Wild Olive Genotypes as a Valuable Source of Resistance to Defoliating Verticillium dahliae

Resistance to the defoliating pathotype of Verticillium dahliae has been evaluated in a pool of 68 wild genotypes of olive belonging to the SILVOLIVE collection. Resistance was evaluated by assessing symptom severity using a 0–4 rating scale, estimating the relative area under the disease progress curve (RAUDPC), determining the percentage of dead plants (PDP), and measuring the evolution of morphological parameters in inoculated plants over time. In addition, the density levels of V. dahliae in the stem of root-inoculated genotypes have been quantified by means of quantitative real-time PCR at 35 and 120 days after inoculation (dai). Fifteen genotypes (22%) were cataloged as resistant to V. dahliae (i.e., disease parameters did not significantly differ from those of the resistant cultivar Frantoio, or were even lower). Resistant genotypes are characterized by presenting fewer symptoms and a lower amount of V. dahliae DNA at 120 dai than at 35 dai, indicating their ability to control the disease and reduce the density of the pathogen. The rest of the evaluated genotypes showed variable levels of susceptibility. Overall analysis of all genotypes showed high correlation between symptomatology and the amount of V. dahliae DNA in the stem of inoculated genotypes at 120 dai, rather than at 35 dai. However, correlation at 120 dai was not observed in the set of resistant genotypes, suggesting that resistance to defoliating V. dahliae in olive is based on the occurrence of different mechanisms such as avoidance or tolerance. These mechanisms are valuable for designing breeding programs and for the identification of target genes and resistant rootstocks to better control Verticillium wilt in the olive grove.

Shortening the Period for Assessing the Resistance of Olive to Verticillium Wilt Using Continuous Lighting

The use of continuous and natural lighting was studied to assess the resistance of 12 olive cultivars to the defoliating pathotype of Verticillium dahliae. The plants were inoculated by dipping their bare root system in a conidial suspension. ‘Frantoio’ was used as a moderately resistant control cultivar. Several evaluated cultivars were susceptible to the defoliating pathotype of Verticillium dahliae. However, six of these cultivars were moderately resistant. In most of the evaluated cultivars, continuous lighting allowed for the identification of resistant genotypes during a period that was 3 weeks shorter than that which is normally required in these assessments. This reduction was even greater for the resistant cultivars. The use of continuous lighting could thus be an important tool for use in olive breeding programs, where it is necessary to evaluate many genotypes in short durations and, therefore, to optimize time, space, and labor.

Response of Olive Cultivars to Stem Puncture Inoculation with a Defoliating Pathotype of Verticillium dahliae

The resistance of 26 olive (Olea europaea L.) cultivars to the defoliating pathotype of Verticillium dahliae Kleb. was evaluated in experiments under controlled conditions by stem puncture inoculation. The area under disease progress curve with reference to the maximum value potentially reached over the assessment period (AUDPCP), calculated from the symptom severity values (0–4 rating scale), was the main parameter for assessing resistance, and the percentage of dead plants (PDP), the final mean symptom severity (FMS), and the disease recovery were used as additional parameters. Five cultivars were catalogued as extremely susceptible showing values of AUDPCP, PDP, and FMS higher than 70%, 50%, and 3.0, respectively. Six other cultivars exhibited AUDPCP values ranging from 50% to 70% and FMS higher than 3.0 and were classified as susceptible. A group of 11 cultivars, including ‘Koroneiki’, was moderately susceptible with AUDPCP ranging from 31% to 50%. Finally, ‘Frantoio’, ‘Grosal de Albocafer’, ‘Kato Drys’, and ‘Manzanilla Picúa’ showed maximum AUDPCP values of 22%, no dead plants, and slight symptom severity, being classified as resistant. Stem puncture inoculation was an effective tool for identifying resistance in olive cultivars, but a reduction or a delay in the disease symptom expression of plants was observed regarding the symptom progress observed in plants root dip-inoculated in previous works of resistance assessment. These differences were more pronounced in susceptible than in resistant cultivars.