OCCURRENCE AND ETIOLOGY OF DEATH OF YOUNG OLIVE TREES IN SOUTHERN SPAIN

New plantations of olive tree in southern Spain are being severely aff ected by wilt or dieback and death, which has been locally called 'Dry ing Syndrome'. To determine the etiology of this problem, a study was carried out in samples of affected young trees collected during a seve n year period (1989-1995), and in two field surveys in 1994-95 and 199 6. Besides some insect damage and agronomic problems, the 'Drying Synd rome' was associated with Verticillium wilt, winter frost and root rot fungi. Although 'Drying Syndrome' can be distinguished from Verticill ium wilt, the latter was included in this study, since, frequently, Ve rticillium wilt symptoms were unspecific and Verticillium dahliae coul d not be always isolated in the diagnostic work that preceded this stu dy. Early winter frost caused a vascular necrosis and wilt of the youn g olive trees. This unusual and severe damage was related with the lac k of frost hardiness due to warm temperatures during the previous autu mn. Root rot fungi were very frequent in the samples of diseased olive trees of field or nursery origin, and they were the main cause of 'Dr ying Syndrome' in the second field survey, when a heavy rainfall level occurred during winter. Pathogenicity tests showed that five fungal s pecies (Cylindrocarpon destructans, Phytophthora megasperma, P. palmiv ora, Pythium irregulare and Sclerotium rolfsii) were pathogenic to oli ve trees and reproduced symptoms of 'Drying syndrome' in rooted cuttin gs of cultivar Picual. Other fungal species associated with root rot o f olive trees in the field or in the nurseries, including Fusarium acu minatum, F. eumartii, F. oxysporum, F. solani, Macrophomina phaseolina and Rhizoctonia solani, were weakly or not pathogenic. Pathogenicity of P. megasperma, P. palmivora and P. irregulare depended on soil wate r content, since isolates tested caused extensive root rot and sudden plant death only when the soil was continuously waterlogged. The high frequency of P. megasperma in waterlogged field soils and its pathogen icity dependence on soil water content suggest that this pathogen may play an important role in the well known sensitivity of young olive tr ees to 'root asphyxiation'.