Plum selections with total resistance to leaf scald (Xylella fastidiosa)
Leaf scald (Xylella fastidiosa) is the most important plum disease in Brazil. It is widespread in the main production areas because of the presence of effective vectors (sharpshooters) and alternative hosts. Most plum cultivars are susceptible, and the main challenge of plum breeding in Brazil is to obtain resistant cultivars. An extensive breeding program has been developed since 1990 in Santa Catarina State, Brazil, using local selections and cultivars from Florida as sources of resistance. Leaf scald resistance has been evaluated in adult plants based on visual symptoms and PCR diagnosis tests for the presence of X. fastidiosa in petiole tissue using silica-based DNA extraction kits. The inheritance of resistance is polygenic and predominantly recessive. However, some recombinant plum genotypes showed resistance levels even higher than those of the resistant parents. One example is the selection 97-37-2-31, where no symptoms have been observed, and the bacteria was not detected by PCR tests even in plants inoculated by grafting. In crosses with 97-38-2-31, a small percentage (5-10%) of the descendants had no symptoms, and leaf drop occurred very late, at the end of the vegetative cycle. Results of PCR tests for Xylella were also negative for these individuals, indicating that they were not infected by natural vectors. However, some of these highly resistant genotypes became PCR-positive and showed some leaf scald symptoms when grafted on infected plants. Apparently, two forms of total resistance were identified. In one, the resistance is complete, and in the other, just natural transmission does not occur.