In Argentina, olives (Olea europaea) are planted on approximately 90,000 ha located primarily in the northwest continental regions. During a 2005 survey, root rot was recorded at several olive plantations in Catamarca, La Rioja, and San Juan provinces (3). Aboveground symptoms associated with root rot were twigs wilting with or without chlorosis, defoliation, and death. Symptoms were initiated on lateral branches and sometimes affected the entire crown. Even if young (5-year-old) trees displayed root rot, aerial symptoms may or may not be seen until years later. Disease incidence varied from 3 to 30%. Rotted rootlets were associated mainly with the infection of Phytophthora palmivora Butler and less frequently with another Phytophthora species. Isolates of this species were heterothallic, had a fluffy growth on carrot agar, and arachnoid growth on potato dextrose agar. Chlamydospores approximately 36 μm in diameter were also produced. The species developed prominent, papillate, noncaducous sporangia of different shapes ranging from ellipsoid to spherical when submerged in saline solution. Sporangia were 35 to 57 × 25 to 45 μm (average 44 × 33 μm), L:B ratio from 1.1 to 1.7. Isolates formed oogonia and amphyginous antheridia following mating type assays. On the basis of morphological features, these isolates were identified as P. nicotianae Breda de Haan. Identity was confirmed by sequencing the rDNA internal transcribed spacer (GenBank Accession No. FJ746693) (1). One-year-old O. europea seedlings were challenged with P. nicotianae (A1 isolates 306G and 339) through soil infestation assay in a growth chamber at 25°C. Infested and uninfested autoclaved millet grains moistened with V8 juice were used to inoculate 15 olive seedlings per isolate and controls, respectively. Fifty days after inoculation, seedlings showed foliar symptoms similar to those observed in the field and had an average of 50% reduction in the root system. Control plants remained healthy. P. nicotianae was always reisolated from symptomatic roots. P. nicotianae was reported on Citrus aurantium in Argentina in 1947 and is currently associated with several hosts (2). In 2002, the same species was reported associated with olive root rot in southern Italy (4). It is possible that P. nicotianae was recently introduced into Argentina through importation of Mediterranean olive varieties. The demonstrated pathogenicity of P. nicotianae on olive together with the recently reported detection of P. palmivora (3) presents a serious threat to olive cultivation in Argentina. References: (1) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (2) M. J. Frezzi. Rev. Investig. Agric. 4:47, 1950. (3) G. Lucero et al. Plant Pathol. 56:728, 2007. (4) F. Nigro and A. Ippolito. Acta Hortic. 586:777, 2002.