RESPONSE TO VERTICILLIUM DAHLIAE INFECTION IN A GENETICALLY RELATED SET OF OLIVE CULTIVARS: PRELIMINARY RESULTS

Verticillium wilt (VW), caused by Verticillium dahliae, is the most significant pathogen in many olive-growing regions all around the world. The most effective and economically efficient control measure for Verticillium wilt in olive is the use of resistant cultivars, forming the foundation of integrated disease management strategies. Despite this, resistance is not widely employed, and the selection of olive cultivars for new orchards is primarily influenced by pedoclimatic conditions, market demands or other phytosanitary considerations. Unfortunately, many cultivars meeting these criteria are susceptible to VW. Due to the outbreak of the olive quick decline syndrome, caused by the quarantine bacterium Xylella fastidiosa, only two olive varieties (Leccino and FS17) are allowed for the new plantations in Apulia (Southern Italy). However, the susceptibility of the cv Leccino to Verticillium wilt is well documented in literature, while not much data is available to date on the behaviour of FS17, which is the result of an open pollination cross with Frantoio as a mother plant. This study characterized the responses of several olive genotypes to Verticillium dahliae infection under controlled conditions, laying the foundation for future genotype-phenotype association studies. The tested olive varieties included two groups of genetically related cultivars, each exhibiting varying degrees of resistance to Verticillium wilt. The first group included the susceptible Leccino, and the related cvs Pendolino, Leccio del Corno, Cima di Mola, and Mignola; the second group included the resistant Frantoio and the related cvs Ogliarola Barese, Taggiasca, Laurina, and FS17 or Favolosa. Six-months-old self-rooted cuttings were utilized, and plantlets were inoculated by immersing the roots in a conidial suspension (2x106 ml) of defoliating and nondefoliating strains of the pathogen for 2 minutes. Disease development was assessed based on the incidence and severity of foliar symptoms, and the area under the disease progress curve (AUDPC) was calculated by using the Percent Severity Index (PSI). The severity of vascular discoloration on the stem section was assessed using an empirical scale (ranging from 0 to 4), after 5 months. The results confirmed the well-known resistance of the Frantoio cultivar and the susceptibility of the Leccino cultivar. However, they also suggested that no clear correlations exist between genetically related cultivars. Notably, among the most relevant findings, FS17 exhibited significantly higher susceptibility compared to the genetically related but resistant Frantoio cultivar.