Richard Buchner, UCCE Farm Advisor Tehama, Butte and Glenn Counties;
Franz Niederholzer, UCCE Farm Advisor Sutter, Yuba and Colusa Counties; and
Katherine Pope, UCCE Farm Advisor Yolo, Solano and Sacramento Counties
As soil treatment options become increasingly limited, more restrictive and less effective, the priority to identify a genetic solution to solve or reduce the replant issue is of increasing interest. One genetic solution is to find or develop rootstocks to help manage soil related problems (soil borne fungi / bacteria, nematodes and soil acidity and excess mineral accumulation). Also of interest are root and tree characteristics imparting canopy size control, good anchorage and little or no root suckering. Recognizing the need for identifying additional rootstocks for California Prune production, University of California Farm Advisors and campus based faculty with funding from the California Dried Plum Board (CDPB) designed and planted 3 large rootstock experiments in 2011 to evaluate 29 prune rootstocks. One experiment is planted near Davis at Wolfskill on Yolo loam, a second in Yuba County on Kilga clay loam and a third in Butte county on Farwell clay adobe alternating with Nord loam. Both Butte and Yuba experienced extensive mortality and were significantly replanted in 2012. As we discuss rootstock attributes, note that trees are still too young for definitive conclusions. Preliminary rootstock data is published in the CDPB research reports. In addition, the Prune production manual (UC ANR # 3507) has a very good chapter describing the traditional rootstock choices.
1) Myrobalan seedling is a seed selection of Prunus cerasifera; Myrobalan 29C is a clonal selection of a vigorous Myrobalan seedling.
2) Marianna 2624 is a clonal rootstock originating in Texas from a Prunus cerasifera by Prunus munsoniana cross. M-30 and M-40 are also clonal selections possibly from the original Texas Marianna or possibly from other Marianna seedlings.
3) Lovell is a peach seedling rootstock (Prunus persica).
4) Krymsk 86 is a plum/peach hybrid (Prunus cerasifera x Prunus persica) that originated in the Krasnodar region of Russia.
5) Atlas and Viking are intraspecific hybrids of peach, almond, apricot and plum developed by Zaiger Genetics.
6) Rootpac-R is a plum/almond hybrid (Prunus cerasifera x Prunus dulcis) developed by Agromillora.
7) Citation is a plum/peach hybrid ( Prunus salicina x Prunus persica) reportedly widely used as a rootstock for fresh market Japanese plum orchards in the San Joaquin Valley.
Rootstocks in the three experiments have only been observed for six growing seasons, but so far, none of the rootstocks have shown rootstock/scion incompatibility.
All rootstocks have been visually rated for vigor. Atlas, Viking, Myrobalan 29c and Lovell impart the greatest vigor with M-30, M-40 and Krymsk 86 close seconds.
Viking, Atlas, M-30, M-40, Lovell, Citation and Krymsk 86 imparted little to no tendency to sucker in the Butte experiment. M-30 and M-40 demonstrated a slightly greater tendency to sucker at the Yuba location.
Tree anchorage was measured as degrees of lean from vertical in 2015. Viking, Atlas, M-30, M-40, Krymsk 86, Lovell, Myrobalan seedling and Myrobalan 29c are all anchored well. Krymsk 86 was the best anchored rootstock in the Yuba experiment. Viking, M-40, Rootpac-R and Marianna 2624 were well anchored at the Yuba County location, but not as well as Krymsk 86.
One interesting observation in the Butte Rootstock experiment is that in 2016, Krymsk 86 rooted trees bloomed about 3 to 5 days later compared to the other rootstocks in the experiment. With only one year of bloom data, it appears rootstock selection in prune may influence full bloom date.
Yield measurements have not favored any particular rootstock yet. Young tree age and poor pollination conditions in 2016 have made yield evaluations questionable.
Nematode resistance is variable depending upon which species of nematode is involved. Scientific comparisons are limited but most of the prune rootstocks do not appear to impart a great deal of resistance to root lesion nematode (Pratylenchus vulnus). Intraspecific hybrids tend to impart some resistance to root knot nematode (Meloidoyne spp) while Krymsk 86 appears to be very susceptible. Lovell peach is susceptible to root knot nematode while plum is variable in susceptibility. Little information is currently available for ring and dagger nematodes.
Crown and root rot (Phytophthora spp.) experience is also limited but in general rootstocks with plum heritage tend to be more resistant while rootstocks with peach heritage tend to be more susceptible. Similarly, plum heritage imparts more crown gall ( Agrobacterium tumefaciens) resistance compared to peach. Krymsk 86 appears to have resistance to oak root fungus ( Armillaria spp.) as does Marianna 2624.
So far, bacterial canker ( Pseudomonas syringae) symptoms have not developed at the Wolfskill or Butte County locations. At the Yuba County experiment, trunk gumming consistent with bacterial canker was observed in 2013. The most effected trees were on M-30 with 61% of the trees showing trunk gumming, followed by Myrobalan 29c (27% trees gumming) and M2624 and Myrobalan seedling, both showing 17% of trunks gumming. Three percent of the trees on M-40 and Rootpac-R showed trunk gumming, while 13% of the Krymsk 86 were symptomatic. Experience in almond rootstock experiments indicates that Lovell and Viking are quite resistant to bacterial canker compared to other rootstocks and no trunk gumming was found on either Lovell or Viking at the Yuba County experiment in 2013.