Olive breeding for Verticillium wilt resistance

Verticillium wilt of olive, caused by the soilborne fungus Verticillium dahliae Kleb., represents currently the main phytosanitary limitaiton in olive orchards for its destructive potential and spreading. The use of resistant cultivars represents one of the most efficient control measurement because of its low cost and environmental friendliness. However, only a few traditional cultivars have showed high levels of resistance to the disease and, in some cases, have negative agronomic traits that limit their use. For this reason, an olive breeding program was initiated to obtain new cultivars with high level of disease resistance and good agronomic characteristics. In this work, seedlings from open-pollination of 26 olive cultivars were evaluated. Bearing of seedlings started from the third year after germination. Agronomic evaluation allowed the selection of interesting genotypes based mainly on the lenght of the juvenile period, productivity and oil content. The selected genotypes were propogated to test their resistance to Veerticillium wilt by dipping the root system in a conidial suspension of the pathogen, using two cultivars of known resistance level as controls, 'Piculal' (susceptible) and 'Frantoio' (resistant). Disease reaction was evaluated periodically assigning to the aerial part of each plant a severity value from 0 to 4 (0: no symptoms, 4: dead plant). Finally, four of the genotypes evaluated showed similar level of resistance than 'Frantoio' open pollination, expressed no symptoms during the experimental period. These four genotypes had showed a good behaviour on the previous agronomic evaluation. The level of resistance and agronomic performance of these genotypes will be confirmed in future studies under both field and controlled conditions.