Grafting Olive to Control Verticillium Wilt: A Study Under Controlled Conditions

The fungus Verticillium dahliae causes Verticillium wilt of olive (Olea europaea), which is nowadays considered the most important disease of this crop. Genetic resistance is probably the best control measure in an integrated production framework. As V. dahliae is a soilborne fungus, the resistance to this pathogen could be provided by either the rootstock or the scion. However, olive cultivars are usually grown in their own roots, and there is a lack of information regarding the resistance to Verticillium wilt provided by rootstocks. In this study, the use of resistant rootstocks to avoid Verticillium wilt disease was tested in controlled conditions. Thus, the susceptible cultivar ‘Picual’ were grafted on plants of the resistant one ‘Frantoio’ (‘Picual’/’Frantoio’). Self-rooted plants of both cultivars were also included in the experiment. All plants were inoculated by dipping their roots in a conidial suspension (10⁷ conidia/ml) of a highly virulent isolate of V. dahliae and incubated in a growth chamber. The severity of disease symptoms was weekly assessed and V. dahliae DNA was quantified both in the rootstock and scion tissues every 3 weeks by real-time PCR. While self-rooted ‘Picual’ plants showed a very susceptible reaction to the infection, all the ‘Frantoio’ and the ‘Picual’/‘Frantoio’ plants remained free of Verticillium wilt symptoms. Quantification of pathogen DNA, however, showed that the fungus was present in both the rootstock and the scion tissues. Moreover, the amount of V. dahliae DNA detected in the scion was in some cases higher than the amount found in the rootstock. Thus, the possible use of resistant genotypes as rootstocks to control Verticillium wilt of olive should be taken carefully, and field experiments are required to test if the resistance provided by the rootstock is stable over the time.