Resistance of olive cultivars to the defoliating pathotype of Verticillium dahliae

The resistance of 33 major olive cultivars to Verticillium dahliae was assessed in four experiments conducted under controlled conditions. Nine-month-old nursery olive plants were inoculated with a defoliating isolate (V117) of V. dahliae. Resistance was evaluated by assessing symptom severity using a 0-4 rating scale and estimating the area under disease progress curves with reference to the maximum value potentially reached over the assessment period (AUDPCP). The percentage of plants killed and that of plants that recovered from the disease were used as additional parameters for classifying the cultivars. Twenty-six of the 33 cultivars were found to be susceptible or extremely susceptible to the defoliating V. dahliae isolate. They exhibited values for AUDPCP higher than 45% with mortality higher than 37.5%. A second group of cultivars, which included 'Cipresino', 'Koroneiki', 'Obionga', and 'Sevillenca', were classified as moderately resistant because they showed an important reduction of the final mean severity values and of values for AUDPCP that ranged from 23% to 42%. 'Changlot Real,, 'Empeltre', and 'Frantoio' were resistant to the defoliating isolate of V. dahliae. The resistance of 'Empeltre' and 'Frantoio', although previously reported, was consistently confirmed, because the two cultivars exhibited no dead plants and had very low vales for AUDPCP and final disease. This is the first report of resistance of 'Changlot Real' to Verticillium wilt based on ability of the plants to recover from infection and the absence of dead plants. These three cultivars will be useful for inclusion in Verticillium wilt-breeding programs.