Molecular Epidemiology of Verticillium Wilt of Olive in Southern and Central Tunisia: Evidence of Host Adaptation Hypothesis

Aims: During the last two decades, verticillium wilt of olive has spread to young olive orchards where highly susceptible crops such as potato, watermelon and tomato are cultivated near to olive orchards.
It was therefore hypothesized that there is an adaptation phenomenon that drives the pathogenicity of V. dahliae in order to infect a broad range of hosts. Therefore, it will be useful to identify the factors that increase the severity of the pathogen which helps growers to implement the appropriate crop rotation program.

Place and Duration of Study: This work was performed in the Laboratory of Phytopathology at the Olive Tree Institute (Tunisia) between December 2013 and March 2015.

Methodology: This study was conducted using 62 isolates of V. dahliae recovered from potato, sunflower and olive. Primarily, a cross pathogenicity tests were performed with the three plant hosts using an artificial infection bioassay. Subsequently, all the isolates were compared at the molecular level using twelve SSR markers and the number of genotypes in each group of isolate was determined by assigning a specific SSR haplotype. The relationship between the isolates groups was defined by comparison of SSR genotypes and pathogenicity data obtained for each isolate.

Results: Olive and potato isolates were pathogenic to both crops with similar disease severity. By contrast, only sunflower isolates were unable to infect sunflower plants. SSR analysis revealed that olive group is the most diversified one with 12 different genotypes followed by the sunflower one. When isolates groups were compared, we concluded that potato and olive groups are highly similar at pathogenic and molecular levels whereas sunflower group was genetically different.

Conclusion: Potato and olive isolates were able to infect both crops with similar pathogenicity level which suggest that these isolates have a well-adapted virulence factors to successfully invade their hosts. By contrast, sunflower isolates were unable to induce wilting on olive plants which could be explained by the fact that sunflower is unusually cultivated near to olive orchards and therefore isolates from this host are not adapted to infect the olive.