Identification of QTL for resistance to plum pox virus strains M and D in Lito and Harcot apricot cultivars

Sharka is a severe apricot viral disease caused by the plum pox virus (PPV) and is responsible for large crop losses in many countries. Among the known PPV strains, both PPV-D (Dideron) and PPV-M (Marcus) are virulent in apricot, the latter being the most threatening. An F1 apricot progeny derived from Lito, described in the literature as resistant, crossed to the susceptible selection BO81604311 (San Castrese x Reale di Imola) was used to study the genetic control of resistance to PPV. A population of 118 individuals was phenotyped by inoculating both PPV-D and PPV-M strains in replicated seedlings and scored for 3 years. An additional set of 231 seedlings from the same cross was also phenotyped for 2 years. SSR-based linkage maps were used for quantitative trait locus (QTL) analysis. A major QTL of resistance to both PPV-M and PPV-D strains was found in the top half of the Lito linkage group 1, where a QTL was previously described in Stark Earli-Orange, the donor of Lito resistance. The LOD score was considerably enhanced when the recovery of plants from infection was taken into account. The results obtained in Lito were compared with those observed in a second apricot cross progeny (Harcot x Reale di Imola) in which QTL of resistance to sharka were also mapped in the same linkage group 1 for both PPV strains. Several models of resistance to sharka disease are discussed considering the segregation frequencies, the QTL alignment in the two maps and the information gathered from the literature.