Problems in the determination of inheritance of Plum pox virus resistance in apricot

In some European countries, breeding for Plum pox virus (PPV) resistance is the most efficient method to control sharka on apricot. Several breeding programs are currently underway in the Czech Republic, Greece, France, Italy and Spain; however, inheritance of resistance to PPV in apricot remains confused. There is not agreement on the hypothesis of the genetic control of PPV. Controversial data have been obtained from the different programs and progenies studied. Hypothesis of the genetic control of PPV resistance involving a single dominant gene, two genes or three genes have been proposed by different authors based on analysis of segregation data from progenies. The lack of agreement might be explained by possible mistakes during the determination of the trait. We have revised some possible sources of errors: parental mislabelling, mistakes in assigning seedling paternity when outcrosses are involved, an inaccurate use of the terms susceptible, tolerant, resistant and immune and lack of a consensus on the procedure for phenotyping the trait in the progenies obtained. Several factors have to be considered: the virus isolate, the source of inoculum, the rootstock, the inoculation method, the time of inoculation, the time of observations, the chilling treatments, the conditions of the greenhouse culture, the number of tested plants per genotype and the number of growing periods studied.