235 Evaluation of Resistance to Phomopsis amygdali in Almond L. Cabrita, A. Neves and J. Leitão Universidade do Algarve- FERN Campus de Gambelas 8005-139 Faro Portugal Keywords: Prunus dulcis, disease resistance, inoculation, hybrids, molecular markers Abstract The disease caused by the fungus Phomopsis amygdali (Del.) Tuset & Portilla is, today, one of the most important in almond growing, since the main commercial cultivars are susceptible to this pathogen. In Algarve (Portugal), the disease has been causing severe damage to introduced foreign commercial cultivars while traditional cultivars are not significantly affected. Field observations have led to the identifica- tion of ‘Barrinho Grado’ as a local cultivar that has shown non-susceptible behaviour in a heavily infected orchard. Inoculation tests with Phomopsis amygdali isolates were performed on young trees obtained from open-pollinated seeds of ‘Barrinho Grado’ (resistant) and ‘Ferragnès’ (susceptible). The ‘Barrinho Grado’ progeny showed higher resistance to the disease than the population descended from ‘Ferragnès’. In order to study the inheritance of resistance to this fungus several controlled crosses were performed involving susceptible and resistant cultivars and the respective prog- eny plants were inoculated with fungus isolates in order to ascertain their phenotype. Moreover, to evaluate genetic variability of parents involved in the crosses, prelim- inary studies using isozymes, microsatellites, RAPD and ISSR markers were per- formed. Genetic similarities between these cultivars were established and suitable molecular markers for genomic mapping were selected. Hybrid populations will be used to construct genetic maps and to identify molecular markers closely linked to the resistance gene(s). INTRODUCTION The fungus Phomopsis amygdali (Del.) Tuset & Portilla (sin: Fusicoccum amygdali Del.) affects almond trees causing cankers on one-year-old branches. Infected twigs and shoots wilt and die, leading, in the most severe attacks, to the death of the entire tree. The main commercial cultivars are susceptible to this pathogen and chemical control is of highly variable efficiency and frequently not effective (Tuset, 1999). For those reasons the disease is one of the most significant limiting factors in the almond industry. In Algarve (Portugal), the disease has been causing severe damage to introduced foreign commercial cultivars while traditional cultivars are not significantly affected. These observations indicate the Portuguese traditional germplasm as a source of resistance to this fungus disease. Field observations have led to the identification of ‘Barrinho Grado’ as a local cultivar that has shown non-susceptible behaviour in a heavily infected orchard. In order to clarify the genetic control and the inheritance of this resistance, several hybrid populations were obtained using resistant and susceptible cultivars as parents, including the traditional cultivar ‘Barrinho Grado’. Studies are currently being carried out using these hybrid progenies with the intention of identifying molecular markers tightly linked to the gene(s) conferring resistance to Phomopsis amygdali and its (their) inclusion in a broader genetic map of molecular markers. MATERIALS AND METHODS Resistance Tests Inoculation tests with Phomopsis amygdali isolates were performed in the spring of 2001 on pot-planted young trees obtained from open pollinated seeds of ‘Barrinho XI th Eucarpia Symp. on Fruit Breed. & Genetics Eds. F. Laurens and K. Evans Acta Hort. 663, ISHS 2004