Plum Breeding for Leaf Scald Resistance in Brazil: State of Art and Perspectives
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.