First report of Phytophthora palmivora on olive trees in Argentina

Olive plantations (Olea europaea) in Argentina cover about 87,000 hectares mainly located in Catamarca, La Rioja and San Juan areas. Following several reports of decline symptoms, a disease survey was conducted during the spring 2005 in 30 olive plantations ranging from 300 to 4000 ha. Crown dieback and root rot symptoms were recorded with an average incidence of 3% in Catamarca, 50% in La Rioja and 10% in San Juan plantations. The disease incidence has increased year by year.

A Phytophthora sp. was consistently isolated from rotted rootlets on selective V8 medium (PARPNH) (Jung et al., 1996). Isolates produced papillate, caducous sporangia of variable shape, mostly elliptical to ovoid, and chlamydospores; all typical characteristics of P. palmivora. PCR amplicons generated using primers ITS 6 and ITS 4 (Cooke et al., 2000) were sequenced and found identical to those of P. palmivora from the NCBI Genbank database, thus confirming the morphological diagnosis.

The pathogenicity of two P. palmivora isolates was tested by soil infestation in a growth chamber at 25°C, using 15 1-year-old olive seedlings per isolate. Uninoculated seedlings were used as negative control. The inoculum was produced on autoclaved millet grains moistened with V8 juice. Fifty days after inoculation the treated plants showed symptoms similar to the ones observed in the field; necrotic leaves, defoliation and a reduction of the root system of up to 40%. Control plants remained healthy. P. palmivora was reisolated from roots of symptomatic plants.

The pathogen may have been introduced through rooted olive plants of Mediterranean varieties currently used in Argentina. P. palmivora has recently been described as the causal agent of root rot of olive in Italy, where it was isolated from collapsed olive trees (Cacciola et al., 2000). In Spain it has been confirmed as a pathogen of olive although P. megasperma is more commonly associated with field symptoms (Hernández et al., 1998). Alternatively, P. palmivora may have been moved to olive trees from other host species. P. palmivora infects more than 200 species of ornamental, shade and hedge plants, mostly from tropical areas. In Argentina, for instance, P. palmivora was recorded for the first time in 1937 associated with Citrus spp.

This is the first report of root rot caused by P. palmivora on olive groves in Argentina. Due to the severe symptoms and the increasing incidence recorded, P. palmivora should be considered a potential threat to olive cultivation in Argentina.