Propagation and field of “BN4 Kr” – a promising sharka resistant rootstock.

Different ‘Mirobolan’ types are still largely used for Prunus sp. propagation. The rootstock resistance has great practical importance in order to limit the spread of Plum pox virus (PPV), given that root suckers represent gateways for the virus. Minoiu and Vlădianu (2001) had developed a mutant of ‘Mirobolan’, named ‘BN 4 Kr’ at the Bistriţa Fruit Research and Development Station, which has been proven resistant to PPV (Minoiu et al., 1997, 1998). Other experiments have confirmed that aphid vectors are not able to transmit PPV to ‘Mirobolan’ ‘BN 4 Kr’ plants (Polák and Kominek, 2014). In order to check the potential of large-scale propagation of ‘BN 4 Kr’, green cuttings and in vitro propagation were studied. Green cuttings propagation was assessed for three consecutive years using different rooting substrates: sand, perlite, and a 1:1 mix of sand and perlite. The results revealed an average rooting between 75.3 and 85.9%, with the highest rooting percentage on the perlite variant. In vitro propagation of ‘BN 4 Kr’ rootstock was tested at Vitroplant Italia. Best culture medium was Murashige and Skoog (1962) supplemented with 2.0 mg L-1 BA and with 0.01 mg L-1 IBA for stabilization, 1.0 mg L-1 BA and 0.01 mg L-1 IBA for multiplication, and 1.5 mg L-1 IBA for rooting. In central Italy (Valdaso, Ancona Province) Polytechnic University of Marche (Ancona, Italy) tested the ‘BN 4 Kr’ rootstocks in replanting conditions, using ‘Myrobalan 29C’ as a control. One-year-old trees of ‘BN 4 Kr’ were planted in November 2007 and grafted with ‘Fortune’ plum in February 2009. The production and fruit quality were recorded from 2011-2015. The ‘Fortune’ on ‘BN 4 Kr’ rootstock showed a production similar to ‘Myrobalan 29C’ but with lower vigour and slightly bigger fruit size. Likely ‘BN 4 Kr’ rootstock induced a lower incidence of phytoplasma symptoms (ESFY), which are a growing problem in plum production. The resistance to PPV correlated with the high potential of vegetative propagation, recommending ‘BN 4 Kr’ as a promising ‘Mirobolan’ rootstock for sharka containment. The exclusive license for propagation and commercialization of ‘BN 4 Kr’ was awarded to Vitroplant Italia, starting with 2016.