Root extracts from Mexican avocado (Persea americana var. drymifolia) inhibit the mycelial growth of the oomycete Phytophthora cinnamomi José D. L. Sánchez-Pérez & Ma. Guadalupe Jaimes-Lara & Rafael Salgado-Garciglia & Joel E. López-Meza Received: 27 August 2008 / Accepted: 19 February 2009 / Published online: 6 March 2009 # KNPV 2009 Abstract Crude root extracts from Mexican avocado trees (Persea americana) were screened for antioomy- cete activity against Phytophthora cinnamomi . Forty- eight accessions from Mexican avocado trees were selected with potential resistance to P. cinnamomi according to environmental and site descriptors. Crude root extracts from these accessions were obtained and tested in vitro against the oomycete P. cinnamomi. Seven crude root extracts inhibited mycelial growth (>50%) and only root extracts from accessions 765-01 and 773-01 showed 100% of inhibition. Extracts from accessions 765-01 and 773-01 were analysed by preparative thin-layer chromatography and six fractions were detected under UV light. In both extracts, fractions IV (R f 0.85) and V (R f 0.9) showed in vitro inhibition (100%) against mycelial growth of P. cinnamomi. Fraction V was subjected to GC–MS analysis and stigmastan-3,5-diene (C 29 H 48 ) was identified as the major compound. In vitro assay showed that stigmastan-3,5-diene (100 ppm) inhibited the mycelial growth of P. cinnamomi. The constitutive presence of this compound in avocado roots offers possibilities to identify and to select potentially resistant plants to P. cinnamomi. Keywords Antioomycete activity . Root extracts . Stigmastan-3,5-diene Introduction The oomycete Phytophthora cinnamomi is the causal agent of avocado root rot, the most destructive and important avocado disease (Persea americana) of orchards worldwide (Pegg et al. 2002). Phytophthora cinnamomi causes massive death of fine feeder roots under wet or flooded conditions as a consequence of its short generation time and high reproductive capacity. The control of this pathogen requires a combination of many practical management proce- dures designed to reduce pathogen activity and increase host resistance during critical infection Eur J Plant Pathol (2009) 124:595–601 DOI 10.1007/s10658-009-9446-y J. D. L. Sánchez-Pérez : M. G. Jaimes-Lara : R. Salgado-Garciglia Instituto de Investigaciones Químico Biológicas, Universidad Michoacana de San Nicolás de Hidalgo (UMSNH), Edif. B3, C.P. 58030, Ciudad Universitaria, Morelia, Michoacán, México J. E. López-Meza (*) Centro Multidisciplinario de Estudios en Biotecnología-Facultad de Medicina Veterinaria y Zootecnia, Universidad Michoacana de San Nicolás de Hidalgo, Apdo. Postal 53, Administración Chapultepec, C.P. 58262, Morelia, Michoacán, México e-mail: elmeza@zeus.umich.mx