Breeding Stone Fruit Rootstocks for High-efficiency Orchards

Productivity of deciduous fruit orchards such as apple were revolutionized by size controlling rootstocks in the early 20^th century, which significantly increased fruit production at the expense of vegetative growth and gave rise to labor efficient training systems. These apple rootstocks were further improved by breeders at the end of the 20^th Century to be tolerant of diseases, insects and edaphic conditions including replant problems. In contrast, rootstock advancements for stone fruit had been hindered due to a lack of productive, size-controlling rootstocks that could be efficiently propagated. The advent of new breeding programs both public and private and the rapid availability of genomics and propagation techniques/facilities has produced many new Prunus hybrid rootstocks for testing and eventual adoption by commercial stone fruit growers. Size-controlling cherry rootstocks have led the way with the Gisela®, Maxma® and Krymsk® hybrids in commercial production and others like the WeiGi and Michigan State series in the testing stage. Peach, nectarine, and plum also have many promising rootstocks such as the Krymsk®, Rootpac®, and Controller™ series currently being widely tested or planted. Others like Viking™, Guardian® and MP-29 have been effective for replant and other pest problems. Fewer size-controlling rootstocks are available with known compatibility with apricot but some of the peach or plum rootstocks may have potential. Gene discovery and mapping among the Prunus genomes has provided information on genes for nematode and fungal resistance, nutrient uptake on calcareous soils, drought, salinity, bacterial canker syndrome, hydraulic conductivity (growth control), and root architecture that serves as a valuable tool to use in marker-assisted selection for important horticultural and disease traits. The incorporation of some of these genes into new rootstock cultivars and their resultant phenotypes will be highlighted as to how they have or can increase production efficiency of stone fruit orchards.