Differential fungal colonization and physiological defense responses of new olive cultivars infected by the necrotrophic fungus Verticillium dahliae

New olive cultivars adapted to Tunisia’s growing conditions were examined for their resistance to verticillium wilt (VWO) to determine whether differences in susceptibility among currently grown cultivars might contribute to the management of this disease. Based on the evaluation of 14 cultivars, 10 were classified as susceptible or extremely susceptible (Chetoui, Chemlali, Rkhami, Jarboui, Zalmati, Jarboui, Oueslati, Manzanille, Picholine and Frangivento), 2 as moderately susceptible (Koroneiki and Coratina), and 2 as resistant (Meski and Sayali) to VWO. Three cultivars with different susceptibility levels were selected to examine the levels of hydrogen peroxide (H₂O₂), soluble sugars (SS), soluble proteins (SP), total polyphenols (TP), lipid peroxidation, activities of antioxidant enzymes, and fungal biomass in planta. V. dahliae DNA occurred early in the roots at 15 dpi and reached a maximum of 3.507 and 2.52 ng/100 ng of plant DNA, respectively, in the extremely susceptible and resistant cultivars. Fungal DNA in the stems occurred at 30 dpi and increased slowly to reach a maximum of 0.23 ng/100 ng of total DNA in the extremely susceptible cultivars. We showed that the amount of fungal DNA in planta was roughly correlated with the susceptibility to VWO (P < 0.0001; r = 0.95). The comparison of cultivars at the physiological level indicated that olive resistance is roughly correlated with the antioxidant enzymes activity, H₂O₂ concentration, and TP and SP contents. The results of this study open new perspectives for olive genetic improvement programs aiming at developing new cultivars resistant to this wilt.