Development of a new method for characterizing olive cultivar resistance to Verticillium dahliae, the causal agent of Verticillium wilt

Verticillium wilt of olive (Olea europaea L.), caused by the soil-borne fungus Verticillium dahliae Kleb., is one of the most important diseases affecting this crop. Using resistant cultivars is among the most effective control measures. Various inoculation methods have been used to assess olive cultivars resistance to Verticillium wilt under controlled conditions, but significant discrepancies often arise when comparing results with field conditions. This study aimed to develop a new method capable of detecting subtle differences in resistance or susceptibility among olive cultivars. Olive cultivars ‘Picual’, ‘Arbequina’, ‘Koroneiki’ and ‘Frantoio’ were inoculated using an artificial substrate containing V. dahliae microsclerotia at two doses (20 and 80 %). For comparison, root seedlings were also immersed in a conidial suspension of the pathogen. The 20 % substrate dose allow distinguished close levels of susceptibility (‘Picual’ and ‘Arbequina’), while the 80 % dose can distinguish between close variations of resistance (‘Koroneiki’ and ‘Frantoio’). To validate these findings under real conditions, a field experiment was conducted over 4 years, demonstrating alignment with the controlled environment results. ‘Frantoio’ consistently showed the highest resistance, ‘Picual’ the greatest susceptibility, and ‘Arbequina’ exhibited intermediate levels, thus confirming the differentiation achieved using the artificial inoculation method. Consequently, this newly developed method offers a significant advancement in the accuracy and reliability of resistance assessments for olive cultivars against Verticillium wilt within breeding programs.