Comparison of screening methods for the evaluation of olive resistance to Verticillium dahliae kleb

Three inoculation methods for screening olive germplasm for resistance to Verticillium dahliae Kleb. were compared: (i) dipping soil-free washed roots into inoculum (WRI); (ii) dipping root balls with soil into inoculum (RBI); (iii) direct introduction of the inoculum into a stem incision (SII). The effectiveness of the methods was compared in their capacity to differentiate levels of resistance/susceptibility to Verticillium wilt in olive germplasm. Genetically homogeneous clones of the olive cvs Frantoio, Coratina and Leccino, which have different degrees of resistance/susceptibility to wilt, were inoculated with one defoliating and two nondefoliating V. dahliae pathotypes. Results indicate that the three screening methods had different sensitivity in identifying variable levels of olive resistance to Verticillium wilt. Inoculation of washed roots (WRI) was the most drastic and may be used to identify higher levels of resistance (such as that of cv. Frantoio), but was not sensitive enough to detect lower levels of resistance. Inoculation of root balls with soil (RBI) was less destructive and allowed detection of lower resistance expression (such as that of cv. Coratina), not detected by WRI. Stem incision inoculation (SII) failed to discriminate between resistant (cvs Frantoio and Coratina) and susceptible (cv. Leccino) germplasm.