Resistance to Verticillium wilt in olive progenies from open-pollination

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Verticillium wilt, a vascular disease caused by the soilborne fungus Verticillium dahliae Kleb., constitutes currently the major cultivation constraint in many olive growing areas. An integrated disease management strategy is recommended for Verticillium wilt control including the use of resistant cultivars as one of the most efficient control measures. However, only a few traditional cultivars have showed high levels of disease resistance. For this reason, an olive breeding program was initiated aiming at obtaining new cultivars displaying both high levels of disease resistance and good agronomic characteristics. In this work, the resistance to the defoliating pathotype of V. dahliae was evaluated in 64 genotypes that included 12 cultivars from the World Olive Germplasm Bank of Córdoba (Spain) and 52 genotypes coming from open-pollination of these cultivar plus three other cultivars. These genotypes were previously selected from a wider initial population on the basis of their favourable agronomic performance, mainly early crop (short juvenile period) and high oil content. Inoculation experiments were carried out under controlled conditions in a growth chamber by dipping the root system of plant cuttings in a conidial suspension of the pathogen. A Relative Susceptibility Index (RSI), which summarized several disease parameters, and plant colonization parameters were used to assess the resistance response. Eight of the evaluated genotypes showed similar resistance level to external development of symptoms than ‘Frantoio’ (the control resistant cultivar). Vascular colonization estimated by isolation of the fungus suggests that a tolerant more than resistant plant defence mechanism may be operating in some genotypes. The correlation between values for female parents and their progenies suggests a high heritability in some disease parameters.