Symptomless Host and Nonhost Responses of Paulownia (Paulownia spp.) to Olive-Defoliating Verticillium dahliae

Symptomless host and nonhost responses of Paulownia spp. to olive-defoliating (D) Verticillium dahliae is reported for the first time. Two paulownia clones, Paulownia elongata 'PC-2' and P. elongata × P. fortunei 'PC-3', were inoculated with a V. dahliae isolate representative of the D pathotype by either root dip or stem injection with a conidial suspension, repeated transplanting to a V. dahliae-infested soil mixture, or root dip in the conidial suspension followed by transplanting to the infested soil mixture. 'Picual' olive and 'Sugar Baby' watermelon were included in all experiments as susceptible standards to show that the inoculation procedures and incubation conditions were successful. Plants were incubated under conditions optimal for Verticillium wilt that caused severe disease in 'Picual' olive and 'Sugar Baby' watermelon in the growth chamber, shade house, and field microplots for 30 to 57 weeks in three independent experiments. No foliar symptoms developed on paulownia, whose stems were found free of V. dahliae both by isolation on semiselective NP-10 medium as well as by a nested-polymerase chain reaction assay using total genomic DNA from inoculated plants that effectively detected D V. dahliae in olive stems. V. dahliae was isolated to a limited extent from roots of PC-3 paulownia plants after 30 weeks of growth in the infested soil mixture but not from those that were root-dip inoculated or from PC-2 plants regardless the method of inoculation. The symptomless host and nonhost responses of Paulownia spp. to D V. dahliae may have practical applications in the use of fertile soils in southern Spain, particularly in those that are highly infested with the highly virulent D pathotype, as well as a replacement crop for Verticillium wilt-affected olive orchards in that region.