Characterization of resistance against the olive-defoliating Verticillium dahliae pathotype in selected clones of wild olive

Verticillium wilt of olive is best managed by resistant cultivars, but those currently available show incomplete resistance to the defoliating (D) Verticillium dahliae pathotype. Moreover, these cultivars do not satisfy consumers' demand for high yields and oil quality. Highly resistant rootstocks would be of paramount importance for production of agronomically adapted and commercially desirable olive cultivars in D V. dahliae-infested soils. In this work, resistance to D V. dahliae in wild olive clones Ac-13, Ac-18, OutVert and StopVert was assessed by quantifying the fungal DNA along the stem using a highly sensitive real-time quantitative polymerase chain reaction (qPCR) protocol and a stem colonization index (SCI) based on isolation of V. dahliae following artificial inoculations under conditions highly conducive for verticillium wilt. Ac-13, Ac-18, OutVert and StopVert showed a symptomless reaction to D V. dahliae. The mean amount of D V. dahliae DNA quantified in stems of the four clones ranged from 3.64 to 28.89 pg/100 ng olive DNA, which was 249 to 1537 times lower than that in susceptible Picual olive. The reduction in the quantitative stem colonization of wild olive clones by D V. dahliae was also indicated by a sharp decrease in the SCI. Overall, there was a pattern of decreasing SCI in acropetal progression along the plant axis, as well as correlation between positive reisolation and quantification of pathogen DNA. The results of this research show that wild olive clones Ac-13, Ac-18, OutVert and StopVert have a valuable potential as rootstocks for the management of verticillium wilt in olive.