Control of Verticillium wilt of olive by resistant rootstocks

Aims The potential of grafting susceptible cultivars onto resistant rootstocks was evaluated for the control of Verticillium wilt in olive. Methods Clones of the cultivars 'Frantoio' (highly resistant), 'Coratina' (susceptible) and 'Leccino' (highly susceptible) were used as scions and rootstocks in all combinations, inoculated by root-dipping with a defoliating (D) pathotype of Verticillium dahliae, and incubated in the greenhouse for 3 months. Results Ninety days after inoculation, non-grafted plants of 'Frantoio' and all cultivars grafted onto 'Frantoio' were devoid of wilt symptoms, while non-grafted plants of 'Coratina' (AUDPC=203.3) and 'Leccino' (AUDPC=167.3) were characterized as susceptible. In contrast, when 'Frantoio' was grafted onto susceptible rootstocks, plants became susceptible (AUDPC of 139.8 and 108.9 on 'Frantoio' grafted onto 'Coratina' and Leccino', respectively). No significant difference occurred for vascular browning severity and frequency of V. dahliae reisolation from trunk tissues, though the pathogen was less abundant in the xylem of 'Frantoio'. Microscopic observations on plugging of xylem vessels are presented. Conclusions Grafting of susceptible olive cultivars 'Leccino' and 'Coratina' onto resistant rootstock 'Frantoio' provided an excellent control of Verticillium wilt. It is postulated that the resistance of 'Frantoio' occurs mainly in the roots and involves biochemical mechanisms rather than plant structural characteristics.