Agricultural factors affecting Verticillium wilt in olive orchards in Spain E. Rodríguez & J. M. García-garrido & P. A. García & M. Campos Received: 25 October 2007 / Accepted: 31 January 2008 / Published online: 23 February 2008 # KNPV 2008 Abstract In recent years, the spread of Verticillium wilt in olive orchards, caused by the soil-borne pathogen Verticillium dahliae, is often related to intensive modern farming of highly productive culti- vars, planted at high densities, usually irrigated, and under a mechanised system. The effects of agricultur- al factors associated with olive orchards were inves- tigated in an important olive-growing area in southern Spain, as tools in predicting outbreaks of the disease. A stratified double-sampling technique was designed to determine the number of olive orchards needed to survey. A sampling survey was conducted from 2002 to 2005 in 873 olive orchards randomly selected, the owners were interviewed for details of agronomic factors, and orchards were inspected for the presence or absence of the disease. Polymerase chain reaction assays were carried out for identifying V. dahliae pathotypes. Pathogen prevalence showed a significant linear correlation with the mean plant density (r 2 = 0.93), associated predominantly with a less virulent non-defoliating pathotype (r 2 =0.96). Overall, irriga- tion × high density caused disease incidence to peak in super-high-density olive-tree-planting systems. Ol- ive orchards that had V. dahliae, however, did not differ in pathogen prevalence regardless of the olive cultivars. Young olive orchards were significantly more affected by V. dahliae than were old ones, particularly orchards with trees 8 to 12 years old. Irrigation increased pathogen prevalence and disease incidence in very young orchards (<7 years old). The prevalence of the non-defoliating pathotype was statistically high in young orchards whereas the prevalence of a highly virulent defoliating pathotype was high in old orchards. Keywords Disease assessment . Irrigation . Olea europaea . Olive landscape Abbreviations ND non-defoliating pathotype D defoliating pathotype VW Verticillium wilt Introduction Spain, with 2,423,841 ha is the largest producer and exporter of olive and olive oil in the world (COI 2005). This crop is concentrated particularly in the region of Andalusia. In recent years, important new information, Eur J Plant Pathol (2008) 122:287–295 DOI 10.1007/s10658-008-9287-0 E. Rodríguez (*) : J. M. García-garrido : M. Campos Departments of Environmental Protection and Microbiology, Estación Experimental del Zaidín (CSIC), Profesor Albareda 1, 18008 Granada, Spain e-mail: estefania.rodriguez@eez.csic.es P. A. García Departments of Statistics, O.R. University of Granada, Campus de Fuente Nueva s/n 18071, Granada, Spain