Plum Rootstock & Scion Selection
Rootstocks are selected based on several traits including graft compatibility, plant vigor, tolerance of different soil conditions, lack of root suckers and resistance to diseases and insects and should be matched to conditions at each orchard location. Peach and plum are closely related within the genus Prunus. Plum cultivars can be grafted onto rootstocks of either species, or interspecific hybrids. Commercial plum cultivars are commonly grown on plum (Myrobalan 29C and Mariana 2624), peach (Nemaguard and Lovell) and peach-plum hybrid rootstocks.
The California Rootstock Breeding program, which includes researchers from UC Davis and the USDA, is focused on developing semi-dwarfing to dwarfing rootstocks for Prunus species. Dwarfing rootstocks could help growers reduce expensive and labor intensive practices (pruning, thinning and harvesting, DeJong et al. 2005).
- Myrobalan 29C (P. cerasifera) is compatible with most cultivars and tolerates a wide range of soil types and climatic conditions. It produces a hardy, vigorous, long lived, standard size tree, but is prone to suckering (Anderson et al. 2006; LaRue 1973).
- Mariana 2624 (P. cerasifera x P. munsoniana) is compatible with most cultivars and produces a semi-dwarf tree (10 to 15 ft). This rootstock is typically used in northern California because it tolerates wet, heavy soils (Norton and Coates 2012). It acclimates well to a wide range of soil types and climatic conditions. It is resistant to oak root fungus, crown rot, crown gall, and root knot nematode (Southwick et al. 1999; LaRue 1973).
- Nemaguard (P. persica x P. davidiana) is a peach rootstock that produces a vigorous, productive tree with good fruit size and little to no suckering (Johnson et al. 2013). It has good resistance to root-knot nematode, but is susceptible to bacterial canker and Armillaria (Lownsbery et al. 1977). Nemaguard is compatible with most plum cultivars.
- Lovell (Prunus persica) is a peach rootstock that produces a standard size tree with early fruit set and consistent crop. It is compatible with most plum cultivars. Lovell does not tolerate heavy soils (Vossen and Silver, no date) and is susceptibility to root-knot nematode, but is partly resistant to bacterial canker (Lownsbery et al. 1977).
- Citation (P. salicina x P. persica) is a peach-plum hybrid rootstock that produces dwarf trees (8 to 12 ft) and is tolerant of wet soils (Layne, 1994). Citation is resistant to root-knot nematode but susceptible to crown gall and bacterial canker (Johnson et al. 2013).
Fruit shape, size and color vary widely among the 100+ Japanese plum cultivars currently grown in California. Fruit skin and flesh colors include: purple, red, black, green, and yellow. Luther Burbank, a legendary plant breeder, introduced the popular commercial plum cultivars ‘Santa Rosa’, ‘Satsuma’ and ‘Elephant Heart’ in the 1900s. ‘Santa Rosa’ was the most popular cultivar from 1907 until the mid-1980s (Day et al. 2013).
Plum breeding in California has been ongoing since World War II. The University of California, the USDA, and private breeders like John Garabedian and Floyd Zaiger have all contributed to popular plum varieties in California (Okie and Ramming 1999). ‘Friar’, the most popular cultivar with black-skinned fruit grown in California, was first introduced in 1968 by USDA plant breeder John Weinberger (Day et al. 2013). Current plum breeding programs are focused on developing improved flavor, trees resistant to the plum pox virus (Hartmann and Neumüller 2010), and hybrids of plum and apricot (P. salicina x P. armeniaca).
- Japanese plum (Prunus salicina) fruit are larger, rounder and firmer than European plum (Prunus domestica). Most Japanese plum cultivars are self-incompatible and require cross pollination by insects to produce a crop. The California Tree Fruit Agreement recorded sales of all Japanese plum cultivars grown commercially in California from the early 1900s through 2010. In 2010 the top six fresh plum cultivars were Santa Rosa, Angeleno, Friar, Owen T, Blackamber and Black Kat.
- Santa Rosa is a medium size, dark red skinned fruit with yellow flesh. It is self-compatible and often used as a pollinator in the Central Valley of California. In Northern California Santa Rosa occasionally requires cross-pollination by another cultivar for adequate fruit set (LaRue and Norton 1989).
- Angeleno is a large, purple skinned fruit with yellow flesh, harvested in late August through September.
- Friar is a dark purple fruit with yellow flesh and a small stone, harvested in early July.
- Owen T is a new cultivar with purple skin and yellow flesh, harvested in late June to early July. This cultivar is self-compatible.
- Blackamber has black skin and yellow flesh, harvested in June.
- Black Kat is a blackish-blue pluot (hybrid of plum and apricot) fruit with yellow flesh, harvested in late August to September (Johnson et al. 2013).
For a more complete list of popular plum cultivars and their descriptions, please go to “University of California Fruit Report: California Varieties” (http://ucanr.edu/sites/fruitreport/Varieties/List/).
Plum-apricot hybrids (P. salicina and P. armeniaca ) date back as far as 1955 with Luther Burbank’s “plumcot”. Later, in the 1980s Floyd Zaiger released pluot and aprium hybrids. The hybridized fruit’s name is indicative of the contribution of each parent. For example a pluot has 75% plum parentage and 25% apricot parentage; while aprium has 75% apricot parentage and 25% plum parentage (Zhivondov and Uzundzhalieva 2012).