New rootstocks with resistance to plum pox virus for prunus domestica and other stone fruit species: The 'docera' and 'dospina' rootstock series

A breeding program with the aim of obtaining Plum pox virus (PPV) resistant rootstocks for stone fruit cultivars was established at Technische Universität München. Genetic crosses between European plum (P. domestica) genotypes with hypersensitivity resistance to PPV were used as one parent and either P. spinosa or P. cerasifera as the other parent resulting in either 'Dospina' or 'Docera' rootstock candidates, respectively. The seedlings were screened for hypersensitivity resistance. The PPV resistant clones were screened for easy vegetative propagation by green cuttings, hardwood cuttings and in vitro propagation. The selected seedlings were retested extensively for PPV resistance with several European plum cultivars differing in their susceptibility to PPV. PPV infected budsticks of these cultivars were grafted onto the rootstock candidates either in spring or in summer. In many cases, the infected buds were rejected by the rootstock before bud break. Otherwise the young shoot which started to grow from the infected bud or budwood was rejected by the rootstock within several weeks after bud break. Due to this selfeliminating effect of PPV infected buds or scions grafted onto the newly developed PPV resistant rootstocks, no PPV infected tree developed to a marketable size. First results obtained from pomological tests indicate that trees grafted onto 'Docera 6' show similar vegetative and generative properties as those grafted onto 'St. Julien A' rootstock. National and international tests for the pomological evaluation of the rootstock are underway. In a first step, the new hypersensitive rootstocks will be used for newly bred hypersensitive plum cultivars. In this way, neither the rootstock nor the scion cultivar can get infected with PPV under natural inoculation conditions. This gives the highest level of security to damages caused by PPV which is available in stone fruits species at the moment. In a second step, their use for PPV sensitive European plum cultivars as well as for apricot, Japanese plum and peach is under evaluation.