83 Plum Breeding for Leaf Scald Resistance in Brazil: State of Art and Perspectives M.A. Dalbó 1 , E.D. Bruna 1 , R.O. Nodari 2 and C. Müller 3 1 Epagri, Est. Experimental de Videira, 89560-000 - Videira, SC, Brazil 2 Univ. Fed. de Santa Catarina, Centro de Ciencias Agrarias 88040-900, Florianopolis, SC, Brazil 3 Universidade de São Paulo, Dept. de Entomologia, 3418-900 - Piracicaba, SP, Brazil Keywords: Prunus salicina, Xylella fastidiosa, disease resistance Abstract Leaf scald (Xylella fastidiosa) is the most important plum disease in Brazil. It is widespread in the main production areas due to the presence of effective vectors and X. fastidiosa alternative hosts. Most of plum cultivars are susceptible to this disease and the main challenge of plum breeding in Brazil is to obtain resistant or tolerant cultivars. An extensive breeding program has been developed since 1990 in the State of Santa Catarina, Brazil. Local selections of resistant materials and cultivars from Florida were used as sources of resistance. These materials were crossed with high fruit quality cultivars. The inheritance of resistance was polygenic and predominantly recessive. However, some recombinant genotypes showed resistance levels even higher than the resistant genotypes. New generations of crosses have been made to combine leaf scald resistance and fruit quality. Two cultivars were released and new promising selections are being evaluated in field tests. Selection trials are carried out at different altitudes in order to develop cultivars adapted to specific climatic conditions. Our breeding program received contributions of genetic studies made in collaboration with other institutions. Extensive genetic mapping work has been done in order to study the inheritance of resistance and possible use of marker-assisted selection. Additional studies have analyzed different haplotypes of Xylella associated with leaf scald in Brazil. Two groups of strains (subsp. multiplex and pauca) were identified. Three plum cultivars were inoculated with one strain of each group. Differences in the ability of host colonization were observed but the rank order of cultivar susceptibility was the same for the two strains. Overall, the advancements of the breeding program will help to recover the cultivated area for plum growing that were abandoned years ago due to infestation of leaf scald. INTRODUCTION Leaf scald (Xylella fastidiosa) is the most important limiting factor for plum production in Brazil. This disease is widespread in the main production areas due to the presence of effective vectors (sharpshooters) and X. fastidiosa alternative hosts. The bacteria was probably introduced from Argentina, where it was first reported in 1935 (Fernandez-Valiela and Bakarcic, 1954). The first report of leaf scald in Brazil was made in the 1950s as an unknown disease devastating plum orchards in South Brazil (French and Katajima, 1978). At that time, ‘Santa Rosa’ was the main cultivated variety, which is extremely susceptible to leaf scald. Nowadays, plum production has been made possible by a series of efforts mainly made by growers. In colder areas, the cultivar ‘Laetitia’, which is moderately resistant, became the most important cultivar and is responsible for almost all late plum production in Brazil. In warmer areas, where early harvest is possible, the attack of Xylella fastidiosa is more severe. In these conditions, ‘Gulf Blaze’ is the most planted cultivar now because of its resistance level but the orchards usually have a very short life. Planting Xylella-free material and spraying insecticides during the whole season to kill disease vectors are commonly used in spite of environmental and consumer restrictions. Since most of plum cultivars are susceptible to this disease – and particularly the Proc. X th IS on Plum & Prune Genetics, Breed. & Pomol. Eds.: T.M. DeJong and C.J. DeBuse Acta Hort. 985, ISHS 2013