Using DNA Markers to Distinguish Scion and Rootstock Tissues on Grafted Pawpaw(Asimina triloba) Trees

The pawpaw [Asimina triloba (L.) Dunal] is a native tree-fruit in Kentucky that is in the early stages of commercial production. Kentucky State University serves as the USDA National Clonal Germplasm Repository for pawpaw; therefore, research concerning pawpaw genetic diversity and DNA fingerprinting are priorities. Pawpaw varieties are propagated by placing a bud, or scion, from a selected variety onto a seedling root system. Scion death and the formation of shoots from the root system result in a tree which is no longer true to the original variety. Usually trees that develop from rootstock shoots produce poor quality fruit that is inferior to the scion variety. The objective of this study was to determine if scions of trees (‘Sunflower’ or ‘Susquehanna’) in the KSU Scion/rootstock trial are genetically true to type or have been replaced by shoots from the rootstock. Leaf samples were collected from varieties ‘Sunflower’, ‘Susquehanna’, and 7 trees that may not be true to type in the KSU pawpaw rootstock trial. Additional leaf samples of wild pawpaw trees in Cove Spring Park in Frankfort, KY were used as a secondary control. DNA was extracted from leaves using the DNAMITE Plant Kit. Primers B3, B103, B129, and G119 were used to amplify SSR products. These products were then separated using a 3130 Applied Biosystems capillary electrophoresis system. Trees in the Cove Spring patch showed a range of pawpaw genotypes. Scions of six of the seven trees were not true to type and were rootstock derived trees that should be removed.