Lack of Evidence for Transmission of Verticillium dahliae by the Olive Bark Beetle Phloeotribus scarabaeoides in Olive Trees

Abstract

Verticillium wilt of olive, caused by Verticillium dahliae Kleb., is one of the most important diseases affecting olive crops in the Mediterranean area. With the aim to evaluate the role of Phloeotribus scarabaeoides (Bernard) (olive bark beetle) as a dispersal vector of V. dahliae, several experiments were conducted in semi-controlled conditions from May 2009 to April 2012. Groups of olive trees (2.5-year-old) certified free from V. dahliae were covered by a mosquito net and exposed to adults of P. scarabaeoides by three different ways: (1) branches or trunks collected in several olive orchards from trees severely affected by Verticillium wilt and showing apparent entry holes (mating galleries) of P. scarabaeoides; (2) adults of olive bark beetle extracted from damaged branches collected in the field; (3) adults from damaged branches that were superficially inoculated with V. dahliae. The fungus V. dahliae was not detected either by microbiological and molecular techniques from shoots of olive trees with galleries of the insect or from any of the tissues of the collected beetle adults from the galleries. Additionally, Verticillium wilt disease symptoms were not observed in olive trees exposed to the olive bark beetles. Moreover, the pathogen was never detected from any beetle adults that were recovered from the mating galleries of branches or trunks collected in several olive orchards from trees severely affected by Verticillium wilt. We conclude that P. scarabaeoides is not a vector of V. dahliae under the investigated experimental conditions.