Recovery of Young Olive Trees from <Emphasis Type="Italic">Verticillium dahliae</Emphasis>

Natural recovery from wilt disease symptoms was evaluated in young olive trees root dip inoculated with Verticillium dahliae in a growth chamber over a 12 week period and, later on, when the trees were transplanted in a V. dahliae-free soil in a lathhouse during a period of 127 weeks. Recovery in an individual tree was considered when a plant showed symptom remission after having reached a maximum value of symptom severity. Recovery accounted for 53% of 464 trees that showed wilt symptoms during observations in the two environments. The remaining trees died. Recurrent wilt symptoms were not observed in recovered trees, and recovery was usually accompanied by the production of new green tissues. Recovery was clearly higher in trees inoculated with a non-defoliating (ND) isolate (86.4%) of the pathogen than in those inoculated with a defoliating (D) isolate (23.9%). The percentage of recovery and the level of resistance were significantly correlated. Recovery accounted for 92.1% of the cases in resistant and moderately susceptible cultivars, reaching 100% in plants inoculated with the ND isolate (Table 2); meanwhile it was three times lower (30.1% of the plants) in susceptible and extremely susceptible diseased trees. In the lathhouse, periodical tissue isolations for monitoring the progress of infections over a period of 127 weeks in recovered trees, showed that the pathogen could only be isolated from trees 19 weeks after inoculation. Pathogen isolation was significantly higher from susceptible and extremely susceptible cultivars (84.6%) than from resistant and moderately susceptible ones (33.3%). Results showed that if a tree overcomes infection by pathogen from a single inoculation, and it is able to begin a recovery process, it will not express wilt symptoms again in a pathogen-free environment. The pathogen remained inactive or dead over time in recovered trees. Thus, new infections from rootlets would be necessary for new symptom expression. Recovery from Verticillium wilt is an important natural mechanism that occurs in a high percentage of infected olive trees, and can complement the resistance of the cultivar, particularly in conditions of low inoculum densities of low virulence isolates of the pathogen in the soil.