Occurrence and etiology of death of young olive trees in southern Spain

New plantations of olive tree in southern Spain are being severely affected by wilt or dieback and death, which has been locally called 'Drying Syndrome'. To determine the etiology of this problem, a study was carried out in samples of affected young trees collected during a seven year period (1989-1995), and in two field surveys in 1994-95 and 1996. Besides some insect damage and agronomic problems, the 'Drying Syndrome' was associated with Verticillium wilt, winter frost and root rot fungi. Although 'Drying Syndrome' can be distinguished from Verticillium wilt, the latter was included in this study, since, frequently, Verticillium wilt symptoms were unspecific and Verticillium dahliae could not be always isolated in the diagnostic work that preceded this study. Early winter frost caused a vascular necrosis and wilt of the young olive trees. This unusual and severe damage was related with the lack of frost hardiness due to warm temperatures during the previous autumn. 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 'Drying Syndrome' in the second field survey, when a heavy rainfall level occurred during winter. Pathogenicity tests showed that five fungal species (Cylindrocarpon destructans, Phytophthora megasperma, P. palmivora, Pythium irregulare and Sclerotium rolfsii) were pathogenic to olive trees and reproduced symptoms of 'Drying syndrome' in rooted cuttings of cultivar Picual. Other fungal species associated with root rot of olive trees in the field or in the nurseries, including Fusarium acuminatum, 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 water 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 pathogenicity dependence on soil water content suggest that this pathogen may play an important role in the well known sensitivity of young olive trees to 'root asphyxiation'.