The Effect of Short Irrigation Frequencies on the Development of Verticillium Wilt in the Susceptible Olive Cultivar 'Picual' under Field Conditions.

The effect of irrigation frequency (daily [T1], biweekly [T2], and dryland [T3]) on Verticillium wilt of olive was studied in two fields that were naturally infested with Verticillium dahliae in southern Spain and planted to 'Picual' olive. Disease onset (average 61 weeks after planting) and disease incidence (average 75.6%) did not differ among irrigation treatments in both fields. Irrigation consistently increased disease development regarding dryland treatment, but this effect varied over time. In experiment I, T1, the relative area under the disease progress curve was greater on all recording dates (ranging from 15.8 to 33.7) in comparison with T3 (average 6.6). Data for experiment II were similar to this on the most favorable dates for disease (March to April). The T2 treatment value varied over time depending on the season and experimental field, being difficult to differentiate from the values of T1 and T3. Significant correlation between disease incidence and severity increments during spring and fall with the soil water content of the same or previous favorable seasons was observed. Through these correlations, we detected soil water contents of 24.3% (experiment I) and 23.6% (experiment II), where the increments of disease parameters remained at zero. Therefore, scheduling irrigation treatments based on rainfall may be a feasible method for maintaining the soil moisture below levels that favor for disease development.