Study of long-distance movement of Plum pox virus (Sharka) as an alternative resistance-evaluation method in Prunus

During the breeding programs for Plum pox virus (PPV, Sharka) resistance in Prunus, the evaluation of the new releases through symptoms observation on leaves has been contradictory and represents one of the main handicaps in these programs. In order to increase the accuracy of this traditional evaluation method, we here analyze an alternative method based on the study of the ability of a genotype to allow the long-distance movement of the virus through its vascular vessels. Two different plant models have been assayed: (a) in Model I, the inoculation was performed in the 'GF305' rootstock with a later grafting of the genotype under evaluation and a scion of healthy control 'GF305', to evaluate the long-distance movement through the studied genotype from the rootstock to the scion (xylem transport), and (b) in Model II, the inoculation with 'GF305' diseased scions was performed by grafting these diseased scions onto the studied genotypes, which were grafted onto healthy 'GF305' peach seedlings, to evaluate the long-distance movement through the studied genotype from the scion to the rootstock (phloem transport). The results show that, regardless of the presence of symptoms, susceptible genotypes allowed the movement of the virus through their vascular vessels in both directions studied. However, the resistant apricot 'Stark Early Orange' did not allow this movement. We propose the study of the ability of a genotype to allow the long-distance movement of the virus as an alternative and more accurate method for the evaluation of PPV resistance. However, this protocol is much more tedious than the traditional one and could be used mainly in the evaluation of a reduced number of more interesting genotypes.