2011).
In recent decades, most of the olive tree-producing countries have developed research lines related to the search for resistance to VWO (López-Escudero and Mercado-Blanco, 2011; Mercado-Blanco and López-Escudero, 2012). Specifically in Spain, significant efforts have been focused on the assessment of olive resistance to VWO. These works have been performed by the Department of Agronomy of Córdoba University together with the Group of Olive Germplasm of the IFAPA ‘Alameda del Obispo’ (Córdoba, Spain). These studies have evaluated the resistance of cultivars in the World Olive Germplasm Bank (WOGB) of the IFAPA (Caballero et al., 2006). Most of the evaluated cultivars, including the main Spanish olive cultivars, exhibit high levels of susceptibility to infection (Garcia-Ruiz et al., 2014; López-Escudero et al., 2004, 2007; Martos-Moreno et al., 2006). However, the cultivars Frantoio from Italy and Empeltre and Changlot Real from Spain became highly resistant after artificial and natural infections (López-Escudero and Mercado-Blanco, 2011; Trapero et al., 2013b). Moreover, the interest in this topic has also been reflected in many studies that were conducted in the center and eastern Mediterranean Basin countries beginning in the 1970s (Cirulli and Montemurro, 1976; Paplomatas and Elena, 2001; Tjamos et al., 1985). Similarly, some countries of the eastern Mediterranean have begun evaluating the resistance of their own olive cultivars to V. dahliae (Al-Ahmad and Mosli, 1993; Erten and Yildiz, 2011; Levin et al., 2003; Sesli et al., 2010).
In previous studies, the resistance of olive genotypes to this disease has usually been assessed in growth chambers. However, such environmental conditions permit the evaluation of only a small number of olive cultivars in each assay (López-Escudero et al., 2004, 2007; Martos-Moreno et al., 2006) as a result of the required time and space (Trapero et al., 2013a). Nevertheless, previous studies have demonstrated that the use of greenhouses in our location (southern Spain) permits the assessment of a higher number of cultivars and could provide an alternative method to the use of growth chambers (Garcia-Ruiz et al., 2014).
The use of different lighting ranges and their influence on the plant growth have been evaluated in several studies of normally herbaceous species (Pegg and Brady, 2002). Moreover, daylength significantly affects disease development in verticillium wilts and plant resistance, reflected mainly in the levels of mycelial colonization, although some reported results are contradictory (Pegg and Brady, 2002). Therefore, in potato (Solanum tuberosum), a short-day photoperiod under field conditions is necessary for tuber formation and increases the disease level (Busch and Edgington, 1967; Tsror et al., 1990). Additionally, Pegg and Jonglaekha (1981) demonstrated that Chrysanthemum (Chrysanthemum sp.) plants that were infected with V. dahliae and grown under long-day conditions showed only slight wilt symptoms, whereas greater mycelial growth and more extensive stem colonization occurred when the plants were grown under reduced light conditions. In contrast, Sackston and Sheppard (1973) reported that sunflower (Helianthus annuus L.) plants developed more severe symptoms of verticillium wilt when grown under long-day than under short-day conditions.
Temperature also significantly influences the mycelial growth of the pathogen and plant colonization. V. dahliae has an optimal growth range of 22 to 27 °C (Pegg and Brady, 2002), and some growth can occur even at 33 °C, but no microsclerotia were found at temperatures greater than 30 °C (Devaux and Sackston, 1966). In cotton (Gossypium sp.), the expression of resistance to a given strain of V. dahliae can vary with the temperature (Barrow, 1970; Bell and Presley, 1969; Xu et al., 2012), which could mask the final recorded disease severity (Garber and Presley, 1971). In tomato (Solanum lycopersicum L.), high temperatures and short-day conditions encourage the development of verticillium wilts in susceptible, tolerant, and resistant cultivars (Jones et al., 1978).
In olive tree, few studies investigated the olive–verticillium–temperature interaction. Indeed, experiments to determine the influence of global climate change on V. dahliae pathotypes and the development of the disease in olive plants exposed that, under the current CO2 concentration, optimal verticillium wilt development occurred from 20 to 24 °C and was more rapid and severe under the most favorable extremely susceptible cultivar–D pathotype combination (Lucena et al., 2013). The aim of this research work was to assess the influence of continuous and natural lighting on VWO onset and development when assessing the resistance of olive cultivars to the defoliating pathotype of Verticillium dahliae under greenhouse conditions.
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